xref: /sof-2.7.6/scripts/kconfig/kconfiglib.py

# SPDX-License-Identifier: ISC
# Copyright (c) 2011-2018, Ulf Magnusson

"""
Overview
========

Kconfiglib is a Python 2/3 library for scripting and extracting information
from Kconfig (https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt)
configuration systems.

See the homepage at https://github.com/ulfalizer/Kconfiglib for a longer
overview.

Using Kconfiglib on the Linux kernel with the Makefile targets
==============================================================

For the Linux kernel, a handy interface is provided by the
scripts/kconfig/Makefile patch, which can be applied with either 'git am' or
the 'patch' utility:

  $ wget -qO- https://raw.githubusercontent.com/ulfalizer/Kconfiglib/master/makefile.patch | git am
  $ wget -qO- https://raw.githubusercontent.com/ulfalizer/Kconfiglib/master/makefile.patch | patch -p1

Warning: Not passing -p1 to patch will cause the wrong file to be patched.

Please tell me if the patch does not apply. It should be trivial to apply
manually, as it's just a block of text that needs to be inserted near the other
*conf: targets in scripts/kconfig/Makefile.

Look further down for a motivation for the Makefile patch and for instructions
on how you can use Kconfiglib without it.

If you do not wish to install Kconfiglib via pip, the Makefile patch is set up
so that you can also just clone Kconfiglib into the kernel root:

  $ git clone git://github.com/ulfalizer/Kconfiglib.git
  $ git am Kconfiglib/makefile.patch  (or 'patch -p1 < Kconfiglib/makefile.patch')

Warning: The directory name Kconfiglib/ is significant in this case, because
it's added to PYTHONPATH by the new targets in makefile.patch.

The targets added by the Makefile patch are described in the following
sections.


make kmenuconfig
----------------

This target runs the curses menuconfig interface with Python 3 (Python 2 is
currently not supported for the menuconfig).


make [ARCH=<arch>] iscriptconfig
--------------------------------

This target gives an interactive Python prompt where a Kconfig instance has
been preloaded and is available in 'kconf'. To change the Python interpreter
used, pass PYTHONCMD=<executable> to make. The default is "python".

To get a feel for the API, try evaluating and printing the symbols in
kconf.defined_syms, and explore the MenuNode menu tree starting at
kconf.top_node by following 'next' and 'list' pointers.

The item contained in a menu node is found in MenuNode.item (note that this can
be one of the constants kconfiglib.MENU and kconfiglib.COMMENT), and all
symbols and choices have a 'nodes' attribute containing their menu nodes
(usually only one). Printing a menu node will print its item, in Kconfig
format.

If you want to look up a symbol by name, use the kconf.syms dictionary.


make scriptconfig SCRIPT=<script> [SCRIPT_ARG=<arg>]
----------------------------------------------------

This target runs the Python script given by the SCRIPT parameter on the
configuration. sys.argv[1] holds the name of the top-level Kconfig file
(currently always "Kconfig" in practice), and sys.argv[2] holds the SCRIPT_ARG
argument, if given.

See the examples/ subdirectory for example scripts.


make dumpvarsconfig
-------------------

This target prints a list of all environment variables referenced from the
Kconfig files, together with their values. See the
Kconfiglib/examples/dumpvars.py script.

Only environment variables that are referenced via the Kconfig preprocessor
$(FOO) syntax are included. The preprocessor was added in Linux 4.18.


Using Kconfiglib without the Makefile targets
=============================================

The make targets are only needed to pick up environment variables exported from
the Kbuild makefiles and referenced inside Kconfig files, via e.g.
'source "arch/$(SRCARCH)/Kconfig" and commands run via '$(shell,...)'.

These variables are referenced as of writing (Linux 4.18), together with sample
values:

  srctree          (.)
  ARCH             (x86)
  SRCARCH          (x86)
  KERNELVERSION    (4.18.0)
  CC               (gcc)
  HOSTCC           (gcc)
  HOSTCXX          (g++)
  CC_VERSION_TEXT  (gcc (Ubuntu 7.3.0-16ubuntu3) 7.3.0)

Older kernels only reference ARCH, SRCARCH, and KERNELVERSION.

If your kernel is recent enough (4.18+), you can get a list of referenced
environment variables via 'make dumpvarsconfig' (see above). Note that this
command is added by the Makefile patch.

To run Kconfiglib without the Makefile patch, set the environment variables
manually:

  $ srctree=. ARCH=x86 SRCARCH=x86 KERNELVERSION=`make kernelversion` ... python(3)
  >>> import kconfiglib
  >>> kconf = kconfiglib.Kconfig()  # filename defaults to "Kconfig"

Search the top-level Makefile for "Additional ARCH settings" to see other
possibilities for ARCH and SRCARCH.


Intro to symbol values
======================

Kconfiglib has the same assignment semantics as the C implementation.

Any symbol can be assigned a value by the user (via Kconfig.load_config() or
Symbol.set_value()), but this user value is only respected if the symbol is
visible, which corresponds to it (currently) being visible in the menuconfig
interface.

For symbols with prompts, the visibility of the symbol is determined by the
condition on the prompt. Symbols without prompts are never visible, so setting
a user value on them is pointless. A warning will be printed by default if
Symbol.set_value() is called on a promptless symbol. Assignments to promptless
symbols are normal within a .config file, so no similar warning will be printed
by load_config().

Dependencies from parents and 'if'/'depends on' are propagated to properties,
including prompts, so these two configurations are logically equivalent:

(1)

  menu "menu"
      depends on A

  if B

  config FOO
      tristate "foo" if D
      default y
      depends on C

  endif

  endmenu

(2)

  menu "menu"
      depends on A

  config FOO
      tristate "foo" if A && B && C && D
      default y if A && B && C

  endmenu

In this example, A && B && C && D (the prompt condition) needs to be non-n for
FOO to be visible (assignable). If its value is m, the symbol can only be
assigned the value m: The visibility sets an upper bound on the value that can
be assigned by the user, and any higher user value will be truncated down.

'default' properties are independent of the visibility, though a 'default' will
often get the same condition as the prompt due to dependency propagation.
'default' properties are used if the symbol is not visible or has no user
value.

Symbols with no user value (or that have a user value but are not visible) and
no (active) 'default' default to n for bool/tristate symbols, and to the empty
string for other symbol types.

'select' works similarly to symbol visibility, but sets a lower bound on the
value of the symbol. The lower bound is determined by the value of the
select*ing* symbol. 'select' does not respect visibility, so non-visible
symbols can be forced to a particular (minimum) value by a select as well.

For non-bool/tristate symbols, it only matters whether the visibility is n or
non-n: m visibility acts the same as y visibility.

Conditions on 'default' and 'select' work in mostly intuitive ways. If the
condition is n, the 'default' or 'select' is disabled. If it is m, the
'default' or 'select' value (the value of the selecting symbol) is truncated
down to m.

When writing a configuration with Kconfig.write_config(), only symbols that are
visible, have an (active) default, or are selected will get written out (note
that this includes all symbols that would accept user values). Kconfiglib
matches the .config format produced by the C implementations down to the
character. This eases testing.

For a visible bool/tristate symbol FOO with value n, this line is written to
.config:

    # CONFIG_FOO is not set

The point is to remember the user n selection (which might differ from the
default value the symbol would get), while at the same sticking to the rule
that undefined corresponds to n (.config uses Makefile format, making the line
above a comment). When the .config file is read back in, this line will be
treated the same as the following assignment:

    CONFIG_FOO=n

In Kconfiglib, the set of (currently) assignable values for a bool/tristate
symbol appear in Symbol.assignable. For other symbol types, just check if
sym.visibility is non-0 (non-n) to see whether the user value will have an
effect.


Intro to the menu tree
======================

The menu structure, as seen in e.g. menuconfig, is represented by a tree of
MenuNode objects. The top node of the configuration corresponds to an implicit
top-level menu, the title of which is shown at the top in the standard
menuconfig interface. (The title is also available in Kconfig.mainmenu_text in
Kconfiglib.)

The top node is found in Kconfig.top_node. From there, you can visit child menu
nodes by following the 'list' pointer, and any following menu nodes by
following the 'next' pointer. Usually, a non-None 'list' pointer indicates a
menu or Choice, but menu nodes for symbols can sometimes have a non-None 'list'
pointer too due to submenus created implicitly from dependencies.

MenuNode.item is either a Symbol or a Choice object, or one of the constants
MENU and COMMENT. The prompt of the menu node can be found in MenuNode.prompt,
which also holds the title for menus and comments. For Symbol and Choice,
MenuNode.help holds the help text (if any, otherwise None).

Most symbols will only have a single menu node. A symbol defined in multiple
locations will have one menu node for each location. The list of menu nodes for
a Symbol or Choice can be found in the Symbol/Choice.nodes attribute.

Note that prompts and help texts for symbols and choices are stored in their
menu node(s) rather than in the Symbol or Choice objects themselves. This makes
it possible to define a symbol in multiple locations with a different prompt or
help text in each location. To get the help text or prompt for a symbol with a
single menu node, do sym.nodes[0].help and sym.nodes[0].prompt, respectively.
The prompt is a (text, condition) tuple, where condition determines the
visibility (see 'Intro to expressions' below).

This organization mirrors the C implementation. MenuNode is called
'struct menu' there, but I thought "menu" was a confusing name.

It is possible to give a Choice a name and define it in multiple locations,
hence why Choice.nodes is also a list.

As a convenience, the properties added at a particular definition location are
available on the MenuNode itself, in e.g. MenuNode.defaults. This is helpful
when generating documentation, so that symbols/choices defined in multiple
locations can be shown with the correct properties at each location.


Intro to expressions
====================

Expressions can be evaluated with the expr_value() function and printed with
the expr_str() function (these are used internally as well). Evaluating an
expression always yields a tristate value, where n, m, and y are represented as
0, 1, and 2, respectively.

The following table should help you figure out how expressions are represented.
A, B, C, ... are symbols (Symbol instances), NOT is the kconfiglib.NOT
constant, etc.

Expression            Representation
----------            --------------
A                     A
"A"                   A (constant symbol)
!A                    (NOT, A)
A && B                (AND, A, B)
A && B && C           (AND, A, (AND, B, C))
A || B                (OR, A, B)
A || (B && C && D)    (OR, A, (AND, B, (AND, C, D)))
A = B                 (EQUAL, A, B)
A != "foo"            (UNEQUAL, A, foo (constant symbol))
A && B = C && D       (AND, A, (AND, (EQUAL, B, C), D))
n                     Kconfig.n (constant symbol)
m                     Kconfig.m (constant symbol)
y                     Kconfig.y (constant symbol)
"y"                   Kconfig.y (constant symbol)

Strings like "foo" in 'default "foo"' or 'depends on SYM = "foo"' are
represented as constant symbols, so the only values that appear in expressions
are symbols***. This mirrors the C implementation.

***For choice symbols, the parent Choice will appear in expressions as well,
but it's usually invisible as the value interfaces of Symbol and Choice are
identical. This mirrors the C implementation and makes different choice modes
"just work".

Manual evaluation examples:

  - The value of A && B is min(A.tri_value, B.tri_value)

  - The value of A || B is max(A.tri_value, B.tri_value)

  - The value of !A is 2 - A.tri_value

  - The value of A = B is 2 (y) if A.str_value == B.str_value, and 0 (n)
    otherwise. Note that str_value is used here instead of tri_value.

    For constant (as well as undefined) symbols, str_value matches the name of
    the symbol. This mirrors the C implementation and explains why
    'depends on SYM = "foo"' above works as expected.

n/m/y are automatically converted to the corresponding constant symbols
"n"/"m"/"y" (Kconfig.n/m/y) during parsing.

Kconfig.const_syms is a dictionary like Kconfig.syms but for constant symbols.

If a condition is missing (e.g., <cond> when the 'if <cond>' is removed from
'default A if <cond>'), it is actually Kconfig.y. The standard __str__()
functions just avoid printing 'if y' conditions to give cleaner output.


Kconfig extensions
==================

Kconfiglib includes a couple of Kconfig extensions:

'source' with relative path
---------------------------

The 'rsource' statement sources Kconfig files with a path relative to directory
of the Kconfig file containing the 'rsource' statement, instead of relative to
the project root.

Consider following directory tree:

  Project
  +--Kconfig
  |
  +--src
     +--Kconfig
     |
     +--SubSystem1
        +--Kconfig
        |
        +--ModuleA
           +--Kconfig

In this example, assume that src/SubSystem1/Kconfig wants to source
src/SubSystem1/ModuleA/Kconfig.

With 'source', this statement would be used:

  source "src/SubSystem1/ModuleA/Kconfig"

With 'rsource', this turns into

  rsource "ModuleA/Kconfig"

If an absolute path is given to 'rsource', it acts the same as 'source'.

'rsource' can be used to create "position-independent" Kconfig trees that can
be moved around freely.


Globbing 'source'
-----------------

'source' and 'rsource' accept glob patterns, sourcing all matching Kconfig
files. They require at least one matching file, throwing a KconfigError
otherwise.

For example, the following statement might source sub1/foofoofoo and
sub2/foobarfoo:

  source "sub[12]/foo*foo"

The glob patterns accepted are the same as for the standard glob.glob()
function.

Two additional statements are provided for cases where it's acceptable for a
pattern to match no files: 'osource' and 'orsource' (the o is for "optional").

For example, the following statements will be no-ops if neither "foo" nor any
files matching "bar*" exist:

  osource "foo"
  osource "bar*"

'orsource' does a relative optional source.

'source' and 'osource' are analogous to 'include' and '-include' in Make.


Generalized def_* keywords
--------------------------

def_int, def_hex, and def_string are available in addition to def_bool and
def_tristate, allowing int, hex, and string symbols to be given a type and a
default at the same time.


Extra optional warnings
-----------------------

Some optional warnings can be controlled via environment variables:

  - KCONFIG_WARN_UNDEF: If set to 'y', warnings will be generated for all
    references to undefined symbols within Kconfig files. The only gotcha is
    that all hex literals must be prefixed with "0x" or "0X", to make it
    possible to distinguish them from symbol references.

    Some projects (e.g. the Linux kernel) use multiple Kconfig trees with many
    shared Kconfig files, leading to some safe undefined symbol references.
    KCONFIG_WARN_UNDEF is useful in projects that only have a single Kconfig
    tree though.

    KCONFIG_STRICT is an older alias for this environment variable, supported
    for backwards compatibility.

  - KCONFIG_WARN_UNDEF_ASSIGN: If set to 'y', warnings will be generated for
    all assignments to undefined symbols within .config files. By default, no
    such warnings are generated.

    This warning can also be enabled/disabled via
    Kconfig.enable/disable_undef_warnings().


Preprocessor user functions defined in Python
---------------------------------------------

Preprocessor functions can be defined in Python, which makes it simple to
integrate information from existing Python tools into Kconfig (e.g. to have
Kconfig symbols depend on hardware information stored in some other format).

Putting a Python module named kconfigfunctions(.py) anywhere in sys.path will
cause it to be imported by Kconfiglib (in Kconfig.__init__()). Note that
sys.path can be customized via PYTHONPATH, and includes the directory of the
module being run by default, as well as installation directories.

If the KCONFIG_FUNCTIONS environment variable is set, it gives a different
module name to use instead of 'kconfigfunctions'.

The imported module is expected to define a global dictionary named 'functions'
that maps function names to Python functions, as follows:

  def my_fn(kconf, name, arg_1, arg_2, ...):
      # kconf:
      #   Kconfig instance
      #
      # name:
      #   Name of the user-defined function ("my-fn"). Think argv[0].
      #
      # arg_1, arg_2, ...:
      #   Arguments passed to the function from Kconfig (strings)
      #
      # Returns a string to be substituted as the result of calling the
      # function
      ...

  def my_other_fn(kconf, name, arg_1, arg_2, ...):
      ...

  functions = {
      "my-fn":       (my_fn,       <min.args>, <max.args>/None),
      "my-other-fn": (my_other_fn, <min.args>, <max.args>/None),
      ...
  }

  ...

<min.args> and <max.args> are the minimum and maximum number of arguments
expected by the function (excluding the implicit 'name' argument). If
<max.args> is None, there is no upper limit to the number of arguments. Passing
an invalid number of arguments will generate a KconfigError exception.

Once defined, user functions can be called from Kconfig in the same way as
other preprocessor functions:

    config FOO
        ...
        depends on $(my-fn,arg1,arg2)

If my_fn() returns "n", this will result in

    config FOO
        ...
        depends on n

Warning
*******

User-defined preprocessor functions are called as they're encountered at parse
time, before all Kconfig files have been processed, and before the menu tree
has been finalized. There are no guarantees that accessing Kconfig symbols or
the menu tree via the 'kconf' parameter will work, and it could potentially
lead to a crash. The 'kconf' parameter is provided for future extension (and
because the predefined functions take it anyway).

Preferably, user-defined functions should be stateless.


Feedback
========

Send bug reports, suggestions, and questions to ulfalizer a.t Google's email
service, or open a ticket on the GitHub page.
"""
import errno
import glob
import importlib
import os
import platform
import re
import subprocess
import sys
import textwrap

# File layout:
#
# Public classes
# Public functions
# Internal functions
# Public global constants
# Internal global constants

# Line length: 79 columns

#
# Public classes
#

class Kconfig(object):
    """
    Represents a Kconfig configuration, e.g. for x86 or ARM. This is the set of
    symbols, choices, and menu nodes appearing in the configuration. Creating
    any number of Kconfig objects (including for different architectures) is
    safe. Kconfiglib doesn't keep any global state.

    The following attributes are available. They should be treated as
    read-only, and some are implemented through @property magic.

    syms:
      A dictionary with all symbols in the configuration, indexed by name. Also
      includes all symbols that are referenced in expressions but never
      defined, except for constant (quoted) symbols.

      Undefined symbols can be recognized by Symbol.nodes being empty -- see
      the 'Intro to the menu tree' section in the module docstring.

    const_syms:
      A dictionary like 'syms' for constant (quoted) symbols

    named_choices:
      A dictionary like 'syms' for named choices (choice FOO)

    defined_syms:
      A list with all defined symbols, in the same order as they appear in the
      Kconfig files. Symbols defined in multiple locations appear multiple
      times.

      Note: You probably want to use 'unique_defined_syms' instead. This
      attribute is mostly maintained for backwards compatibility.

    unique_defined_syms:
      A list like 'defined_syms', but with duplicates removed. Just the first
      instance is kept for symbols defined in multiple locations. Kconfig order
      is preserved otherwise.

      Using this attribute instead of 'defined_syms' can save work, and
      automatically gives reasonable behavior when writing configuration output
      (symbols defined in multiple locations only generate output once, while
      still preserving Kconfig order for readability).

    choices:
      A list with all choices, in the same order as they appear in the Kconfig
      files.

      Note: You probably want to use 'unique_choices' instead. This attribute
      is mostly maintained for backwards compatibility.

    unique_choices:
      Analogous to 'unique_defined_syms', for choices. Named choices can have
      multiple definition locations.

    menus:
      A list with all menus, in the same order as they appear in the Kconfig
      files

    comments:
      A list with all comments, in the same order as they appear in the Kconfig
      files

    kconfig_filenames:
      A list with the filenames of all Kconfig files included in the
      configuration, relative to $srctree (or relative to the current directory
      if $srctree isn't set).

      The files are listed in the order they are source'd, starting with the
      top-level Kconfig file. If a file is source'd multiple times, it will
      appear multiple times. Use set() to get unique filenames.

      Note: Using this for incremental builds is redundant. Kconfig.sync_deps()
      already indirectly catches any file modifications that change the
      configuration output.

    env_vars:
      A set() with the names of all environment variables referenced in the
      Kconfig files.

      Only environment variables referenced with the preprocessor $(FOO) syntax
      will be registered. The older $FOO syntax is only supported for backwards
      compatibility.

      Also note that $(FOO) won't be registered unless the environment variable
      $FOO is actually set. If it isn't, $(FOO) is an expansion of an unset
      preprocessor variable (which gives the empty string).

      Another gotcha is that environment variables referenced in the values of
      recursively expanded preprocessor variables (those defined with =) will
      only be registered if the variable is actually used (expanded) somewhere.

      The note from the 'kconfig_filenames' documentation applies here too.

    n/m/y:
      The predefined constant symbols n/m/y. Also available in const_syms.

    modules:
      The Symbol instance for the modules symbol. Currently hardcoded to
      MODULES, which is backwards compatible. Kconfiglib will warn if
      'option modules' is set on some other symbol. Tell me if you need proper
      'option modules' support.

      'modules' is never None. If the MODULES symbol is not explicitly defined,
      its tri_value will be 0 (n), as expected.

      A simple way to enable modules is to do 'kconf.modules.set_value(2)'
      (provided the MODULES symbol is defined and visible). Modules are
      disabled by default in the kernel Kconfig files as of writing, though
      nearly all defconfig files enable them (with 'CONFIG_MODULES=y').

    defconfig_list:
      The Symbol instance for the 'option defconfig_list' symbol, or None if no
      defconfig_list symbol exists. The defconfig filename derived from this
      symbol can be found in Kconfig.defconfig_filename.

    defconfig_filename:
      The filename given by the defconfig_list symbol. This is taken from the
      first 'default' with a satisfied condition where the specified file
      exists (can be opened for reading). If a defconfig file foo/defconfig is
      not found and $srctree was set when the Kconfig was created,
      $srctree/foo/defconfig is looked up as well.

      'defconfig_filename' is None if either no defconfig_list symbol exists,
      or if the defconfig_list symbol has no 'default' with a satisfied
      condition that specifies a file that exists.

      Gotcha: scripts/kconfig/Makefile might pass --defconfig=<defconfig> to
      scripts/kconfig/conf when running e.g. 'make defconfig'. This option
      overrides the defconfig_list symbol, meaning defconfig_filename might not
      always match what 'make defconfig' would use.

    top_node:
      The menu node (see the MenuNode class) of the implicit top-level menu.
      Acts as the root of the menu tree.

    mainmenu_text:
      The prompt (title) of the top menu (top_node). Defaults to "Main menu".
      Can be changed with the 'mainmenu' statement (see kconfig-language.txt).

    variables:
      A dictionary with all preprocessor variables, indexed by name. See the
      Variable class.

    warnings:
      A list of strings containing all warnings that have been generated. This
      allows flexibility in how warnings are printed and processed.

      See the 'warn_to_stderr' parameter to Kconfig.__init__() and the
      Kconfig.enable/disable_stderr_warnings() functions as well. Note that
      warnings still get added to Kconfig.warnings when 'warn_to_stderr' is
      True.

      Just as for warnings printed to stderr, only optional warnings that are
      enabled will get added to Kconfig.warnings. See the various
      Kconfig.enable/disable_*_warnings() functions.

    srctree:
      The value of the $srctree environment variable when the configuration was
      loaded, or the empty string if $srctree wasn't set. This gives nice
      behavior with os.path.join(), which treats "" as the current directory,
      without adding "./".

      Kconfig files are looked up relative to $srctree (unless absolute paths
      are used), and .config files are looked up relative to $srctree if they
      are not found in the current directory. This is used to support
      out-of-tree builds. The C tools use this environment variable in the same
      way.

      Changing $srctree after creating the Kconfig instance has no effect. Only
      the value when the configuration is loaded matters. This avoids surprises
      if multiple configurations are loaded with different values for $srctree.

    config_prefix:
      The value of the $CONFIG_ environment variable when the configuration was
      loaded. This is the prefix used (and expected) on symbol names in .config
      files and C headers. Defaults to "CONFIG_". Used in the same way in the C
      tools.

      Like for srctree, only the value of $CONFIG_ when the configuration is
      loaded matters.
    """
    __slots__ = (
        "_encoding",
        "_functions",
        "_set_match",
        "_unset_match",
        "_warn_for_no_prompt",
        "_warn_for_override",
        "_warn_for_redun_assign",
        "_warn_for_undef_assign",
        "_warn_to_stderr",
        "_warnings_enabled",
        "choices",
        "comments",
        "config_prefix",
        "const_syms",
        "defconfig_list",
        "defined_syms",
        "env_vars",
        "kconfig_filenames",
        "m",
        "mainmenu_text",
        "menus",
        "modules",
        "n",
        "named_choices",
        "srctree",
        "syms",
        "top_node",
        "unique_choices",
        "unique_defined_syms",
        "variables",
        "warnings",
        "y",

        # Parsing-related
        "_parsing_kconfigs",
        "_readline",
        "_filename",
        "_linenr",
        "_include_path",
        "_filestack",
        "_line",
        "_tokens",
        "_tokens_i",
        "_reuse_tokens",
    )

    #
    # Public interface
    #

    def __init__(self, filename="Kconfig", warn=True, warn_to_stderr=True,
                 encoding="utf-8"):
        """
        Creates a new Kconfig object by parsing Kconfig files.
        Note that Kconfig files are not the same as .config files (which store
        configuration symbol values).

        See the module docstring for some environment variables that influence
        default warning settings (KCONFIG_WARN_UNDEF and
        KCONFIG_WARN_UNDEF_ASSIGN).

        Raises KconfigError on syntax errors, and (possibly a subclass of)
        IOError on IO errors ('errno', 'strerror', and 'filename' are
        available). Note that IOError can be caught as OSError on Python 3.

        filename (default: "Kconfig"):
          The Kconfig file to load. For the Linux kernel, you'll want "Kconfig"
          from the top-level directory, as environment variables will make sure
          the right Kconfig is included from there (arch/$SRCARCH/Kconfig as of
          writing).

          If $srctree is set, 'filename' will be looked up relative to it.
          $srctree is also used to look up source'd files within Kconfig files.
          See the class documentation.

          If you are using Kconfiglib via 'make scriptconfig', the filename of
          the base base Kconfig file will be in sys.argv[1]. It's currently
          always "Kconfig" in practice.

        warn (default: True):
          True if warnings related to this configuration should be generated.
          This can be changed later with Kconfig.enable/disable_warnings(). It
          is provided as a constructor argument since warnings might be
          generated during parsing.

          See the other Kconfig.enable_*_warnings() functions as well, which
          enable or suppress certain warnings when warnings are enabled.

          All generated warnings are added to the Kconfig.warnings list. See
          the class documentation.

        warn_to_stderr (default: True):
          True if warnings should be printed to stderr in addition to being
          added to Kconfig.warnings.

          This can be changed later with
          Kconfig.enable/disable_stderr_warnings().

        encoding (default: "utf-8"):
          The encoding to use when reading and writing files. If None, the
          encoding specified in the current locale will be used.

          The "utf-8" default avoids exceptions on systems that are configured
          to use the C locale, which implies an ASCII encoding.

          This parameter has no effect on Python 2, due to implementation
          issues (regular strings turning into Unicode strings, which are
          distinct in Python 2). Python 2 doesn't decode regular strings
          anyway.

          Related PEP: https://www.python.org/dev/peps/pep-0538/
        """
        self.srctree = os.environ.get("srctree", "")
        self.config_prefix = os.environ.get("CONFIG_", "CONFIG_")

        # Regular expressions for parsing .config files
        self._set_match = _re_match(self.config_prefix + r"([^=]+)=(.*)")
        self._unset_match = \
            _re_match(r"# {}([^ ]+) is not set".format(self.config_prefix))


        self.warnings = []

        self._warnings_enabled = warn
        self._warn_to_stderr = warn_to_stderr
        self._warn_for_undef_assign = \
            os.environ.get("KCONFIG_WARN_UNDEF_ASSIGN") == "y"
        self._warn_for_redun_assign = self._warn_for_override = True


        self._encoding = encoding


        self.syms = {}
        self.const_syms = {}
        self.defined_syms = []

        self.named_choices = {}
        self.choices = []

        self.menus = []
        self.comments = []

        for nmy in "n", "m", "y":
            sym = Symbol()
            sym.kconfig = self
            sym.name = nmy
            sym.is_constant = True
            sym.orig_type = TRISTATE
            sym._cached_tri_val = STR_TO_TRI[nmy]

            self.const_syms[nmy] = sym

        self.n = self.const_syms["n"]
        self.m = self.const_syms["m"]
        self.y = self.const_syms["y"]

        # Make n/m/y well-formed symbols
        for nmy in "n", "m", "y":
            sym = self.const_syms[nmy]
            sym.rev_dep = sym.weak_rev_dep = sym.direct_dep = self.n


        # Maps preprocessor variables names to Variable instances
        self.variables = {}

        # Predefined preprocessor functions, with min/max number of arguments
        self._functions = {
            "info":       (_info_fn,       1, 1),
            "error-if":   (_error_if_fn,   2, 2),
            "filename":   (_filename_fn,   0, 0),
            "lineno":     (_lineno_fn,     0, 0),
            "shell":      (_shell_fn,      1, 1),
            "warning-if": (_warning_if_fn, 2, 2),
        }

        # Add any user-defined preprocessor functions
        try:
            self._functions.update(
                importlib.import_module(
                    os.environ.get("KCONFIG_FUNCTIONS", "kconfigfunctions")
                ).functions)
        except ImportError:
            pass


        # This is used to determine whether previously unseen symbols should be
        # registered. They shouldn't be if we parse expressions after parsing,
        # as part of Kconfig.eval_string().
        self._parsing_kconfigs = True

        self.modules = self._lookup_sym("MODULES")
        self.defconfig_list = None

        self.top_node = MenuNode()
        self.top_node.kconfig = self
        self.top_node.item = MENU
        self.top_node.is_menuconfig = True
        self.top_node.visibility = self.y
        self.top_node.prompt = ("Main menu", self.y)
        self.top_node.parent = None
        self.top_node.dep = self.y
        self.top_node.filename = filename
        self.top_node.linenr = 1
        self.top_node.include_path = ()

        # Parse the Kconfig files

        # Not used internally. Provided as a convenience.
        self.kconfig_filenames = [filename]
        self.env_vars = set()

        # Used to avoid retokenizing lines when we discover that they're not
        # part of the construct currently being parsed. This is kinda like an
        # unget operation.
        self._reuse_tokens = False

        # Keeps track of the location in the parent Kconfig files. Kconfig
        # files usually source other Kconfig files. See _enter_file().
        self._filestack = []
        self._include_path = ()

        # The current parsing location
        self._filename = filename
        self._linenr = 0

        # Open the top-level Kconfig file. Store the readline() method directly
        # as a small optimization.
        self._readline = \
            self._open(os.path.join(self.srctree, filename), "r").readline

        try:
            # Parse everything
            self._parse_block(None, self.top_node, self.top_node)
        except UnicodeDecodeError as e:
            _decoding_error(e, self._filename)

        # Close the top-level Kconfig file. __self__ fetches the 'file' object
        # for the method.
        self._readline.__self__.close()

        self.top_node.list = self.top_node.next
        self.top_node.next = None

        self._parsing_kconfigs = False

        self.unique_defined_syms = _ordered_unique(self.defined_syms)
        self.unique_choices = _ordered_unique(self.choices)

        # Do various post-processing of the menu tree
        self._finalize_tree(self.top_node, self.y)


        # Do sanity checks. Some of these depend on everything being finalized.
        self._check_sym_sanity()
        self._check_choice_sanity()

        # KCONFIG_STRICT is an older alias for KCONFIG_WARN_UNDEF, supported
        # for backwards compatibility
        if os.environ.get("KCONFIG_WARN_UNDEF") == "y" or \
           os.environ.get("KCONFIG_STRICT") == "y":

            self._check_undef_syms()


        # Build Symbol._dependents for all symbols and choices
        self._build_dep()

        # Check for dependency loops
        check_dep_loop_sym = _check_dep_loop_sym  # Micro-optimization
        for sym in self.unique_defined_syms:
            check_dep_loop_sym(sym, False)

        # Add extra dependencies from choices to choice symbols that get
        # awkward during dependency loop detection
        self._add_choice_deps()


        self._warn_for_no_prompt = True

        self.mainmenu_text = self.top_node.prompt[0]

    @property
    def defconfig_filename(self):
        """
        See the class documentation.
        """
        if self.defconfig_list:
            for filename, cond in self.defconfig_list.defaults:
                if expr_value(cond):
                    try:
                        with self._open_config(filename.str_value) as f:
                            return f.name
                    except IOError:
                        continue

        return None

    def load_config(self, filename=None, replace=True, verbose=True):
        """
        Loads symbol values from a file in the .config format. Equivalent to
        calling Symbol.set_value() to set each of the values.

        "# CONFIG_FOO is not set" within a .config file sets the user value of
        FOO to n. The C tools work the same way.

        The Symbol.user_value attribute can be inspected afterwards to see what
        value the symbol was assigned in the .config file (if any). The user
        value might differ from Symbol.str/tri_value if there are unsatisfied
        dependencies.

        Raises (possibly a subclass of) IOError on IO errors ('errno',
        'strerror', and 'filename' are available). Note that IOError can be
        caught as OSError on Python 3.

        filename (default: None):
          Path to load configuration from (a string). Respects $srctree if set
          (see the class documentation).

          If 'filename' is None (the default), the configuration file to load
          (if any) is calculated automatically, giving the behavior you'd
          usually want:

            1. If the KCONFIG_CONFIG environment variable is set, it gives the
               path to the configuration file to load. Otherwise, ".config" is
               used. See standard_config_filename().

            2. If the path from (1.) doesn't exist, the configuration file
               given by kconf.defconfig_filename is loaded instead, which is
               derived from the 'option defconfig_list' symbol.

            3. If (1.) and (2.) fail to find a configuration file to load, no
               configuration file is loaded, and symbols retain their current
               values (e.g., their default values). This is not an error.

           See the return value as well.

        replace (default: True):
          If True, all existing user values will be cleared before loading the
          .config. Pass False to merge configurations.

        verbose (default: True):
          If True and filename is None (automatically infer configuration
          file), a message will be printed to stdout telling which file got
          loaded (or that no file got loaded). This is meant to reduce
          boilerplate in tools.

        Returns True if an existing configuration was loaded (that didn't come
        from the 'option defconfig_list' symbol), and False otherwise. This is
        mostly useful in conjunction with filename=None, as True will always be
        returned otherwise.
        """
        loaded_existing = True
        if filename is None:
            filename = standard_config_filename()
            if os.path.exists(filename):
                if verbose:
                    print("Using existing configuration '{}' as base"
                          .format(filename))
            else:
                filename = self.defconfig_filename
                if filename is None:
                    if verbose:
                        print("Using default symbol values as base")
                    return False

                if verbose:
                    print("Using default configuration found in '{}' as "
                          "base".format(filename))

                loaded_existing = False

        # Disable the warning about assigning to symbols without prompts. This
        # is normal and expected within a .config file.
        self._warn_for_no_prompt = False

        # This stub only exists to make sure _warn_for_no_prompt gets reenabled
        try:
            self._load_config(filename, replace)
        except UnicodeDecodeError as e:
            _decoding_error(e, filename)
        finally:
            self._warn_for_no_prompt = True

        return loaded_existing

    def _load_config(self, filename, replace):
        with self._open_config(filename) as f:
            if replace:
                # If we're replacing the configuration, keep track of which
                # symbols and choices got set so that we can unset the rest
                # later. This avoids invalidating everything and is faster.
                # Another benefit is that invalidation must be rock solid for
                # it to work, making it a good test.

                for sym in self.unique_defined_syms:
                    sym._was_set = False

                for choice in self.unique_choices:
                    choice._was_set = False

            # Small optimizations
            set_match = self._set_match
            unset_match = self._unset_match
            syms = self.syms

            for linenr, line in enumerate(f, 1):
                # The C tools ignore trailing whitespace
                line = line.rstrip()

                match = set_match(line)
                if match:
                    name, val = match.groups()
                    if name not in syms:
                        self._warn_undef_assign_load(name, val, filename,
                                                     linenr)
                        continue

                    sym = syms[name]
                    if not sym.nodes:
                        self._warn_undef_assign_load(name, val, filename,
                                                     linenr)
                        continue

                    if sym.orig_type in _BOOL_TRISTATE:
                        # The C implementation only checks the first character
                        # to the right of '=', for whatever reason
                        if not ((sym.orig_type is BOOL and
                                 val.startswith(("y", "n"))) or
                                (sym.orig_type is TRISTATE and
                                 val.startswith(("y", "m", "n")))):
                            self._warn("'{}' is not a valid value for the {} "
                                       "symbol {}. Assignment ignored."
                                       .format(val, TYPE_TO_STR[sym.orig_type],
                                               _name_and_loc(sym)),
                                       filename, linenr)
                            continue

                        val = val[0]

                        if sym.choice and val != "n":
                            # During .config loading, we infer the mode of the
                            # choice from the kind of values that are assigned
                            # to the choice symbols

                            prev_mode = sym.choice.user_value
                            if prev_mode is not None and \
                               TRI_TO_STR[prev_mode] != val:

                                self._warn("both m and y assigned to symbols "
                                           "within the same choice",
                                           filename, linenr)

                            # Set the choice's mode
                            sym.choice.set_value(val)

                    elif sym.orig_type is STRING:
                        match = _conf_string_match(val)
                        if not match:
                            self._warn("malformed string literal in "
                                       "assignment to {}. Assignment ignored."
                                       .format(_name_and_loc(sym)),
                                       filename, linenr)
                            continue

                        val = unescape(match.group(1))

                else:
                    match = unset_match(line)
                    if not match:
                        # Print a warning for lines that match neither
                        # set_match() nor unset_match() and that are not blank
                        # lines or comments. 'line' has already been
                        # rstrip()'d, so blank lines show up as "" here.
                        if line and not line.lstrip().startswith("#"):
                            self._warn("ignoring malformed line '{}'"
                                       .format(line),
                                       filename, linenr)

                        continue

                    name = match.group(1)
                    if name not in syms:
                        self._warn_undef_assign_load(name, "n", filename,
                                                     linenr)
                        continue

                    sym = syms[name]
                    if sym.orig_type not in _BOOL_TRISTATE:
                        continue

                    val = "n"

                # Done parsing the assignment. Set the value.

                if sym._was_set:
                    # Use strings for bool/tristate user values in the warning
                    if sym.orig_type in _BOOL_TRISTATE:
                        display_user_val = TRI_TO_STR[sym.user_value]
                    else:
                        display_user_val = sym.user_value

                    msg = '{} set more than once. Old value: "{}", new value: "{}".'.format(
                        _name_and_loc(sym), display_user_val, val
                    )

                    if display_user_val == val:
                        self._warn_redun_assign(msg, filename, linenr)
                    else:
                        self._warn_override(msg, filename, linenr)

                sym.set_value(val)

        if replace:
            # If we're replacing the configuration, unset the symbols that
            # didn't get set

            for sym in self.unique_defined_syms:
                if not sym._was_set:
                    sym.unset_value()

            for choice in self.unique_choices:
                if not choice._was_set:
                    choice.unset_value()

    def write_autoconf(self, filename,
                       header="/* Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib) */\n"):
        r"""
        Writes out symbol values as a C header file, matching the format used
        by include/generated/autoconf.h in the kernel.

        The ordering of the #defines matches the one generated by
        write_config(). The order in the C implementation depends on the hash
        table implementation as of writing, and so won't match.

        filename:
          Self-explanatory.

        header (default: "/* Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib) */\n"):
          Text that will be inserted verbatim at the beginning of the file. You
          would usually want it enclosed in '/* */' to make it a C comment,
          and include a final terminating newline.
        """
        with self._open(filename, "w") as f:
            f.write(header)

            for sym in self.unique_defined_syms:
                # Note: _write_to_conf is determined when the value is
                # calculated. This is a hidden function call due to
                # property magic.
                val = sym.str_value
                if sym._write_to_conf:
                    if sym.orig_type in _BOOL_TRISTATE:
                        if val != "n":
                            f.write("#define {}{}{} 1\n"
                                    .format(self.config_prefix, sym.name,
                                            "_MODULE" if val == "m" else ""))

                    elif sym.orig_type is STRING:
                        f.write('#define {}{} "{}"\n'
                                .format(self.config_prefix, sym.name,
                                        escape(val)))

                    else:  # sym.orig_type in _INT_HEX:
                        if sym.orig_type is HEX and \
                           not val.startswith(("0x", "0X")):
                            val = "0x" + val

                        f.write("#define {}{} {}\n"
                                .format(self.config_prefix, sym.name, val))

    def write_config(self, filename=None,
                     header="# Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)\n",
                     save_old=True, verbose=True):
        r"""
        Writes out symbol values in the .config format. The format matches the
        C implementation, including ordering.

        Symbols appear in the same order in generated .config files as they do
        in the Kconfig files. For symbols defined in multiple locations, a
        single assignment is written out corresponding to the first location
        where the symbol is defined.

        See the 'Intro to symbol values' section in the module docstring to
        understand which symbols get written out.

        filename (default: None):
          Filename to save configuration to (a string).

          If None (the default), the filename in the the environment variable
          KCONFIG_CONFIG is used if set, and ".config" otherwise. See
          standard_config_filename().

        header (default: "# Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)\n"):
          Text that will be inserted verbatim at the beginning of the file. You
          would usually want each line to start with '#' to make it a comment,
          and include a final terminating newline.

        save_old (default: True):
          If True and <filename> already exists, a copy of it will be saved to
          .<filename>.old in the same directory before the new configuration is
          written. The leading dot is added only if the filename doesn't
          already start with a dot.

          Errors are silently ignored if .<filename>.old cannot be written
          (e.g. due to being a directory).

        verbose (default: True):
          If True and filename is None (automatically infer configuration
          file), a message will be printed to stdout telling which file got
          written. This is meant to reduce boilerplate in tools.
        """
        if filename is None:
            filename = standard_config_filename()
        else:
            verbose = False

        if save_old:
            _save_old(filename)

        with self._open(filename, "w") as f:
            f.write(header)

            for node in self.node_iter(unique_syms=True):
                item = node.item

                if item.__class__ is Symbol:
                    f.write(item.config_string)

                elif expr_value(node.dep) and \
                     ((item is MENU and expr_value(node.visibility)) or
                       item is COMMENT):

                    f.write("\n#\n# {}\n#\n".format(node.prompt[0]))

        if verbose:
            print("Configuration written to '{}'".format(filename))

    def write_min_config(self, filename,
                         header="# Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)\n"):
        """
        Writes out a "minimal" configuration file, omitting symbols whose value
        matches their default value. The format matches the one produced by
        'make savedefconfig'.

        The resulting configuration file is incomplete, but a complete
        configuration can be derived from it by loading it. Minimal
        configuration files can serve as a more manageable configuration format
        compared to a "full" .config file, especially when configurations files
        are merged or edited by hand.

        filename:
          Self-explanatory.

        header (default: "# Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)\n"):
          Text that will be inserted verbatim at the beginning of the file. You
          would usually want each line to start with '#' to make it a comment,
          and include a final terminating newline.
        """
        with self._open(filename, "w") as f:
            f.write(header)

            for sym in self.unique_defined_syms:
                # Skip symbols that cannot be changed. Only check
                # non-choice symbols, as selects don't affect choice
                # symbols.
                if not sym.choice and \
                   sym.visibility <= expr_value(sym.rev_dep):
                    continue

                # Skip symbols whose value matches their default
                if sym.str_value == sym._str_default():
                    continue

                # Skip symbols that would be selected by default in a
                # choice, unless the choice is optional or the symbol type
                # isn't bool (it might be possible to set the choice mode
                # to n or the symbol to m in those cases).
                if sym.choice and \
                   not sym.choice.is_optional and \
                   sym.choice._get_selection_from_defaults() is sym and \
                   sym.orig_type is BOOL and \
                   sym.tri_value == 2:
                    continue

                f.write(sym.config_string)

    def sync_deps(self, path):
        """
        Creates or updates a directory structure that can be used to avoid
        doing a full rebuild whenever the configuration is changed, mirroring
        include/config/ in the kernel.

        This function is intended to be called during each build, before
        compiling source files that depend on configuration symbols.

        path:
          Path to directory

        sync_deps(path) does the following:

          1. If the directory <path> does not exist, it is created.

          2. If <path>/auto.conf exists, old symbol values are loaded from it,
             which are then compared against the current symbol values. If a
             symbol has changed value (would generate different output in
             autoconf.h compared to before), the change is signaled by
             touch'ing a file corresponding to the symbol.

             The first time sync_deps() is run on a directory, <path>/auto.conf
             won't exist, and no old symbol values will be available. This
             logically has the same effect as updating the entire
             configuration.

             The path to a symbol's file is calculated from the symbol's name
             by replacing all '_' with '/' and appending '.h'. For example, the
             symbol FOO_BAR_BAZ gets the file <path>/foo/bar/baz.h, and FOO
             gets the file <path>/foo.h.

             This scheme matches the C tools. The point is to avoid having a
             single directory with a huge number of files, which the underlying
             filesystem might not handle well.

          3. A new auto.conf with the current symbol values is written, to keep
             track of them for the next build.


        The last piece of the puzzle is knowing what symbols each source file
        depends on. Knowing that, dependencies can be added from source files
        to the files corresponding to the symbols they depends on. The source
        file will then get recompiled (only) when the symbol value changes
        (provided sync_deps() is run first during each build).

        The tool in the kernel that extracts symbol dependencies from source
        files is scripts/basic/fixdep.c. Missing symbol files also correspond
        to "not changed", which fixdep deals with by using the $(wildcard) Make
        function when adding symbol prerequisites to source files.

        In case you need a different scheme for your project, the sync_deps()
        implementation can be used as a template.
        """
        if not os.path.exists(path):
            os.mkdir(path, 0o755)

        # This setup makes sure that at least the current working directory
        # gets reset if things fail
        prev_dir = os.getcwd()
        try:
            # cd'ing into the symbol file directory simplifies
            # _sync_deps() and saves some work
            os.chdir(path)
            self._sync_deps()
        finally:
            os.chdir(prev_dir)

    def _sync_deps(self):
        # Load old values from auto.conf, if any
        self._load_old_vals()

        for sym in self.unique_defined_syms:
            # Note: _write_to_conf is determined when the value is
            # calculated. This is a hidden function call due to
            # property magic.
            val = sym.str_value

            # Note: n tristate values do not get written to auto.conf and
            # autoconf.h, making a missing symbol logically equivalent to n

            if sym._write_to_conf:
                if sym._old_val is None and \
                   sym.orig_type in _BOOL_TRISTATE and \
                   val == "n":
                    # No old value (the symbol was missing or n), new value n.
                    # No change.
                    continue

                if val == sym._old_val:
                    # New value matches old. No change.
                    continue

            elif sym._old_val is None:
                # The symbol wouldn't appear in autoconf.h (because
                # _write_to_conf is false), and it wouldn't have appeared in
                # autoconf.h previously either (because it didn't appear in
                # auto.conf). No change.
                continue

            # 'sym' has a new value. Flag it.
            _touch_dep_file(sym.name)

        # Remember the current values as the "new old" values.
        #
        # This call could go anywhere after the call to _load_old_vals(), but
        # putting it last means _sync_deps() can be safely rerun if it fails
        # before this point.
        self._write_old_vals()

    def _write_old_vals(self):
        # Helper for writing auto.conf. Basically just a simplified
        # write_config() that doesn't write any comments (including
        # '# CONFIG_FOO is not set' comments). The format matches the C
        # implementation, though the ordering is arbitrary there (depends on
        # the hash table implementation).
        #
        # A separate helper function is neater than complicating write_config()
        # by passing a flag to it, plus we only need to look at symbols here.

        with self._open("auto.conf", "w") as f:
            for sym in self.unique_defined_syms:
                if not (sym.orig_type in _BOOL_TRISTATE and not sym.tri_value):
                    f.write(sym.config_string)

    def _load_old_vals(self):
        # Loads old symbol values from auto.conf into a dedicated
        # Symbol._old_val field. Mirrors load_config().
        #
        # The extra field could be avoided with some trickery involving dumping
        # symbol values and restoring them later, but this is simpler and
        # faster. The C tools also use a dedicated field for this purpose.

        for sym in self.unique_defined_syms:
            sym._old_val = None

        if not os.path.exists("auto.conf"):
            # No old values
            return

        with self._open("auto.conf", "r") as f:
            for line in f:
                match = self._set_match(line)
                if not match:
                    # We only expect CONFIG_FOO=... (and possibly a header
                    # comment) in auto.conf
                    continue

                name, val = match.groups()
                if name in self.syms:
                    sym = self.syms[name]

                    if sym.orig_type is STRING:
                        match = _conf_string_match(val)
                        if not match:
                            continue
                        val = unescape(match.group(1))

                    self.syms[name]._old_val = val
                else:
                    # Flag that the symbol no longer exists, in
                    # case something still depends on it
                    _touch_dep_file(name)

    def node_iter(self, unique_syms=False):
        """
        Returns a generator for iterating through all MenuNode's in the Kconfig
        tree. The iteration is done in Kconfig definition order (each node is
        visited before its children, and the children of a node are visited
        before the next node).

        The Kconfig.top_node menu node is skipped. It contains an implicit menu
        that holds the top-level items.

        As an example, the following code will produce a list equal to
        Kconfig.defined_syms:

          defined_syms = [node.item for node in kconf.node_iter()
                          if isinstance(node.item, Symbol)]

        unique_syms (default: False):
          If True, only the first MenuNode will be included for symbols defined
          in multiple locations.

          Using kconf.node_iter(True) in the example above would give a list
          equal to unique_defined_syms.
        """
        if unique_syms:
            for sym in self.unique_defined_syms:
                sym._visited = False

        node = self.top_node
        while 1:
            # Jump to the next node with an iterative tree walk
            if node.list:
                node = node.list
            elif node.next:
                node = node.next
            else:
                while node.parent:
                    node = node.parent
                    if node.next:
                        node = node.next
                        break
                else:
                    # No more nodes
                    return

            if unique_syms and node.item.__class__ is Symbol:
                if node.item._visited:
                    continue
                node.item._visited = True

            yield node

    def eval_string(self, s):
        """
        Returns the tristate value of the expression 's', represented as 0, 1,
        and 2 for n, m, and y, respectively. Raises KconfigError if syntax
        errors are detected in 's'. Warns if undefined symbols are referenced.

        As an example, if FOO and BAR are tristate symbols at least one of
        which has the value y, then config.eval_string("y && (FOO || BAR)")
        returns 2 (y).

        To get the string value of non-bool/tristate symbols, use
        Symbol.str_value. eval_string() always returns a tristate value, and
        all non-bool/tristate symbols have the tristate value 0 (n).

        The expression parsing is consistent with how parsing works for
        conditional ('if ...') expressions in the configuration, and matches
        the C implementation. m is rewritten to 'm && MODULES', so
        eval_string("m") will return 0 (n) unless modules are enabled.
        """
        # The parser is optimized to be fast when parsing Kconfig files (where
        # an expression can never appear at the beginning of a line). We have
        # to monkey-patch things a bit here to reuse it.

        self._filename = None

        # Don't include the "if " from below to avoid giving confusing error
        # messages
        self._line = s
        self._tokens = self._tokenize("if " + s)
        self._tokens_i = 1  # Skip the 'if' token

        return expr_value(self._expect_expr_and_eol())

    def unset_values(self):
        """
        Resets the user values of all symbols, as if Kconfig.load_config() or
        Symbol.set_value() had never been called.
        """
        self._warn_for_no_prompt = False
        try:
            # set_value() already rejects undefined symbols, and they don't
            # need to be invalidated (because their value never changes), so we
            # can just iterate over defined symbols
            for sym in self.unique_defined_syms:
                sym.unset_value()

            for choice in self.unique_choices:
                choice.unset_value()
        finally:
            self._warn_for_no_prompt = True

    def enable_warnings(self):
        """
        See Kconfig.__init__().
        """
        self._warnings_enabled = True

    def disable_warnings(self):
        """
        See Kconfig.__init__().
        """
        self._warnings_enabled = False

    def enable_stderr_warnings(self):
        """
        See Kconfig.__init__().
        """
        self._warn_to_stderr = True

    def disable_stderr_warnings(self):
        """
        See Kconfig.__init__().
        """
        self._warn_to_stderr = False

    def enable_undef_warnings(self):
        """
        Enables warnings for assignments to undefined symbols. Disabled by
        default unless the KCONFIG_WARN_UNDEF_ASSIGN environment variable was
        set to 'y' when the Kconfig instance was created.
        """
        self._warn_for_undef_assign = True

    def disable_undef_warnings(self):
        """
        See enable_undef_assign().
        """
        self._warn_for_undef_assign = False

    def enable_override_warnings(self):
        """
        Enables warnings for duplicated assignments in .config files that set
        different values (e.g. CONFIG_FOO=m followed by CONFIG_FOO=y, where
        the last value set is used).

        These warnings are enabled by default. Disabling them might be helpful
        in certain cases when merging configurations.
        """
        self._warn_for_override = True

    def disable_override_warnings(self):
        """
        See enable_override_warnings().
        """
        self._warn_for_override = False

    def enable_redun_warnings(self):
        """
        Enables warnings for duplicated assignments in .config files that all
        set the same value.

        These warnings are enabled by default. Disabling them might be helpful
        in certain cases when merging configurations.
        """
        self._warn_for_redun_assign = True

    def disable_redun_warnings(self):
        """
        See enable_redun_warnings().
        """
        self._warn_for_redun_assign = False

    def __repr__(self):
        """
        Returns a string with information about the Kconfig object when it is
        evaluated on e.g. the interactive Python prompt.
        """
        return "<{}>".format(", ".join((
            "configuration with {} symbols".format(len(self.syms)),
            'main menu prompt "{}"'.format(self.mainmenu_text),
            "srctree is current directory" if not self.srctree else
                'srctree "{}"'.format(self.srctree),
            'config symbol prefix "{}"'.format(self.config_prefix),
            "warnings " +
                ("enabled" if self._warnings_enabled else "disabled"),
            "printing of warnings to stderr " +
                ("enabled" if self._warn_to_stderr else "disabled"),
            "undef. symbol assignment warnings " +
                ("enabled" if self._warn_for_undef_assign else "disabled"),
            "redundant symbol assignment warnings " +
                ("enabled" if self._warn_for_redun_assign else "disabled")
        )))

    #
    # Private methods
    #


    #
    # File reading
    #

    def _open_config(self, filename):
        # Opens a .config file. First tries to open 'filename', then
        # '$srctree/filename' if $srctree was set when the configuration was
        # loaded.

        try:
            return self._open(filename, "r")
        except IOError as e:
            # This will try opening the same file twice if $srctree is unset,
            # but it's not a big deal
            try:
                return self._open(os.path.join(self.srctree, filename), "r")
            except IOError as e2:
                # This is needed for Python 3, because e2 is deleted after
                # the try block:
                #
                # https://docs.python.org/3/reference/compound_stmts.html#the-try-statement
                e = e2

            raise _KconfigIOError(e, "\n" + textwrap.fill(
                "Could not open '{}' ({}: {}){}".format(
                    filename, errno.errorcode[e.errno], e.strerror,
                    self._srctree_hint()),
                80))

    def _enter_file(self, full_filename, rel_filename):
        # Jumps to the beginning of a sourced Kconfig file, saving the previous
        # position and file object.
        #
        # full_filename:
        #   Actual path to the file.
        #
        # rel_filename:
        #   File path with $srctree prefix stripped, stored in e.g.
        #   self._filename (which makes it indirectly show up in
        #   MenuNode.filename). Equals full_filename for absolute paths.

        self.kconfig_filenames.append(rel_filename)

        # The parent Kconfig files are represented as a list of
        # (<include path>, <Python 'file' object for Kconfig file>) tuples.
        #
        # <include path> is immutable and holds a *tuple* of
        # (<filename>, <linenr>) tuples, giving the locations of the 'source'
        # statements in the parent Kconfig files. The current include path is
        # also available in Kconfig._include_path.
        #
        # The point of this redundant setup is to allow Kconfig._include_path
        # to be assigned directly to MenuNode.include_path without having to
        # copy it, sharing it wherever possible.

        # Save include path and 'file' object (via its 'readline' function)
        # before entering the file
        self._filestack.append((self._include_path, self._readline))

        # _include_path is a tuple, so this rebinds the variable instead of
        # doing in-place modification
        self._include_path += ((self._filename, self._linenr),)

        # Check for recursive 'source'
        for name, _ in self._include_path:
            if name == rel_filename:
                raise KconfigError(
                    "\n{}:{}: Recursive 'source' of '{}' detected. Check that "
                    "environment variables are set correctly.\n"
                    "Include path:\n{}"
                    .format(self._filename, self._linenr, rel_filename,
                            "\n".join("{}:{}".format(name, linenr)
                                      for name, linenr in self._include_path)))

        # Note: We already know that the file exists

        try:
            self._readline = self._open(full_filename, "r").readline
        except IOError as e:
            raise _KconfigIOError(
                e, "{}:{}: Could not open '{}' ({}: {})"
                   .format(self._filename, self._linenr, full_filename,
                           errno.errorcode[e.errno], e.strerror))

        self._filename = rel_filename
        self._linenr = 0

    def _leave_file(self):
        # Returns from a Kconfig file to the file that sourced it. See
        # _enter_file().

        # __self__ fetches the 'file' object for the method
        self._readline.__self__.close()
        # Restore location from parent Kconfig file
        self._filename, self._linenr = self._include_path[-1]
        # Restore include path and 'file' object
        self._include_path, self._readline = self._filestack.pop()

    def _next_line(self):
        # Fetches and tokenizes the next line from the current Kconfig file.
        # Returns False at EOF and True otherwise.

        # We might already have tokens from parsing a line and discovering that
        # it's part of a different construct
        if self._reuse_tokens:
            self._reuse_tokens = False
            # self._tokens_i is known to be 1 here, because _parse_properties()
            # leaves it like that when it can't recognize a line (or parses
            # a help text)
            return True

        # Note: readline() returns '' over and over at EOF, which we rely on
        # for help texts at the end of files (see _line_after_help())
        line = self._readline()
        if not line:
            return False
        self._linenr += 1

        # Handle line joining
        while line.endswith("\\\n"):
            line = line[:-2] + self._readline()
            self._linenr += 1

        self._line = line  # Used for error reporting
        self._tokens = self._tokenize(line)
        # Initialize to 1 instead of 0 to factor out code from _parse_block()
        # and _parse_properties(). They immediately fetch self._tokens[0].
        self._tokens_i = 1

        return True

    def _line_after_help(self, line):
        # Tokenizes a line after a help text. This case is special in that the
        # line has already been fetched (to discover that it isn't part of the
        # help text).
        #
        # An earlier version used a _saved_line variable instead that was
        # checked in _next_line(). This special-casing gets rid of it and makes
        # _reuse_tokens alone sufficient to handle unget.

        # Handle line joining
        while line.endswith("\\\n"):
            line = line[:-2] + self._readline()
            self._linenr += 1

        self._line = line
        self._tokens = self._tokenize(line)
        self._reuse_tokens = True


    #
    # Tokenization
    #

    def _lookup_sym(self, name):
        # Fetches the symbol 'name' from the symbol table, creating and
        # registering it if it does not exist. If '_parsing_kconfigs' is False,
        # it means we're in eval_string(), and new symbols won't be registered.

        if name in self.syms:
            return self.syms[name]

        sym = Symbol()
        sym.kconfig = self
        sym.name = name
        sym.is_constant = False
        sym.rev_dep = sym.weak_rev_dep = sym.direct_dep = self.n

        if self._parsing_kconfigs:
            self.syms[name] = sym
        else:
            self._warn("no symbol {} in configuration".format(name))

        return sym

    def _lookup_const_sym(self, name):
        # Like _lookup_sym(), for constant (quoted) symbols

        if name in self.const_syms:
            return self.const_syms[name]

        sym = Symbol()
        sym.kconfig = self
        sym.name = name
        sym.is_constant = True
        sym.rev_dep = sym.weak_rev_dep = sym.direct_dep = self.n

        if self._parsing_kconfigs:
            self.const_syms[name] = sym

        return sym

    def _tokenize(self, s):
        # Parses 's', returning a None-terminated list of tokens. Registers any
        # new symbols encountered with _lookup(_const)_sym().
        #
        # Tries to be reasonably speedy by processing chunks of text via
        # regexes and string operations where possible. This is the biggest
        # hotspot during parsing.
        #
        # Note: It might be possible to rewrite this to 'yield' tokens instead,
        # working across multiple lines. The 'option env' lookback thing below
        # complicates things though.

        # Initial token on the line
        match = _command_match(s)
        if not match:
            if s.isspace() or s.lstrip().startswith("#"):
                return (None,)
            self._parse_error("unknown token at start of line")

        # Tricky implementation detail: While parsing a token, 'token' refers
        # to the previous token. See _STRING_LEX for why this is needed.
        token = _get_keyword(match.group(1))
        if not token:
            # Backwards compatibility with old versions of the C tools, which
            # (accidentally) accepted stuff like "--help--" and "-help---".
            # This was fixed in the C tools by commit c2264564 ("kconfig: warn
            # of unhandled characters in Kconfig commands"), committed in July
            # 2015, but it seems people still run Kconfiglib on older kernels.
            if s.strip(" \t\n-") == "help":
                return (_T_HELP, None)

            # If the first token is not a keyword (and not a weird help token),
            # we have a preprocessor variable assignment (or a bare macro on a
            # line)
            self._parse_assignment(s)
            return (None,)

        tokens = [token]
        # The current index in the string being tokenized
        i = match.end()

        # Main tokenization loop (for tokens past the first one)
        while i < len(s):
            # Test for an identifier/keyword first. This is the most common
            # case.
            match = _id_keyword_match(s, i)
            if match:
                # We have an identifier or keyword

                # Check what it is. lookup_sym() will take care of allocating
                # new symbols for us the first time we see them. Note that
                # 'token' still refers to the previous token.

                name = match.group(1)
                keyword = _get_keyword(name)
                if keyword:
                    # It's a keyword
                    token = keyword
                    # Jump past it
                    i = match.end()

                elif token not in _STRING_LEX:
                    # It's a non-const symbol, except we translate n, m, and y
                    # into the corresponding constant symbols, like the C
                    # implementation

                    if "$" in name:
                        # Macro expansion within symbol name
                        name, s, i = self._expand_name(s, i)
                    else:
                        i = match.end()

                    token = self.const_syms[name] \
                            if name in ("y", "m", "n") else \
                            self._lookup_sym(name)

                else:
                    # It's a case of missing quotes. For example, the
                    # following is accepted:
                    #
                    #   menu unquoted_title
                    #
                    #   config A
                    #       tristate unquoted_prompt
                    #
                    #   endmenu
                    token = name
                    i = match.end()

            else:
                # Neither a keyword nor a non-const symbol

                # We always strip whitespace after tokens, so it is safe to
                # assume that s[i] is the start of a token here.
                c = s[i]

                if c in "\"'":
                    if "$" not in s and "\\" not in s:
                        # Fast path for lines without $ and \. Find the
                        # matching quote.
                        end_i = s.find(c, i + 1) + 1
                        if not end_i:
                            self._parse_error("unterminated string")
                        val = s[i + 1:end_i - 1]
                        i = end_i
                    else:
                        # Slow path
                        s, end_i = self._expand_str(s, i)

                        # os.path.expandvars() and the $UNAME_RELEASE replace()
                        # is a backwards compatibility hack, which should be
                        # reasonably safe as expandvars() leaves references to
                        # undefined env. vars. as is.
                        #
                        # The preprocessor functionality changed how
                        # environment variables are referenced, to $(FOO).
                        val = os.path.expandvars(
                            s[i + 1:end_i - 1].replace("$UNAME_RELEASE",
                                                       platform.uname()[2]))

                        i = end_i

                    # This is the only place where we don't survive with a
                    # single token of lookback: 'option env="FOO"' does not
                    # refer to a constant symbol named "FOO".
                    token = \
                        val if token in _STRING_LEX or tokens[0] is _T_OPTION \
                        else self._lookup_const_sym(val)

                elif s.startswith("&&", i):
                    token = _T_AND
                    i += 2

                elif s.startswith("||", i):
                    token = _T_OR
                    i += 2

                elif c == "=":
                    token = _T_EQUAL
                    i += 1

                elif s.startswith("!=", i):
                    token = _T_UNEQUAL
                    i += 2

                elif c == "!":
                    token = _T_NOT
                    i += 1

                elif c == "(":
                    token = _T_OPEN_PAREN
                    i += 1

                elif c == ")":
                    token = _T_CLOSE_PAREN
                    i += 1

                elif c == "#":
                    break


                # Very rare

                elif s.startswith("<=", i):
                    token = _T_LESS_EQUAL
                    i += 2

                elif c == "<":
                    token = _T_LESS
                    i += 1

                elif s.startswith(">=", i):
                    token = _T_GREATER_EQUAL
                    i += 2

                elif c == ">":
                    token = _T_GREATER
                    i += 1


                else:
                    self._parse_error("unknown tokens in line")


                # Skip trailing whitespace
                while i < len(s) and s[i].isspace():
                    i += 1


            # Add the token
            tokens.append(token)

        # None-terminating the token list makes token fetching simpler/faster
        tokens.append(None)

        return tokens

    # Helpers for syntax checking and token fetching. See the
    # 'Intro to expressions' section for what a constant symbol is.
    #
    # More of these could be added, but the single-use cases are inlined as an
    # optimization.

    def _expect_sym(self):
        token = self._tokens[self._tokens_i]
        self._tokens_i += 1

        if token.__class__ is not Symbol:
            self._parse_error("expected symbol")

        return token

    def _expect_nonconst_sym(self):
        # Used for 'select' and 'imply' only. We know the token indices.

        token = self._tokens[1]
        self._tokens_i = 2

        if token.__class__ is not Symbol or token.is_constant:
            self._parse_error("expected nonconstant symbol")

        return token

    def _expect_str_and_eol(self):
        token = self._tokens[self._tokens_i]
        self._tokens_i += 1

        if token.__class__ is not str:
            self._parse_error("expected string")

        if self._tokens[self._tokens_i] is not None:
            self._trailing_tokens_error()

        return token

    def _expect_expr_and_eol(self):
        expr = self._parse_expr(True)

        if self._tokens[self._tokens_i] is not None:
            self._trailing_tokens_error()

        return expr

    def _check_token(self, token):
        # If the next token is 'token', removes it and returns True

        if self._tokens[self._tokens_i] is token:
            self._tokens_i += 1
            return True
        return False


    #
    # Preprocessor logic
    #

    def _parse_assignment(self, s):
        # Parses a preprocessor variable assignment, registering the variable
        # if it doesn't already exist. Also takes care of bare macros on lines
        # (which are allowed, and can be useful for their side effects).

        # Expand any macros in the left-hand side of the assignment (the
        # variable name)
        s = s.lstrip()
        i = 0
        while 1:
            i = _assignment_lhs_fragment_match(s, i).end()
            if s.startswith("$(", i):
                s, i = self._expand_macro(s, i, ())
            else:
                break

        if s.isspace():
            # We also accept a bare macro on a line (e.g.
            # $(warning-if,$(foo),ops)), provided it expands to a blank string
            return

        # Assigned variable
        name = s[:i]


        # Extract assignment operator (=, :=, or +=) and value
        rhs_match = _assignment_rhs_match(s, i)
        if not rhs_match:
            self._parse_error("syntax error")

        op, val = rhs_match.groups()


        if name in self.variables:
            # Already seen variable
            var = self.variables[name]
        else:
            # New variable
            var = Variable()
            var.kconfig = self
            var.name = name
            var._n_expansions = 0
            self.variables[name] = var

            # += acts like = on undefined variables (defines a recursive
            # variable)
            if op == "+=":
                op = "="

        if op == "=":
            var.is_recursive = True
            var.value = val
        elif op == ":=":
            var.is_recursive = False
            var.value = self._expand_whole(val, ())
        else:  # op == "+="
            # += does immediate expansion if the variable was last set
            # with :=
            var.value += " " + (val if var.is_recursive else
                                self._expand_whole(val, ()))

    def _expand_whole(self, s, args):
        # Expands preprocessor macros in all of 's'. Used whenever we don't
        # have to worry about delimiters. See _expand_macro() re. the 'args'
        # parameter.
        #
        # Returns the expanded string.

        i = 0
        while 1:
            i = s.find("$(", i)
            if i == -1:
                break
            s, i = self._expand_macro(s, i, args)
        return s

    def _expand_name(self, s, i):
        # Expands a symbol name starting at index 'i' in 's'.
        #
        # Returns the expanded name, the expanded 's' (including the part
        # before the name), and the index of the first character in the next
        # token after the name.

        s, end_i = self._expand_name_iter(s, i)
        name = s[i:end_i]
        # isspace() is False for empty strings
        if not name.strip():
            # Avoid creating a Kconfig symbol with a blank name. It's almost
            # guaranteed to be an error.
            self._parse_error("macro expanded to blank string")

        # Skip trailing whitespace
        while end_i < len(s) and s[end_i].isspace():
            end_i += 1

        return name, s, end_i

    def _expand_name_iter(self, s, i):
        # Expands a symbol name starting at index 'i' in 's'.
        #
        # Returns the expanded 's' (including the part before the name) and the
        # index of the first character after the expanded name in 's'.

        while 1:
            match = _name_special_search(s, i)

            if match.group() == "$(":
                s, i = self._expand_macro(s, match.start(), ())
            else:
                return (s, match.start())

    def _expand_str(self, s, i):
        # Expands a quoted string starting at index 'i' in 's'. Handles both
        # backslash escapes and macro expansion.
        #
        # Returns the expanded 's' (including the part before the string) and
        # the index of the first character after the expanded string in 's'.

        quote = s[i]
        i += 1  # Skip over initial "/'
        while 1:
            match = _string_special_search(s, i)
            if not match:
                self._parse_error("unterminated string")


            if match.group() == quote:
                # Found the end of the string
                return (s, match.end())

            elif match.group() == "\\":
                # Replace '\x' with 'x'. 'i' ends up pointing to the character
                # after 'x', which allows macros to be canceled with '\$(foo)'.
                i = match.end()
                s = s[:match.start()] + s[i:]

            elif match.group() == "$(":
                # A macro call within the string
                s, i = self._expand_macro(s, match.start(), ())

            else:
                # A ' quote within " quotes or vice versa
                i += 1

    def _expand_macro(self, s, i, args):
        # Expands a macro starting at index 'i' in 's'. If this macro resulted
        # from the expansion of another macro, 'args' holds the arguments
        # passed to that macro.
        #
        # Returns the expanded 's' (including the part before the macro) and
        # the index of the first character after the expanded macro in 's'.

        start = i
        i += 2  # Skip over "$("

        # Start of current macro argument
        arg_start = i

        # Arguments of this macro call
        new_args = []

        while 1:
            match = _macro_special_search(s, i)
            if not match:
                self._parse_error("missing end parenthesis in macro expansion")


            if match.group() == ")":
                # Found the end of the macro

                new_args.append(s[arg_start:match.start()])

                prefix = s[:start]

                # $(1) is replaced by the first argument to the function, etc.,
                # provided at least that many arguments were passed

                try:
                    # Does the macro look like an integer, with a corresponding
                    # argument? If so, expand it to the value of the argument.
                    prefix += args[int(new_args[0])]
                except (ValueError, IndexError):
                    # Regular variables are just functions without arguments,
                    # and also go through the function value path
                    prefix += self._fn_val(new_args)

                return (prefix + s[match.end():],
                        len(prefix))

            elif match.group() == ",":
                # Found the end of a macro argument
                new_args.append(s[arg_start:match.start()])
                arg_start = i = match.end()

            else:  # match.group() == "$("
                # A nested macro call within the macro
                s, i = self._expand_macro(s, match.start(), args)

    def _fn_val(self, args):
        # Returns the result of calling the function args[0] with the arguments
        # args[1..len(args)-1]. Plain variables are treated as functions
        # without arguments.

        fn = args[0]

        if fn in self.variables:
            var = self.variables[fn]

            if len(args) == 1:
                # Plain variable
                if var._n_expansions:
                    self._parse_error("Preprocessor variable {} recursively "
                                      "references itself".format(var.name))
            elif var._n_expansions > 100:
                # Allow functions to call themselves, but guess that functions
                # that are overly recursive are stuck
                self._parse_error("Preprocessor function {} seems stuck "
                                  "in infinite recursion".format(var.name))

            var._n_expansions += 1
            res = self._expand_whole(self.variables[fn].value, args)
            var._n_expansions -= 1
            return res

        if fn in self._functions:
            # Built-in or user-defined function

            py_fn, min_arg, max_arg = self._functions[fn]

            if len(args) - 1 < min_arg or \
               (max_arg is not None and len(args) - 1 > max_arg):

                if min_arg == max_arg:
                    expected_args = min_arg
                elif max_arg is None:
                    expected_args = "{} or more".format(min_arg)
                else:
                    expected_args = "{}-{}".format(min_arg, max_arg)

                raise KconfigError("{}:{}: bad number of arguments in call "
                                   "to {}, expected {}, got {}"
                                   .format(self._filename, self._linenr, fn,
                                           expected_args, len(args) - 1))

            return py_fn(self, *args)

        # Environment variables are tried last
        if fn in os.environ:
            self.env_vars.add(fn)
            return os.environ[fn]

        return ""


    #
    # Parsing
    #

    def _make_and(self, e1, e2):
        # Constructs an AND (&&) expression. Performs trivial simplification.

        if e1 is self.y:
            return e2

        if e2 is self.y:
            return e1

        if e1 is self.n or e2 is self.n:
            return self.n

        return (AND, e1, e2)

    def _make_or(self, e1, e2):
        # Constructs an OR (||) expression. Performs trivial simplification.

        if e1 is self.n:
            return e2

        if e2 is self.n:
            return e1

        if e1 is self.y or e2 is self.y:
            return self.y

        return (OR, e1, e2)

    def _parse_block(self, end_token, parent, prev):
        # Parses a block, which is the contents of either a file or an if,
        # menu, or choice statement.
        #
        # end_token:
        #   The token that ends the block, e.g. _T_ENDIF ("endif") for ifs.
        #   None for files.
        #
        # parent:
        #   The parent menu node, corresponding to a menu, Choice, or 'if'.
        #   'if's are flattened after parsing.
        #
        # prev:
        #   The previous menu node. New nodes will be added after this one (by
        #   modifying their 'next' pointer).
        #
        #   'prev' is reused to parse a list of child menu nodes (for a menu or
        #   Choice): After parsing the children, the 'next' pointer is assigned
        #   to the 'list' pointer to "tilt up" the children above the node.
        #
        # Returns the final menu node in the block (or 'prev' if the block is
        # empty). This allows chaining.

        while self._next_line():
            t0 = self._tokens[0]

            if t0 is _T_CONFIG or t0 is _T_MENUCONFIG:
                # The tokenizer allocates Symbol objects for us
                sym = self._tokens[1]

                if sym.__class__ is not Symbol or sym.is_constant:
                    self._parse_error("missing or bad symbol name")

                if self._tokens[2] is not None:
                    self._trailing_tokens_error()

                self.defined_syms.append(sym)

                node = MenuNode()
                node.kconfig = self
                node.item = sym
                node.is_menuconfig = (t0 is _T_MENUCONFIG)
                node.prompt = node.help = node.list = None
                node.parent = parent
                node.filename = self._filename
                node.linenr = self._linenr
                node.include_path = self._include_path

                sym.nodes.append(node)

                self._parse_properties(node)

                if node.is_menuconfig and not node.prompt:
                    self._warn("the menuconfig symbol {} has no prompt"
                               .format(_name_and_loc(sym)))

                # Equivalent to
                #
                #   prev.next = node
                #   prev = node
                #
                # due to tricky Python semantics. The order matters.
                prev.next = prev = node

            elif t0 is None:
                # Blank line
                continue

            elif t0 in _SOURCE_TOKENS:
                pattern = self._expect_str_and_eol()

                # Check if the pattern is absolute and avoid stripping srctree
                # from it below in that case. We must do the check before
                # join()'ing, as srctree might be an absolute path.
                isabs = os.path.isabs(pattern)

                if t0 in _REL_SOURCE_TOKENS:
                    # Relative source
                    pattern = os.path.join(os.path.dirname(self._filename),
                                           pattern)

                # Sort the glob results to ensure a consistent ordering of
                # Kconfig symbols, which indirectly ensures a consistent
                # ordering in e.g. .config files
                filenames = \
                    sorted(glob.iglob(os.path.join(self.srctree, pattern)))

                if not filenames and t0 in _OBL_SOURCE_TOKENS:
                    raise KconfigError("\n" + textwrap.fill(
                        "{}:{}: '{}' does not exist{}".format(
                            self._filename, self._linenr, pattern,
                            self._srctree_hint()),
                        80))

                for filename in filenames:
                    self._enter_file(
                        filename,
                        # Unless an absolute path is passed to *source, strip
                        # the $srctree prefix from the filename. That way it
                        # appears without a $srctree prefix in
                        # MenuNode.filename, which is nice e.g. when generating
                        # documentation.
                        filename if isabs else
                            os.path.relpath(filename, self.srctree))

                    prev = self._parse_block(None, parent, prev)

                    self._leave_file()

            elif t0 is end_token:
                # We have reached the end of the block. Terminate the final
                # node and return it.

                if self._tokens[1] is not None:
                    self._trailing_tokens_error()

                prev.next = None
                return prev

            elif t0 is _T_IF:
                node = MenuNode()
                node.item = node.prompt = None
                node.parent = parent
                node.dep = self._expect_expr_and_eol()

                self._parse_block(_T_ENDIF, node, node)
                node.list = node.next

                prev.next = prev = node

            elif t0 is _T_MENU:
                node = MenuNode()
                node.kconfig = self
                node.item = t0  # _T_MENU == MENU
                node.is_menuconfig = True
                node.prompt = (self._expect_str_and_eol(), self.y)
                node.visibility = self.y
                node.parent = parent
                node.filename = self._filename
                node.linenr = self._linenr
                node.include_path = self._include_path

                self.menus.append(node)

                self._parse_properties(node)
                self._parse_block(_T_ENDMENU, node, node)
                node.list = node.next

                prev.next = prev = node

            elif t0 is _T_COMMENT:
                node = MenuNode()
                node.kconfig = self
                node.item = t0  # _T_COMMENT == COMMENT
                node.is_menuconfig = False
                node.prompt = (self._expect_str_and_eol(), self.y)
                node.list = None
                node.parent = parent
                node.filename = self._filename
                node.linenr = self._linenr
                node.include_path = self._include_path

                self.comments.append(node)

                self._parse_properties(node)

                prev.next = prev = node

            elif t0 is _T_CHOICE:
                if self._tokens[1] is None:
                    choice = Choice()
                    choice.direct_dep = self.n
                else:
                    # Named choice
                    name = self._expect_str_and_eol()
                    choice = self.named_choices.get(name)
                    if not choice:
                        choice = Choice()
                        choice.name = name
                        choice.direct_dep = self.n
                        self.named_choices[name] = choice

                self.choices.append(choice)

                choice.kconfig = self

                node = MenuNode()
                node.kconfig = self
                node.item = choice
                node.is_menuconfig = True
                node.prompt = node.help = None
                node.parent = parent
                node.filename = self._filename
                node.linenr = self._linenr
                node.include_path = self._include_path

                choice.nodes.append(node)

                self._parse_properties(node)
                self._parse_block(_T_ENDCHOICE, node, node)
                node.list = node.next

                prev.next = prev = node

            elif t0 is _T_MAINMENU:
                self.top_node.prompt = (self._expect_str_and_eol(), self.y)
                self.top_node.filename = self._filename
                self.top_node.linenr = self._linenr

            else:
                # A valid endchoice/endif/endmenu is caught by the 'end_token'
                # check above
                self._parse_error(
                    "no corresponding 'choice'" if t0 is _T_ENDCHOICE else
                    "no corresponding 'if'"     if t0 is _T_ENDIF else
                    "no corresponding 'menu'"   if t0 is _T_ENDMENU else
                    "unrecognized construct")

        # End of file reached. Terminate the final node and return it.

        if end_token:
            raise KconfigError(
                "expected '{}' at end of '{}'"
                .format("endchoice" if end_token is _T_ENDCHOICE else
                        "endif"     if end_token is _T_ENDIF else
                        "endmenu",
                        self._filename))

        prev.next = None
        return prev

    def _parse_cond(self):
        # Parses an optional 'if <expr>' construct and returns the parsed
        # <expr>, or self.y if the next token is not _T_IF

        expr = self._parse_expr(True) if self._check_token(_T_IF) else self.y

        if self._tokens[self._tokens_i] is not None:
            self._trailing_tokens_error()

        return expr

    def _parse_properties(self, node):
        # Parses and adds properties to the MenuNode 'node' (type, 'prompt',
        # 'default's, etc.) Properties are later copied up to symbols and
        # choices in a separate pass after parsing, in e.g.
        # _add_props_to_sym().
        #
        # An older version of this code added properties directly to symbols
        # and choices instead of to their menu nodes (and handled dependency
        # propagation simultaneously), but that loses information on where a
        # property is added when a symbol or choice is defined in multiple
        # locations. Some Kconfig configuration systems rely heavily on such
        # symbols, and better docs can be generated by keeping track of where
        # properties are added.
        #
        # node:
        #   The menu node we're parsing properties on

        # Dependencies from 'depends on'. Will get propagated to the properties
        # below.
        node.dep = self.y

        while self._next_line():
            t0 = self._tokens[0]

            if t0 in _TYPE_TOKENS:
                self._set_type(node, t0)
                if self._tokens[1] is not None:
                    self._parse_prompt(node)

            elif t0 is _T_DEPENDS:
                if not self._check_token(_T_ON):
                    self._parse_error("expected 'on' after 'depends'")

                node.dep = self._make_and(node.dep,
                                          self._expect_expr_and_eol())

            elif t0 is _T_HELP:
                self._parse_help(node)

            elif t0 is _T_SELECT:
                if node.item.__class__ is not Symbol:
                    self._parse_error("only symbols can select")

                node.selects.append((self._expect_nonconst_sym(),
                                     self._parse_cond()))

            elif t0 is None:
                # Blank line
                continue

            elif t0 is _T_DEFAULT:
                node.defaults.append((self._parse_expr(False),
                                      self._parse_cond()))

            elif t0 in _DEF_TYPE_TOKENS:
                self._set_type(node, t0)
                node.defaults.append((self._parse_expr(False),
                                      self._parse_cond()))

            elif t0 is _T_PROMPT:
                self._parse_prompt(node)

            elif t0 is _T_RANGE:
                node.ranges.append((self._expect_sym(), self._expect_sym(),
                                    self._parse_cond()))

            elif t0 is _T_IMPLY:
                if node.item.__class__ is not Symbol:
                    self._parse_error("only symbols can imply")

                node.implies.append((self._expect_nonconst_sym(),
                                     self._parse_cond()))

            elif t0 is _T_VISIBLE:
                if not self._check_token(_T_IF):
                    self._parse_error("expected 'if' after 'visible'")

                node.visibility = self._make_and(node.visibility,
                                                 self._expect_expr_and_eol())

            elif t0 is _T_OPTION:
                if self._check_token(_T_ENV):
                    if not self._check_token(_T_EQUAL):
                        self._parse_error("expected '=' after 'env'")

                    env_var = self._expect_str_and_eol()
                    node.item.env_var = env_var

                    if env_var in os.environ:
                        node.defaults.append(
                            (self._lookup_const_sym(os.environ[env_var]),
                             self.y))
                    else:
                        self._warn("{1} has 'option env=\"{0}\"', "
                                   "but the environment variable {0} is not "
                                   "set".format(node.item.name, env_var),
                                   self._filename, self._linenr)

                    if env_var != node.item.name:
                        self._warn("Kconfiglib expands environment variables "
                                   "in strings directly, meaning you do not "
                                   "need 'option env=...' \"bounce\" symbols. "
                                   "For compatibility with the C tools, "
                                   "rename {} to {} (so that the symbol name "
                                   "matches the environment variable name)."
                                   .format(node.item.name, env_var),
                                   self._filename, self._linenr)

                elif self._check_token(_T_DEFCONFIG_LIST):
                    if not self.defconfig_list:
                        self.defconfig_list = node.item
                    else:
                        self._warn("'option defconfig_list' set on multiple "
                                   "symbols ({0} and {1}). Only {0} will be "
                                   "used.".format(self.defconfig_list.name,
                                                  node.item.name),
                                   self._filename, self._linenr)

                elif self._check_token(_T_MODULES):
                    # To reduce warning spam, only warn if 'option modules' is
                    # set on some symbol that isn't MODULES, which should be
                    # safe. I haven't run into any projects that make use
                    # modules besides the kernel yet, and there it's likely to
                    # keep being called "MODULES".
                    if node.item is not self.modules:
                        self._warn("the 'modules' option is not supported. "
                                   "Let me know if this is a problem for you, "
                                   "as it wouldn't be that hard to implement. "
                                   "Note that modules are supported -- "
                                   "Kconfiglib just assumes the symbol name "
                                   "MODULES, like older versions of the C "
                                   "implementation did when 'option modules' "
                                   "wasn't used.",
                                   self._filename, self._linenr)

                elif self._check_token(_T_ALLNOCONFIG_Y):
                    if node.item.__class__ is not Symbol:
                        self._parse_error("the 'allnoconfig_y' option is only "
                                          "valid for symbols")

                    node.item.is_allnoconfig_y = True

                else:
                    self._parse_error("unrecognized option")

            elif t0 is _T_OPTIONAL:
                if node.item.__class__ is not Choice:
                    self._parse_error('"optional" is only valid for choices')

                node.item.is_optional = True

            else:
                # Reuse the tokens for the non-property line later
                self._reuse_tokens = True
                return

    def _set_type(self, node, type_token):
        new_type = _TOKEN_TO_TYPE[type_token]

        # The 'is not UNKNOWN' comparison will usually fail, since single-def
        # symbols/choices are more common
        if node.item.orig_type is not UNKNOWN and \
           node.item.orig_type is not new_type:

            self._warn("{} defined with multiple types, {} will be used"
                       .format(_name_and_loc(node.item),
                               TYPE_TO_STR[new_type]))

        node.item.orig_type = new_type

    def _parse_prompt(self, node):
        # 'prompt' properties override each other within a single definition of
        # a symbol, but additional prompts can be added by defining the symbol
        # multiple times

        if node.prompt:
            self._warn(_name_and_loc(node.item) +
                       " defined with multiple prompts in single location")

        prompt = self._tokens[1]
        self._tokens_i = 2

        if prompt.__class__ is not str:
            self._parse_error("expected prompt string")

        if prompt != prompt.strip():
            self._warn(_name_and_loc(node.item) +
                       " has leading or trailing whitespace in its prompt")

            # This avoid issues for e.g. reStructuredText documentation, where
            # '*prompt *' is invalid
            prompt = prompt.strip()

        node.prompt = (prompt, self._parse_cond())

    def _parse_help(self, node):
        if node.help is not None:
            self._warn(_name_and_loc(node.item) + " defined with more than "
                       "one help text -- only the last one will be used")

        # Micro-optimization. This code is pretty hot.
        readline = self._readline

        # Find first non-blank (not all-space) line and get its
        # indentation

        while 1:
            line = readline()
            self._linenr += 1
            if not line:
                self._empty_help(node, line)
                return
            if not line.isspace():
                break

        len_ = len  # Micro-optimization

        # Use a separate 'expline' variable here and below to avoid stomping on
        # any tabs people might've put deliberately into the first line after
        # the help text
        expline = line.expandtabs()
        indent = len_(expline) - len_(expline.lstrip())
        if not indent:
            self._empty_help(node, line)
            return

        # The help text goes on till the first non-blank line with less indent
        # than the first line

        # Add the first line
        lines = [expline[indent:]]
        add_line = lines.append  # Micro-optimization

        while 1:
            line = readline()
            if line.isspace():
                # No need to preserve the exact whitespace in these
                add_line("\n")
            elif not line:
                # End of file
                break
            else:
                expline = line.expandtabs()
                if len_(expline) - len_(expline.lstrip()) < indent:
                    break
                add_line(expline[indent:])

        self._linenr += len_(lines)
        node.help = "".join(lines).rstrip()
        if line:
            self._line_after_help(line)

    def _empty_help(self, node, line):
        self._warn(_name_and_loc(node.item) +
                   " has 'help' but empty help text")
        node.help = ""
        if line:
            self._line_after_help(line)

    def _parse_expr(self, transform_m):
        # Parses an expression from the tokens in Kconfig._tokens using a
        # simple top-down approach. See the module docstring for the expression
        # format.
        #
        # transform_m:
        #   True if m should be rewritten to m && MODULES. See the
        #   Kconfig.eval_string() documentation.

        # Grammar:
        #
        #   expr:     and_expr ['||' expr]
        #   and_expr: factor ['&&' and_expr]
        #   factor:   <symbol> ['='/'!='/'<'/... <symbol>]
        #             '!' factor
        #             '(' expr ')'
        #
        # It helps to think of the 'expr: and_expr' case as a single-operand OR
        # (no ||), and of the 'and_expr: factor' case as a single-operand AND
        # (no &&). Parsing code is always a bit tricky.

        # Mind dump: parse_factor() and two nested loops for OR and AND would
        # work as well. The straightforward implementation there gives a
        # (op, (op, (op, A, B), C), D) parse for A op B op C op D. Representing
        # expressions as (op, [list of operands]) instead goes nicely with that
        # version, but is wasteful for short expressions and complicates
        # expression evaluation and other code that works on expressions (more
        # complicated code likely offsets any performance gain from less
        # recursion too). If we also try to optimize the list representation by
        # merging lists when possible (e.g. when ANDing two AND expressions),
        # we end up allocating a ton of lists instead of reusing expressions,
        # which is bad.

        and_expr = self._parse_and_expr(transform_m)

        # Return 'and_expr' directly if we have a "single-operand" OR.
        # Otherwise, parse the expression on the right and make an OR node.
        # This turns A || B || C || D into (OR, A, (OR, B, (OR, C, D))).
        return and_expr \
               if not self._check_token(_T_OR) else \
               (OR, and_expr, self._parse_expr(transform_m))

    def _parse_and_expr(self, transform_m):
        factor = self._parse_factor(transform_m)

        # Return 'factor' directly if we have a "single-operand" AND.
        # Otherwise, parse the right operand and make an AND node. This turns
        # A && B && C && D into (AND, A, (AND, B, (AND, C, D))).
        return factor \
               if not self._check_token(_T_AND) else \
               (AND, factor, self._parse_and_expr(transform_m))

    def _parse_factor(self, transform_m):
        token = self._tokens[self._tokens_i]
        self._tokens_i += 1

        if token.__class__ is Symbol:
            # Plain symbol or relation

            if self._tokens[self._tokens_i] not in _RELATIONS:
                # Plain symbol

                # For conditional expressions ('depends on <expr>',
                # '... if <expr>', etc.), m is rewritten to m && MODULES.
                if transform_m and token is self.m:
                    return (AND, self.m, self.modules)

                return token

            # Relation
            #
            # _T_EQUAL, _T_UNEQUAL, etc., deliberately have the same values as
            # EQUAL, UNEQUAL, etc., so we can just use the token directly
            self._tokens_i += 1
            return (self._tokens[self._tokens_i - 1], token,
                    self._expect_sym())

        if token is _T_NOT:
            # token == _T_NOT == NOT
            return (token, self._parse_factor(transform_m))

        if token is _T_OPEN_PAREN:
            expr_parse = self._parse_expr(transform_m)
            if self._check_token(_T_CLOSE_PAREN):
                return expr_parse

        self._parse_error("malformed expression")

    #
    # Caching and invalidation
    #

    def _build_dep(self):
        # Populates the Symbol/Choice._dependents sets, which contain all other
        # items (symbols and choices) that immediately depend on the item in
        # the sense that changing the value of the item might affect the value
        # of the dependent items. This is used for caching/invalidation.
        #
        # The calculated sets might be larger than necessary as we don't do any
        # complex analysis of the expressions.

        make_depend_on = _make_depend_on  # Micro-optimization

        # Only calculate _dependents for defined symbols. Constant and
        # undefined symbols could theoretically be selected/implied, but it
        # wouldn't change their value, so it's not a true dependency.
        for sym in self.unique_defined_syms:
            # Symbols depend on the following:

            # The prompt conditions
            for node in sym.nodes:
                if node.prompt:
                    make_depend_on(sym, node.prompt[1])

            # The default values and their conditions
            for value, cond in sym.defaults:
                make_depend_on(sym, value)
                make_depend_on(sym, cond)

            # The reverse and weak reverse dependencies
            make_depend_on(sym, sym.rev_dep)
            make_depend_on(sym, sym.weak_rev_dep)

            # The ranges along with their conditions
            for low, high, cond in sym.ranges:
                make_depend_on(sym, low)
                make_depend_on(sym, high)
                make_depend_on(sym, cond)

            # The direct dependencies. This is usually redundant, as the direct
            # dependencies get propagated to properties, but it's needed to get
            # invalidation solid for 'imply', which only checks the direct
            # dependencies (even if there are no properties to propagate it
            # to).
            make_depend_on(sym, sym.direct_dep)

            # In addition to the above, choice symbols depend on the choice
            # they're in, but that's handled automatically since the Choice is
            # propagated to the conditions of the properties before
            # _build_dep() runs.

        for choice in self.unique_choices:
            # Choices depend on the following:

            # The prompt conditions
            for node in choice.nodes:
                if node.prompt:
                    make_depend_on(choice, node.prompt[1])

            # The default symbol conditions
            for _, cond in choice.defaults:
                make_depend_on(choice, cond)

    def _add_choice_deps(self):
        # Choices also depend on the choice symbols themselves, because the
        # y-mode selection of the choice might change if a choice symbol's
        # visibility changes.
        #
        # We add these dependencies separately after dependency loop detection.
        # The invalidation algorithm can handle the resulting
        # <choice symbol> <-> <choice> dependency loops, but they make loop
        # detection awkward.

        for choice in self.unique_choices:
            for sym in choice.syms:
                sym._dependents.add(choice)

    def _invalidate_all(self):
        # Undefined symbols never change value and don't need to be
        # invalidated, so we can just iterate over defined symbols.
        # Invalidating constant symbols would break things horribly.
        for sym in self.unique_defined_syms:
            sym._invalidate()

        for choice in self.unique_choices:
            choice._invalidate()


    #
    # Post-parsing menu tree processing, including dependency propagation and
    # implicit submenu creation
    #

    def _finalize_tree(self, node, visible_if):
        # Propagates properties and dependencies, creates implicit menus (see
        # kconfig-language.txt), removes 'if' nodes, and finalizes choices.
        # This pretty closely mirrors menu_finalize() from the C
        # implementation, with some minor tweaks (MenuNode holds lists of
        # properties instead of each property having a MenuNode pointer, for
        # example).
        #
        # node:
        #   The current "parent" menu node, from which we propagate
        #   dependencies
        #
        # visible_if:
        #   Dependencies from 'visible if' on parent menus. These are added to
        #   the prompts of symbols and choices.

        if node.list:
            # The menu node is a choice, menu, or if. Finalize each child in
            # it.

            if node.item is MENU:
                visible_if = self._make_and(visible_if, node.visibility)

            # Propagate the menu node's dependencies to each child menu node.
            #
            # The recursive _finalize_tree() calls assume that the current
            # "level" in the tree has already had dependencies propagated. This
            # makes e.g. implicit submenu creation easier, because it needs to
            # look ahead.
            self._propagate_deps(node, visible_if)

            # Finalize the children
            cur = node.list
            while cur:
                self._finalize_tree(cur, visible_if)
                cur = cur.next

        elif node.item.__class__ is Symbol:
            # Add the node's non-node-specific properties (defaults, ranges,
            # etc.) to the Symbol
            self._add_props_to_sym(node)

            # See if we can create an implicit menu rooted at the Symbol and
            # finalize each child menu node in that menu if so, like for the
            # choice/menu/if case above
            cur = node
            while cur.next and _auto_menu_dep(node, cur.next):
                # This also makes implicit submenu creation work recursively,
                # with implicit menus inside implicit menus
                self._finalize_tree(cur.next, visible_if)
                cur = cur.next
                cur.parent = node

            if cur is not node:
                # Found symbols that should go in an implicit submenu. Tilt
                # them up above us.
                node.list = node.next
                node.next = cur.next
                cur.next = None


        if node.list:
            # We have a parent node with individually finalized child nodes. Do
            # final steps to finalize this "level" in the menu tree.
            _flatten(node.list)
            _remove_ifs(node)

        # Empty choices (node.list None) are possible, so this needs to go
        # outside
        if node.item.__class__ is Choice:
            # Add the node's non-node-specific properties to the choice, like
            # _add_props_to_sym() does
            choice = node.item
            choice.direct_dep = self._make_or(choice.direct_dep, node.dep)
            choice.defaults += node.defaults

            _finalize_choice(node)

    def _propagate_deps(self, node, visible_if):
        # Propagates 'node's dependencies to its child menu nodes

        # If the parent node holds a Choice, we use the Choice itself as the
        # parent dependency. This makes sense as the value (mode) of the choice
        # limits the visibility of the contained choice symbols. The C
        # implementation works the same way.
        #
        # Due to the similar interface, Choice works as a drop-in replacement
        # for Symbol here.
        basedep = node.item if node.item.__class__ is Choice else node.dep

        cur = node.list
        while cur:
            cur.dep = dep = self._make_and(cur.dep, basedep)

            # Propagate dependencies to prompt
            if cur.prompt:
                cur.prompt = (cur.prompt[0],
                              self._make_and(cur.prompt[1], dep))

            if cur.item.__class__ in _SYMBOL_CHOICE:
                # Propagate 'visible if' dependencies to the prompt
                if cur.prompt:
                    cur.prompt = (cur.prompt[0],
                                  self._make_and(cur.prompt[1], visible_if))

                # Propagate dependencies to defaults
                if cur.defaults:
                    cur.defaults = [(default, self._make_and(cond, dep))
                                    for default, cond in cur.defaults]

                # Propagate dependencies to ranges
                if cur.ranges:
                    cur.ranges = [(low, high, self._make_and(cond, dep))
                                  for low, high, cond in cur.ranges]

                # Propagate dependencies to selects
                if cur.selects:
                    cur.selects = [(target, self._make_and(cond, dep))
                                   for target, cond in cur.selects]

                # Propagate dependencies to implies
                if cur.implies:
                    cur.implies = [(target, self._make_and(cond, dep))
                                   for target, cond in cur.implies]


            cur = cur.next

    def _add_props_to_sym(self, node):
        # Copies properties from the menu node 'node' up to its contained
        # symbol, and adds (weak) reverse dependencies to selected/implied
        # symbols.
        #
        # This can't be rolled into _propagate_deps(), because that function
        # traverses the menu tree roughly breadth-first, meaning properties on
        # symbols defined in multiple locations could end up in the wrong
        # order.

        sym = node.item

        # See the Symbol class docstring
        sym.direct_dep = self._make_or(sym.direct_dep, node.dep)

        sym.defaults += node.defaults
        sym.ranges += node.ranges
        sym.selects += node.selects
        sym.implies += node.implies

        # Modify the reverse dependencies of the selected symbol
        for target, cond in node.selects:
            target.rev_dep = self._make_or(
                target.rev_dep,
                self._make_and(sym, cond))

        # Modify the weak reverse dependencies of the implied
        # symbol
        for target, cond in node.implies:
            target.weak_rev_dep = self._make_or(
                target.weak_rev_dep,
                self._make_and(sym, cond))


    #
    # Misc.
    #

    def _check_sym_sanity(self):
        # Checks various symbol properties that are handiest to check after
        # parsing. Only generates errors and warnings.

        def num_ok(sym, type_):
            # Returns True if the (possibly constant) symbol 'sym' is valid as a value
            # for a symbol of type type_ (INT or HEX)

            # 'not sym.nodes' implies a constant or undefined symbol, e.g. a plain
            # "123"
            if not sym.nodes:
                return _is_base_n(sym.name, _TYPE_TO_BASE[type_])

            return sym.orig_type is type_

        for sym in self.unique_defined_syms:
            if sym.orig_type in _BOOL_TRISTATE:
                # A helper function could be factored out here, but keep it
                # speedy/straightforward

                for target_sym, _ in sym.selects:
                    if target_sym.orig_type not in _BOOL_TRISTATE_UNKNOWN:
                        self._warn("{} selects the {} symbol {}, which is not "
                                   "bool or tristate"
                                   .format(_name_and_loc(sym),
                                           TYPE_TO_STR[target_sym.orig_type],
                                           _name_and_loc(target_sym)))

                for target_sym, _ in sym.implies:
                    if target_sym.orig_type not in _BOOL_TRISTATE_UNKNOWN:
                        self._warn("{} implies the {} symbol {}, which is not "
                                   "bool or tristate"
                                   .format(_name_and_loc(sym),
                                           TYPE_TO_STR[target_sym.orig_type],
                                           _name_and_loc(target_sym)))

            elif sym.orig_type in _STRING_INT_HEX:
                for default, _ in sym.defaults:
                    if default.__class__ is not Symbol:
                        raise KconfigError(
                            "the {} symbol {} has a malformed default {} -- expected "
                            "a single symbol"
                            .format(TYPE_TO_STR[sym.orig_type], _name_and_loc(sym),
                                    expr_str(default)))

                    if sym.orig_type is STRING:
                        if not default.is_constant and not default.nodes and \
                           not default.name.isupper():
                            # 'default foo' on a string symbol could be either a symbol
                            # reference or someone leaving out the quotes. Guess that
                            # the quotes were left out if 'foo' isn't all-uppercase
                            # (and no symbol named 'foo' exists).
                            self._warn("style: quotes recommended around "
                                       "default value for string symbol "
                                       + _name_and_loc(sym))

                    elif not num_ok(default, sym.orig_type):  # INT/HEX
                        self._warn("the {0} symbol {1} has a non-{0} default {2}"
                                   .format(TYPE_TO_STR[sym.orig_type],
                                           _name_and_loc(sym),
                                           _name_and_loc(default)))

                if sym.selects or sym.implies:
                    self._warn("the {} symbol {} has selects or implies"
                               .format(TYPE_TO_STR[sym.orig_type],
                                       _name_and_loc(sym)))

            else:  # UNKNOWN
                self._warn("{} defined without a type"
                           .format(_name_and_loc(sym)))


            if sym.ranges:
                if sym.orig_type not in _INT_HEX:
                    self._warn(
                        "the {} symbol {} has ranges, but is not int or hex"
                        .format(TYPE_TO_STR[sym.orig_type],
                                _name_and_loc(sym)))
                else:
                    for low, high, _ in sym.ranges:
                        if not num_ok(low, sym.orig_type) or \
                           not num_ok(high, sym.orig_type):

                            self._warn("the {0} symbol {1} has a non-{0} "
                                       "range [{2}, {3}]"
                                       .format(TYPE_TO_STR[sym.orig_type],
                                               _name_and_loc(sym),
                                               _name_and_loc(low),
                                               _name_and_loc(high)))

    def _check_choice_sanity(self):
        # Checks various choice properties that are handiest to check after
        # parsing. Only generates errors and warnings.

        def warn_select_imply(sym, expr, expr_type):
            msg = "the choice symbol {} is {} by the following symbols, but " \
                  "select/imply has no effect on choice symbols" \
                  .format(_name_and_loc(sym), expr_type)

            # si = select/imply
            for si in split_expr(expr, OR):
                msg += "\n - " + _name_and_loc(split_expr(si, AND)[0])

            self._warn(msg)

        for choice in self.unique_choices:
            if choice.orig_type not in _BOOL_TRISTATE:
                self._warn("{} defined with type {}"
                           .format(_name_and_loc(choice),
                                   TYPE_TO_STR[choice.orig_type]))

            for node in choice.nodes:
                if node.prompt:
                    break
            else:
                self._warn(_name_and_loc(choice) + " defined without a prompt")

            for default, _ in choice.defaults:
                if default.__class__ is not Symbol:
                    raise KconfigError(
                        "{} has a malformed default {}"
                        .format(_name_and_loc(choice), expr_str(default)))

                if default.choice is not choice:
                    self._warn("the default selection {} of {} is not "
                               "contained in the choice"
                               .format(_name_and_loc(default),
                                       _name_and_loc(choice)))

            for sym in choice.syms:
                if sym.defaults:
                    self._warn("default on the choice symbol {} will have "
                               "no effect, as defaults do not affect choice "
                               "symbols".format(_name_and_loc(sym)))

                if sym.rev_dep is not sym.kconfig.n:
                    warn_select_imply(sym, sym.rev_dep, "selected")

                if sym.weak_rev_dep is not sym.kconfig.n:
                    warn_select_imply(sym, sym.weak_rev_dep, "implied")

                for node in sym.nodes:
                    if node.parent.item is choice:
                        if not node.prompt:
                            self._warn("the choice symbol {} has no prompt"
                                       .format(_name_and_loc(sym)))

                    elif node.prompt:
                        self._warn("the choice symbol {} is defined with a "
                                   "prompt outside the choice"
                                   .format(_name_and_loc(sym)))

    def _parse_error(self, msg):
        raise KconfigError("{}couldn't parse '{}': {}".format(
            "" if self._filename is None else
                "{}:{}: ".format(self._filename, self._linenr),
            self._line.strip(), msg))

    def _trailing_tokens_error(self):
        self._parse_error("extra tokens at end of line")

    def _open(self, filename, mode):
        # open() wrapper:
        #
        # - Enable universal newlines mode on Python 2 to ease
        #   interoperability between Linux and Windows. It's already the
        #   default on Python 3.
        #
        #   The "U" flag would currently work for both Python 2 and 3, but it's
        #   deprecated on Python 3, so play it future-safe.
        #
        #   A simpler solution would be to use io.open(), which defaults to
        #   universal newlines on both Python 2 and 3 (and is an alias for
        #   open() on Python 3), but it's appreciably slower on Python 2:
        #
        #     Parsing x86 Kconfigs on Python 2
        #
        #     with open(..., "rU"):
        #
        #       real  0m0.930s
        #       user  0m0.905s
        #       sys   0m0.025s
        #
        #     with io.open():
        #
        #       real  0m1.069s
        #       user  0m1.040s
        #       sys   0m0.029s
        #
        #   There's no appreciable performance difference between "r" and
        #   "rU" for parsing performance on Python 2.
        #
        # - For Python 3, force the encoding. Forcing the encoding on Python 2
        #   turns strings into Unicode strings, which gets messy. Python 2
        #   doesn't decode regular strings anyway.
        return open(filename, "rU" if mode == "r" else mode) if _IS_PY2 else \
               open(filename, mode, encoding=self._encoding)

    def _check_undef_syms(self):
        # Prints warnings for all references to undefined symbols within the
        # Kconfig files

        def is_num(s):
            # Returns True if the string 's' looks like a number.
            #
            # Internally, all operands in Kconfig are symbols, only undefined symbols
            # (which numbers usually are) get their name as their value.
            #
            # Only hex numbers that start with 0x/0X are classified as numbers.
            # Otherwise, symbols whose names happen to contain only the letters A-F
            # would trigger false positives.

            try:
                int(s)
            except ValueError:
                if not s.startswith(("0x", "0X")):
                    return False

                try:
                    int(s, 16)
                except ValueError:
                    return False

            return True

        for sym in (self.syms.viewvalues if _IS_PY2 else self.syms.values)():
            # - sym.nodes empty means the symbol is undefined (has no
            #   definition locations)
            #
            # - Due to Kconfig internals, numbers show up as undefined Kconfig
            #   symbols, but shouldn't be flagged
            #
            # - The MODULES symbol always exists
            if not sym.nodes and not is_num(sym.name) and \
               sym.name != "MODULES":

                msg = "undefined symbol {}:".format(sym.name)

                for node in self.node_iter():
                    if sym in node.referenced:
                        msg += "\n\n- Referenced at {}:{}:\n\n{}" \
                               .format(node.filename, node.linenr, node)

                self._warn(msg)

    def _warn(self, msg, filename=None, linenr=None):
        # For printing general warnings

        if self._warnings_enabled:
            msg = "warning: " + msg
            if filename is not None:
                msg = "{}:{}: {}".format(filename, linenr, msg)

            self.warnings.append(msg)
            if self._warn_to_stderr:
                sys.stderr.write(msg + "\n")

    def _warn_undef_assign(self, msg, filename, linenr):
        # See the class documentation

        if self._warn_for_undef_assign:
            self._warn(msg, filename, linenr)

    def _warn_undef_assign_load(self, name, val, filename, linenr):
        # Special version for load_config()

        self._warn_undef_assign(
            'attempt to assign the value "{}" to the undefined symbol {}'
            .format(val, name), filename, linenr)

    def _warn_override(self, msg, filename, linenr):
        # See the class documentation

        if self._warn_for_override:
            self._warn(msg, filename, linenr)

    def _warn_redun_assign(self, msg, filename, linenr):
        # See the class documentation

        if self._warn_for_redun_assign:
            self._warn(msg, filename, linenr)

    def _srctree_hint(self):
        # Hint printed when Kconfig files can't be found or .config files can't
        # be opened

        return ". Perhaps the $srctree environment variable ({}) " \
               "is set incorrectly. Note that the current value of $srctree " \
               "is saved when the Kconfig instance is created (for " \
               "consistency and to cleanly separate instances)." \
               .format("set to '{}'".format(self.srctree) if self.srctree
                           else "unset or blank")

class Symbol(object):
    """
    Represents a configuration symbol:

      (menu)config FOO
          ...

    The following attributes are available. They should be viewed as read-only,
    and some are implemented through @property magic (but are still efficient
    to access due to internal caching).

    Note: Prompts, help texts, and locations are stored in the Symbol's
    MenuNode(s) rather than in the Symbol itself. Check the MenuNode class and
    the Symbol.nodes attribute. This organization matches the C tools.

    name:
      The name of the symbol, e.g. "FOO" for 'config FOO'.

    type:
      The type of the symbol. One of BOOL, TRISTATE, STRING, INT, HEX, UNKNOWN.
      UNKNOWN is for undefined symbols, (non-special) constant symbols, and
      symbols defined without a type.

      When running without modules (MODULES having the value n), TRISTATE
      symbols magically change type to BOOL. This also happens for symbols
      within choices in "y" mode. This matches the C tools, and makes sense for
      menuconfig-like functionality.

    orig_type:
      The type as given in the Kconfig file, without any magic applied. Used
      when printing the symbol.

    str_value:
      The value of the symbol as a string. Gives the value for string/int/hex
      symbols. For bool/tristate symbols, gives "n", "m", or "y".

      This is the symbol value that's used in relational expressions
      (A = B, A != B, etc.)

      Gotcha: For int/hex symbols, the exact format of the value must often be
      preserved (e.g., when writing a .config file), hence why you can't get it
      directly as an int. Do int(int_sym.str_value) or
      int(hex_sym.str_value, 16) to get the integer value.

    tri_value:
      The tristate value of the symbol as an integer. One of 0, 1, 2,
      representing n, m, y. Always 0 (n) for non-bool/tristate symbols.

      This is the symbol value that's used outside of relation expressions
      (A, !A, A && B, A || B).

    assignable:
      A tuple containing the tristate user values that can currently be
      assigned to the symbol (that would be respected), ordered from lowest (0,
      representing n) to highest (2, representing y). This corresponds to the
      selections available in the menuconfig interface. The set of assignable
      values is calculated from the symbol's visibility and selects/implies.

      Returns the empty set for non-bool/tristate symbols and for symbols with
      visibility n. The other possible values are (0, 2), (0, 1, 2), (1, 2),
      (1,), and (2,). A (1,) or (2,) result means the symbol is visible but
      "locked" to m or y through a select, perhaps in combination with the
      visibility. menuconfig represents this as -M- and -*-, respectively.

      For string/hex/int symbols, check if Symbol.visibility is non-0 (non-n)
      instead to determine if the value can be changed.

      Some handy 'assignable' idioms:

        # Is 'sym' an assignable (visible) bool/tristate symbol?
        if sym.assignable:
            # What's the highest value it can be assigned? [-1] in Python
            # gives the last element.
            sym_high = sym.assignable[-1]

            # The lowest?
            sym_low = sym.assignable[0]

            # Can the symbol be set to at least m?
            if sym.assignable[-1] >= 1:
                ...

        # Can the symbol be set to m?
        if 1 in sym.assignable:
            ...

    visibility:
      The visibility of the symbol. One of 0, 1, 2, representing n, m, y. See
      the module documentation for an overview of symbol values and visibility.

    user_value:
      The user value of the symbol. None if no user value has been assigned
      (via Kconfig.load_config() or Symbol.set_value()).

      Holds 0, 1, or 2 for bool/tristate symbols, and a string for the other
      symbol types.

      WARNING: Do not assign directly to this. It will break things. Use
      Symbol.set_value().

    config_string:
      The .config assignment string that would get written out for the symbol
      by Kconfig.write_config(). Returns the empty string if no .config
      assignment would get written out.

      In general, visible symbols, symbols with (active) defaults, and selected
      symbols get written out. This includes all non-n-valued bool/tristate
      symbols, and all visible string/int/hex symbols.

      Symbols with the (no longer needed) 'option env=...' option generate no
      configuration output, and neither does the special
      'option defconfig_list' symbol.

      Tip: This field is useful when generating custom configuration output,
      even for non-.config-like formats. To write just the symbols that would
      get written out to .config files, do this:

        if sym.config_string:
            *Write symbol, e.g. by looking sym.str_value*

      This is a superset of the symbols written out by write_autoconf().
      That function skips all n-valued symbols.

      There usually won't be any great harm in just writing all symbols either,
      though you might get some special symbols and possibly some "redundant"
      n-valued symbol entries in there.

    nodes:
      A list of MenuNodes for this symbol. Will contain a single MenuNode for
      most symbols. Undefined and constant symbols have an empty nodes list.
      Symbols defined in multiple locations get one node for each location.

    choice:
      Holds the parent Choice for choice symbols, and None for non-choice
      symbols. Doubles as a flag for whether a symbol is a choice symbol.

    defaults:
      List of (default, cond) tuples for the symbol's 'default' properties. For
      example, 'default A && B if C || D' is represented as
      ((AND, A, B), (OR, C, D)). If no condition was given, 'cond' is
      self.kconfig.y.

      Note that 'depends on' and parent dependencies are propagated to
      'default' conditions.

    selects:
      List of (symbol, cond) tuples for the symbol's 'select' properties. For
      example, 'select A if B && C' is represented as (A, (AND, B, C)). If no
      condition was given, 'cond' is self.kconfig.y.

      Note that 'depends on' and parent dependencies are propagated to 'select'
      conditions.

    implies:
      Like 'selects', for imply.

    ranges:
      List of (low, high, cond) tuples for the symbol's 'range' properties. For
      example, 'range 1 2 if A' is represented as (1, 2, A). If there is no
      condition, 'cond' is self.config.y.

      Note that 'depends on' and parent dependencies are propagated to 'range'
      conditions.

      Gotcha: 1 and 2 above will be represented as (undefined) Symbols rather
      than plain integers. Undefined symbols get their name as their string
      value, so this works out. The C tools work the same way.

    rev_dep:
      Reverse dependency expression from other symbols selecting this symbol.
      Multiple selections get ORed together. A condition on a select is ANDed
      with the selecting symbol.

      For example, if A has 'select FOO' and B has 'select FOO if C', then
      FOO's rev_dep will be (OR, A, (AND, B, C)).

    weak_rev_dep:
      Like rev_dep, for imply.

    direct_dep:
      The 'depends on' dependencies. If a symbol is defined in multiple
      locations, the dependencies at each location are ORed together.

      Internally, this is used to implement 'imply', which only applies if the
      implied symbol has expr_value(self.direct_dep) != 0. 'depends on' and
      parent dependencies are automatically propagated to the conditions of
      properties, so normally it's redundant to check the direct dependencies.

    referenced:
      A set() with all symbols and choices referenced in the properties and
      property conditions of the symbol.

      Also includes dependencies inherited from surrounding menus and if's.
      Choices appear in the dependencies of choice symbols.

    env_var:
      If the Symbol has an 'option env="FOO"' option, this contains the name
      ("FOO") of the environment variable. None for symbols without no
      'option env'.

      'option env="FOO"' acts like a 'default' property whose value is the
      value of $FOO.

      Symbols with 'option env' are never written out to .config files, even if
      they are visible. env_var corresponds to a flag called SYMBOL_AUTO in the
      C implementation.

    is_allnoconfig_y:
      True if the symbol has 'option allnoconfig_y' set on it. This has no
      effect internally (except when printing symbols), but can be checked by
      scripts.

    is_constant:
      True if the symbol is a constant (quoted) symbol.

    kconfig:
      The Kconfig instance this symbol is from.
    """
    __slots__ = (
        "_cached_assignable",
        "_cached_str_val",
        "_cached_tri_val",
        "_cached_vis",
        "_dependents",
        "_old_val",
        "_visited",
        "_was_set",
        "_write_to_conf",
        "choice",
        "defaults",
        "direct_dep",
        "env_var",
        "implies",
        "is_allnoconfig_y",
        "is_constant",
        "kconfig",
        "name",
        "nodes",
        "orig_type",
        "ranges",
        "rev_dep",
        "selects",
        "user_value",
        "weak_rev_dep",
    )

    #
    # Public interface
    #

    @property
    def type(self):
        """
        See the class documentation.
        """
        if self.orig_type is TRISTATE and \
           ((self.choice and self.choice.tri_value == 2) or
            not self.kconfig.modules.tri_value):

            return BOOL

        return self.orig_type

    @property
    def str_value(self):
        """
        See the class documentation.
        """
        if self._cached_str_val is not None:
            return self._cached_str_val

        if self.orig_type in _BOOL_TRISTATE:
            # Also calculates the visibility, so invalidation safe
            self._cached_str_val = TRI_TO_STR[self.tri_value]
            return self._cached_str_val

        # As a quirk of Kconfig, undefined symbols get their name as their
        # string value. This is why things like "FOO = bar" work for seeing if
        # FOO has the value "bar".
        if not self.orig_type:  # UNKNOWN
            self._cached_str_val = self.name
            return self.name

        val = ""
        # Warning: See Symbol._rec_invalidate(), and note that this is a hidden
        # function call (property magic)
        vis = self.visibility

        self._write_to_conf = (vis != 0)

        if self.orig_type in _INT_HEX:
            # The C implementation checks the user value against the range in a
            # separate code path (post-processing after loading a .config).
            # Checking all values here instead makes more sense for us. It
            # requires that we check for a range first.

            base = _TYPE_TO_BASE[self.orig_type]

            # Check if a range is in effect
            for low_expr, high_expr, cond in self.ranges:
                if expr_value(cond):
                    has_active_range = True

                    # The zeros are from the C implementation running strtoll()
                    # on empty strings
                    low = int(low_expr.str_value, base) if \
                      _is_base_n(low_expr.str_value, base) else 0
                    high = int(high_expr.str_value, base) if \
                      _is_base_n(high_expr.str_value, base) else 0

                    break
            else:
                has_active_range = False

            # Defaults are used if the symbol is invisible, lacks a user value,
            # or has an out-of-range user value
            use_defaults = True

            if vis and self.user_value:
                user_val = int(self.user_value, base)
                if has_active_range and not low <= user_val <= high:
                    num2str = str if base == 10 else hex
                    self.kconfig._warn(
                        "user value {} on the {} symbol {} ignored due to "
                        "being outside the active range ([{}, {}]) -- falling "
                        "back on defaults"
                        .format(num2str(user_val), TYPE_TO_STR[self.orig_type],
                                _name_and_loc(self),
                                num2str(low), num2str(high)))
                else:
                    # If the user value is well-formed and satisfies range
                    # contraints, it is stored in exactly the same form as
                    # specified in the assignment (with or without "0x", etc.)
                    val = self.user_value
                    use_defaults = False

            if use_defaults:
                # No user value or invalid user value. Look at defaults.

                # Used to implement the warning below
                has_default = False

                for sym, cond in self.defaults:
                    if expr_value(cond):
                        has_default = self._write_to_conf = True

                        val = sym.str_value

                        if _is_base_n(val, base):
                            val_num = int(val, base)
                        else:
                            val_num = 0  # strtoll() on empty string

                        break
                else:
                    val_num = 0  # strtoll() on empty string

                # This clamping procedure runs even if there's no default
                if has_active_range:
                    clamp = None
                    if val_num < low:
                        clamp = low
                    elif val_num > high:
                        clamp = high

                    if clamp is not None:
                        # The value is rewritten to a standard form if it is
                        # clamped
                        val = str(clamp) \
                              if self.orig_type is INT else \
                              hex(clamp)

                        if has_default:
                            num2str = str if base == 10 else hex
                            self.kconfig._warn(
                                "default value {} on {} clamped to {} due to "
                                "being outside the active range ([{}, {}])"
                                .format(val_num, _name_and_loc(self),
                                        num2str(clamp), num2str(low),
                                        num2str(high)))

        elif self.orig_type is STRING:
            if vis and self.user_value is not None:
                # If the symbol is visible and has a user value, use that
                val = self.user_value
            else:
                # Otherwise, look at defaults
                for sym, cond in self.defaults:
                    if expr_value(cond):
                        val = sym.str_value
                        self._write_to_conf = True
                        break

        # env_var corresponds to SYMBOL_AUTO in the C implementation, and is
        # also set on the defconfig_list symbol there. Test for the
        # defconfig_list symbol explicitly instead here, to avoid a nonsensical
        # env_var setting and the defconfig_list symbol being printed
        # incorrectly. This code is pretty cold anyway.
        if self.env_var is not None or self is self.kconfig.defconfig_list:
            self._write_to_conf = False

        self._cached_str_val = val
        return val

    @property
    def tri_value(self):
        """
        See the class documentation.
        """
        if self._cached_tri_val is not None:
            return self._cached_tri_val

        if self.orig_type not in _BOOL_TRISTATE:
            if self.orig_type:  # != UNKNOWN
                # Would take some work to give the location here
                self.kconfig._warn(
                    "The {} symbol {} is being evaluated in a logical context "
                    "somewhere. It will always evaluate to n."
                    .format(TYPE_TO_STR[self.orig_type], _name_and_loc(self)))

            self._cached_tri_val = 0
            return 0

        # Warning: See Symbol._rec_invalidate(), and note that this is a hidden
        # function call (property magic)
        vis = self.visibility
        self._write_to_conf = (vis != 0)

        val = 0

        if not self.choice:
            # Non-choice symbol

            if vis and self.user_value is not None:
                # If the symbol is visible and has a user value, use that
                val = min(self.user_value, vis)

            else:
                # Otherwise, look at defaults and weak reverse dependencies
                # (implies)

                for default, cond in self.defaults:
                    dep_val = expr_value(cond)
                    if dep_val:
                        val = min(expr_value(default), dep_val)
                        if val:
                            self._write_to_conf = True
                        break

                # Weak reverse dependencies are only considered if our
                # direct dependencies are met
                dep_val = expr_value(self.weak_rev_dep)
                if dep_val and expr_value(self.direct_dep):
                    val = max(dep_val, val)
                    self._write_to_conf = True

            # Reverse (select-related) dependencies take precedence
            dep_val = expr_value(self.rev_dep)
            if dep_val:
                if expr_value(self.direct_dep) < dep_val:
                    self._warn_select_unsatisfied_deps()

                val = max(dep_val, val)
                self._write_to_conf = True

            # m is promoted to y for (1) bool symbols and (2) symbols with a
            # weak_rev_dep (from imply) of y
            if val == 1 and \
               (self.type is BOOL or expr_value(self.weak_rev_dep) == 2):
                val = 2

        elif vis == 2:
            # Visible choice symbol in y-mode choice. The choice mode limits
            # the visibility of choice symbols, so it's sufficient to just
            # check the visibility of the choice symbols themselves.
            val = 2 if self.choice.selection is self else 0

        elif vis and self.user_value:
            # Visible choice symbol in m-mode choice, with set non-0 user value
            val = 1

        self._cached_tri_val = val
        return val

    @property
    def assignable(self):
        """
        See the class documentation.
        """
        if self._cached_assignable is None:
            self._cached_assignable = self._assignable()

        return self._cached_assignable

    @property
    def visibility(self):
        """
        See the class documentation.
        """
        if self._cached_vis is None:
            self._cached_vis = _visibility(self)

        return self._cached_vis

    @property
    def config_string(self):
        """
        See the class documentation.
        """
        # Note: _write_to_conf is determined when the value is calculated. This
        # is a hidden function call due to property magic.
        val = self.str_value
        if not self._write_to_conf:
            return ""

        if self.orig_type in _BOOL_TRISTATE:
            return "{}{}={}\n" \
                   .format(self.kconfig.config_prefix, self.name, val) \
                   if val != "n" else \
                   "# {}{} is not set\n" \
                   .format(self.kconfig.config_prefix, self.name)

        if self.orig_type in _INT_HEX:
            return "{}{}={}\n" \
                   .format(self.kconfig.config_prefix, self.name, val)

        # sym.orig_type is STRING
        return '{}{}="{}"\n' \
               .format(self.kconfig.config_prefix, self.name, escape(val))

    def set_value(self, value):
        """
        Sets the user value of the symbol.

        Equal in effect to assigning the value to the symbol within a .config
        file. For bool and tristate symbols, use the 'assignable' attribute to
        check which values can currently be assigned. Setting values outside
        'assignable' will cause Symbol.user_value to differ from
        Symbol.str/tri_value (be truncated down or up).

        Setting a choice symbol to 2 (y) sets Choice.user_selection to the
        choice symbol in addition to setting Symbol.user_value.
        Choice.user_selection is considered when the choice is in y mode (the
        "normal" mode).

        Other symbols that depend (possibly indirectly) on this symbol are
        automatically recalculated to reflect the assigned value.

        value:
          The user value to give to the symbol. For bool and tristate symbols,
          n/m/y can be specified either as 0/1/2 (the usual format for tristate
          values in Kconfiglib) or as one of the strings "n"/"m"/"y". For other
          symbol types, pass a string.

          Values that are invalid for the type (such as "foo" or 1 (m) for a
          BOOL or "0x123" for an INT) are ignored and won't be stored in
          Symbol.user_value. Kconfiglib will print a warning by default for
          invalid assignments, and set_value() will return False.

        Returns True if the value is valid for the type of the symbol, and
        False otherwise. This only looks at the form of the value. For BOOL and
        TRISTATE symbols, check the Symbol.assignable attribute to see what
        values are currently in range and would actually be reflected in the
        value of the symbol. For other symbol types, check whether the
        visibility is non-n.
        """
        # If the new user value matches the old, nothing changes, and we can
        # save some work.
        #
        # This optimization is skipped for choice symbols: Setting a choice
        # symbol's user value to y might change the state of the choice, so it
        # wouldn't be safe (symbol user values always match the values set in a
        # .config file or via set_value(), and are never implicitly updated).
        if value == self.user_value and not self.choice:
            self._was_set = True
            return True

        # Check if the value is valid for our type
        if not (self.orig_type is BOOL     and value in (2, 0, "y", "n")         or
                self.orig_type is TRISTATE and value in (2, 1, 0, "y", "m", "n") or
                (value.__class__ is str    and
                 (self.orig_type is STRING                        or
                  self.orig_type is INT and _is_base_n(value, 10) or
                  self.orig_type is HEX and _is_base_n(value, 16)
                                        and int(value, 16) >= 0))):

            # Display tristate values as n, m, y in the warning
            self.kconfig._warn(
                "the value {} is invalid for {}, which has type {} -- "
                "assignment ignored"
                .format(TRI_TO_STR[value] if value in (0, 1, 2) else
                            "'{}'".format(value),
                        _name_and_loc(self), TYPE_TO_STR[self.orig_type]))

            return False

        if self.orig_type in _BOOL_TRISTATE and value in ("y", "m", "n"):
            value = STR_TO_TRI[value]

        self.user_value = value
        self._was_set = True

        if self.choice and value == 2:
            # Setting a choice symbol to y makes it the user selection of the
            # choice. Like for symbol user values, the user selection is not
            # guaranteed to match the actual selection of the choice, as
            # dependencies come into play.
            self.choice.user_selection = self
            self.choice._was_set = True
            self.choice._rec_invalidate()
        else:
            self._rec_invalidate_if_has_prompt()

        return True

    def unset_value(self):
        """
        Resets the user value of the symbol, as if the symbol had never gotten
        a user value via Kconfig.load_config() or Symbol.set_value().
        """
        if self.user_value is not None:
            self.user_value = None
            self._rec_invalidate_if_has_prompt()

    @property
    def referenced(self):
        """
        See the class documentation.
        """
        res = set()
        for node in self.nodes:
            res |= node.referenced

        return res

    def __repr__(self):
        """
        Returns a string with information about the symbol (including its name,
        value, visibility, and location(s)) when it is evaluated on e.g. the
        interactive Python prompt.
        """
        fields = []

        fields.append("symbol " + self.name)
        fields.append(TYPE_TO_STR[self.type])

        for node in self.nodes:
            if node.prompt:
                fields.append('"{}"'.format(node.prompt[0]))

        # Only add quotes for non-bool/tristate symbols
        fields.append("value " +
                      (self.str_value
                       if self.orig_type in _BOOL_TRISTATE else
                       '"{}"'.format(self.str_value)))

        if not self.is_constant:
            # These aren't helpful to show for constant symbols

            if self.user_value is not None:
                # Only add quotes for non-bool/tristate symbols
                fields.append("user value " +
                              (TRI_TO_STR[self.user_value]
                               if self.orig_type in _BOOL_TRISTATE else
                               '"{}"'.format(self.user_value)))

            fields.append("visibility " + TRI_TO_STR[self.visibility])

            if self.choice:
                fields.append("choice symbol")

            if self.is_allnoconfig_y:
                fields.append("allnoconfig_y")

            if self is self.kconfig.defconfig_list:
                fields.append("is the defconfig_list symbol")

            if self.env_var is not None:
                fields.append("from environment variable " + self.env_var)

            if self is self.kconfig.modules:
                fields.append("is the modules symbol")

            fields.append("direct deps " +
                          TRI_TO_STR[expr_value(self.direct_dep)])

        if self.nodes:
            for node in self.nodes:
                fields.append("{}:{}".format(node.filename, node.linenr))
        else:
            if self.is_constant:
                fields.append("constant")
            else:
                fields.append("undefined")

        return "<{}>".format(", ".join(fields))

    def __str__(self):
        """
        Returns a string representation of the symbol when it is printed,
        matching the Kconfig format, with parent dependencies propagated.

        The string is constructed by joining the strings returned by
        MenuNode.__str__() for each of the symbol's menu nodes, so symbols
        defined in multiple locations will return a string with all
        definitions.

        The returned string does not end in a newline. An empty string is
        returned for undefined and constant symbols.
        """
        return self.custom_str(standard_sc_expr_str)

    def custom_str(self, sc_expr_str_fn):
        """
        Works like Symbol.__str__(), but allows a custom format to be used for
        all symbol/choice references. See expr_str().
        """
        return "\n\n".join(node.custom_str(sc_expr_str_fn)
                           for node in self.nodes)

    #
    # Private methods
    #

    def __init__(self):
        """
        Symbol constructor -- not intended to be called directly by Kconfiglib
        clients.
        """
        # These attributes are always set on the instance from outside and
        # don't need defaults:
        #   kconfig
        #   direct_dep
        #   is_constant
        #   name
        #   rev_dep
        #   weak_rev_dep

        self.orig_type = UNKNOWN
        self.defaults = []
        self.selects = []
        self.implies = []
        self.ranges = []

        self.nodes = []

        self.user_value = \
        self.choice = \
        self.env_var = \
        self._cached_str_val = self._cached_tri_val = self._cached_vis = \
        self._cached_assignable = None

        # _write_to_conf is calculated along with the value. If True, the
        # Symbol gets a .config entry.

        self.is_allnoconfig_y = \
        self._was_set = \
        self._write_to_conf = False

        # See Kconfig._build_dep()
        self._dependents = set()

        # Used during dependency loop detection and (independently) in
        # node_iter()
        self._visited = 0

    def _assignable(self):
        # Worker function for the 'assignable' attribute

        if self.orig_type not in _BOOL_TRISTATE:
            return ()

        # Warning: See Symbol._rec_invalidate(), and note that this is a hidden
        # function call (property magic)
        vis = self.visibility

        if not vis:
            return ()

        rev_dep_val = expr_value(self.rev_dep)

        if vis == 2:
            if self.choice:
                return (2,)

            if not rev_dep_val:
                if self.type is BOOL or expr_value(self.weak_rev_dep) == 2:
                    return (0, 2)
                return (0, 1, 2)

            if rev_dep_val == 2:
                return (2,)

            # rev_dep_val == 1

            if self.type is BOOL or expr_value(self.weak_rev_dep) == 2:
                return (2,)
            return (1, 2)

        # vis == 1

        # Must be a tristate here, because bool m visibility gets promoted to y

        if not rev_dep_val:
            return (0, 1) if expr_value(self.weak_rev_dep) != 2 else (0, 2)

        if rev_dep_val == 2:
            return (2,)

        # vis == rev_dep_val == 1

        return (1,)

    def _invalidate(self):
        # Marks the symbol as needing to be recalculated

        self._cached_str_val = self._cached_tri_val = self._cached_vis = \
            self._cached_assignable = None

    def _rec_invalidate(self):
        # Invalidates the symbol and all items that (possibly) depend on it

        if self is self.kconfig.modules:
            # Invalidating MODULES has wide-ranging effects
            self.kconfig._invalidate_all()
        else:
            self._invalidate()

            for item in self._dependents:
                # _cached_vis doubles as a flag that tells us whether 'item'
                # has cached values, because it's calculated as a side effect
                # of calculating all other (non-constant) cached values.
                #
                # If item._cached_vis is None, it means there can't be cached
                # values on other items that depend on 'item', because if there
                # were, some value on 'item' would have been calculated and
                # item._cached_vis set as a side effect. It's therefore safe to
                # stop the invalidation at symbols with _cached_vis None.
                #
                # This approach massively speeds up scripts that set a lot of
                # values, vs simply invalidating all possibly dependent symbols
                # (even when you already have a list of all the dependent
                # symbols, because some symbols get huge dependency trees).
                #
                # This gracefully handles dependency loops too, which is nice
                # for choices, where the choice depends on the choice symbols
                # and vice versa.
                if item._cached_vis is not None:
                    item._rec_invalidate()

    def _rec_invalidate_if_has_prompt(self):
        # Invalidates the symbol and its dependent symbols, but only if the
        # symbol has a prompt. User values never have an effect on promptless
        # symbols, so we skip invalidation for them as an optimization.
        #
        # This also prevents constant (quoted) symbols from being invalidated
        # if set_value() is called on them, which would cause them to lose
        # their value and break things.
        #
        # Prints a warning if the symbol has no prompt. In some contexts (e.g.
        # when loading a .config files) assignments to promptless symbols are
        # normal and expected, so the warning can be disabled.

        for node in self.nodes:
            if node.prompt:
                self._rec_invalidate()
                return

        if self.kconfig._warn_for_no_prompt:
            self.kconfig._warn(_name_and_loc(self) + " has no prompt, meaning "
                               "user values have no effect on it")

    def _str_default(self):
        # write_min_config() helper function. Returns the value the symbol
        # would get from defaults if it didn't have a user value. Uses exactly
        # the same algorithm as the C implementation (though a bit cleaned up),
        # for compatibility.

        if self.orig_type in _BOOL_TRISTATE:
            val = 0

            # Defaults, selects, and implies do not affect choice symbols
            if not self.choice:
                for default, cond in self.defaults:
                    cond_val = expr_value(cond)
                    if cond_val:
                        val = min(expr_value(default), cond_val)
                        break

                val = max(expr_value(self.rev_dep),
                          expr_value(self.weak_rev_dep),
                          val)

                # Transpose mod to yes if type is bool (possibly due to modules
                # being disabled)
                if val == 1 and self.type is BOOL:
                    val = 2

            return TRI_TO_STR[val]

        if self.orig_type in _STRING_INT_HEX:
            for default, cond in self.defaults:
                if expr_value(cond):
                    return default.str_value

        return ""

    def _warn_select_unsatisfied_deps(self):
        # Helper for printing an informative warning when a symbol with
        # unsatisfied direct dependencies (dependencies from 'depends on', ifs,
        # and menus) is selected by some other symbol. Also warn if a symbol
        # whose direct dependencies evaluate to m is selected to y.

        msg = "{} has direct dependencies {} with value {}, but is " \
              "currently being {}-selected by the following symbols:" \
              .format(_name_and_loc(self), expr_str(self.direct_dep),
                      TRI_TO_STR[expr_value(self.direct_dep)],
                      TRI_TO_STR[expr_value(self.rev_dep)])

        # The reverse dependencies from each select are ORed together
        for select in split_expr(self.rev_dep, OR):
            if expr_value(select) <= expr_value(self.direct_dep):
                # Only include selects that exceed the direct dependencies
                continue

            # - 'select A if B' turns into A && B
            # - 'select A' just turns into A
            #
            # In both cases, we can split on AND and pick the first operand
            selecting_sym = split_expr(select, AND)[0]

            msg += "\n - {}, with value {}, direct dependencies {} " \
                   "(value: {})" \
                   .format(_name_and_loc(selecting_sym),
                           selecting_sym.str_value,
                           expr_str(selecting_sym.direct_dep),
                           TRI_TO_STR[expr_value(selecting_sym.direct_dep)])

            if select.__class__ is tuple:
                msg += ", and select condition {} (value: {})" \
                       .format(expr_str(select[2]),
                               TRI_TO_STR[expr_value(select[2])])

        self.kconfig._warn(msg)

class Choice(object):
    """
    Represents a choice statement:

      choice
          ...
      endchoice

    The following attributes are available on Choice instances. They should be
    treated as read-only, and some are implemented through @property magic (but
    are still efficient to access due to internal caching).

    Note: Prompts, help texts, and locations are stored in the Choice's
    MenuNode(s) rather than in the Choice itself. Check the MenuNode class and
    the Choice.nodes attribute. This organization matches the C tools.

    name:
      The name of the choice, e.g. "FOO" for 'choice FOO', or None if the
      Choice has no name.

    type:
      The type of the choice. One of BOOL, TRISTATE, UNKNOWN. UNKNOWN is for
      choices defined without a type where none of the contained symbols have a
      type either (otherwise the choice inherits the type of the first symbol
      defined with a type).

      When running without modules (CONFIG_MODULES=n), TRISTATE choices
      magically change type to BOOL. This matches the C tools, and makes sense
      for menuconfig-like functionality.

    orig_type:
      The type as given in the Kconfig file, without any magic applied. Used
      when printing the choice.

    tri_value:
      The tristate value (mode) of the choice. A choice can be in one of three
      modes:

        0 (n) - The choice is disabled and no symbols can be selected. For
                visible choices, this mode is only possible for choices with
                the 'optional' flag set (see kconfig-language.txt).

        1 (m) - Any number of choice symbols can be set to m, the rest will
                be n.

        2 (y) - One symbol will be y, the rest n.

      Only tristate choices can be in m mode. The visibility of the choice is
      an upper bound on the mode, and the mode in turn is an upper bound on the
      visibility of the choice symbols.

      To change the mode, use Choice.set_value().

      Implementation note:
        The C tools internally represent choices as a type of symbol, with
        special-casing in many code paths. This is why there is a lot of
        similarity to Symbol. The value (mode) of a choice is really just a
        normal symbol value, and an implicit reverse dependency forces its
        lower bound to m for visible non-optional choices (the reverse
        dependency is 'm && <visibility>').

        Symbols within choices get the choice propagated as a dependency to
        their properties. This turns the mode of the choice into an upper bound
        on e.g. the visibility of choice symbols, and explains the gotcha
        related to printing choice symbols mentioned in the module docstring.

        Kconfiglib uses a separate Choice class only because it makes the code
        and interface less confusing (especially in a user-facing interface).
        Corresponding attributes have the same name in the Symbol and Choice
        classes, for consistency and compatibility.

    assignable:
      See the symbol class documentation. Gives the assignable values (modes).

    visibility:
      See the Symbol class documentation. Acts on the value (mode).

    selection:
      The Symbol instance of the currently selected symbol. None if the Choice
      is not in y mode or has no selected symbol (due to unsatisfied
      dependencies on choice symbols).

      WARNING: Do not assign directly to this. It will break things. Call
      sym.set_value(2) on the choice symbol you want to select instead.

    user_value:
      The value (mode) selected by the user through Choice.set_value(). Either
      0, 1, or 2, or None if the user hasn't selected a mode. See
      Symbol.user_value.

      WARNING: Do not assign directly to this. It will break things. Use
      Choice.set_value() instead.

    user_selection:
      The symbol selected by the user (by setting it to y). Ignored if the
      choice is not in y mode, but still remembered so that the choice "snaps
      back" to the user selection if the mode is changed back to y. This might
      differ from 'selection' due to unsatisfied dependencies.

      WARNING: Do not assign directly to this. It will break things. Call
      sym.set_value(2) on the choice symbol to be selected instead.

    syms:
      List of symbols contained in the choice.

      Obscure gotcha: If a symbol depends on the previous symbol within a
      choice so that an implicit menu is created, it won't be a choice symbol,
      and won't be included in 'syms'.

    nodes:
      A list of MenuNodes for this choice. In practice, the list will probably
      always contain a single MenuNode, but it is possible to give a choice a
      name and define it in multiple locations.

    defaults:
      List of (symbol, cond) tuples for the choice's 'defaults' properties. For
      example, 'default A if B && C' is represented as (A, (AND, B, C)). If
      there is no condition, 'cond' is self.config.y.

      Note that 'depends on' and parent dependencies are propagated to
      'default' conditions.

    direct_dep:
      See Symbol.direct_dep.

    referenced:
      A set() with all symbols referenced in the properties and property
      conditions of the choice.

      Also includes dependencies inherited from surrounding menus and if's.

    is_optional:
      True if the choice has the 'optional' flag set on it and can be in
      n mode.

    kconfig:
      The Kconfig instance this choice is from.
    """
    __slots__ = (
        "_cached_assignable",
        "_cached_selection",
        "_cached_vis",
        "_dependents",
        "_visited",
        "_was_set",
        "defaults",
        "direct_dep",
        "is_constant",
        "is_optional",
        "kconfig",
        "name",
        "nodes",
        "orig_type",
        "syms",
        "user_selection",
        "user_value",
    )

    #
    # Public interface
    #

    @property
    def type(self):
        """
        Returns the type of the choice. See Symbol.type.
        """
        if self.orig_type is TRISTATE and not self.kconfig.modules.tri_value:
            return BOOL

        return self.orig_type

    @property
    def str_value(self):
        """
        See the class documentation.
        """
        return TRI_TO_STR[self.tri_value]

    @property
    def tri_value(self):
        """
        See the class documentation.
        """
        # This emulates a reverse dependency of 'm && visibility' for
        # non-optional choices, which is how the C implementation does it

        val = 0 if self.is_optional else 1

        if self.user_value is not None:
            val = max(val, self.user_value)

        # Warning: See Symbol._rec_invalidate(), and note that this is a hidden
        # function call (property magic)
        val = min(val, self.visibility)

        # Promote m to y for boolean choices
        return 2 if val == 1 and self.type is BOOL else val

    @property
    def assignable(self):
        """
        See the class documentation.
        """
        if self._cached_assignable is None:
            self._cached_assignable = self._assignable()

        return self._cached_assignable

    @property
    def visibility(self):
        """
        See the class documentation.
        """
        if self._cached_vis is None:
            self._cached_vis = _visibility(self)

        return self._cached_vis

    @property
    def selection(self):
        """
        See the class documentation.
        """
        if self._cached_selection is _NO_CACHED_SELECTION:
            self._cached_selection = self._selection()

        return self._cached_selection

    def set_value(self, value):
        """
        Sets the user value (mode) of the choice. Like for Symbol.set_value(),
        the visibility might truncate the value. Choices without the 'optional'
        attribute (is_optional) can never be in n mode, but 0/"n" is still
        accepted since it's not a malformed value (though it will have no
        effect).

        Returns True if the value is valid for the type of the choice, and
        False otherwise. This only looks at the form of the value. Check the
        Choice.assignable attribute to see what values are currently in range
        and would actually be reflected in the mode of the choice.
        """
        if value == self.user_value:
            # We know the value must be valid if it was successfully set
            # previously
            self._was_set = True
            return True

        if not ((self.orig_type is BOOL     and value in (2, 0, "y", "n")        ) or
                (self.orig_type is TRISTATE and value in (2, 1, 0, "y", "m", "n"))):

            # Display tristate values as n, m, y in the warning
            self.kconfig._warn(
                "the value {} is invalid for {}, which has type {} -- "
                "assignment ignored"
                .format(TRI_TO_STR[value] if value in (0, 1, 2) else
                            "'{}'".format(value),
                        _name_and_loc(self),
                        TYPE_TO_STR[self.orig_type]))

            return False

        if value in ("y", "m", "n"):
            value = STR_TO_TRI[value]

        self.user_value = value
        self._was_set = True
        self._rec_invalidate()

        return True

    def unset_value(self):
        """
        Resets the user value (mode) and user selection of the Choice, as if
        the user had never touched the mode or any of the choice symbols.
        """
        if self.user_value is not None or self.user_selection:
            self.user_value = self.user_selection = None
            self._rec_invalidate()

    @property
    def referenced(self):
        """
        See the class documentation.
        """
        res = set()
        for node in self.nodes:
            res |= node.referenced

        return res

    def __repr__(self):
        """
        Returns a string with information about the choice when it is evaluated
        on e.g. the interactive Python prompt.
        """
        fields = []

        fields.append("choice " + self.name if self.name else "choice")
        fields.append(TYPE_TO_STR[self.type])

        for node in self.nodes:
            if node.prompt:
                fields.append('"{}"'.format(node.prompt[0]))

        fields.append("mode " + self.str_value)

        if self.user_value is not None:
            fields.append('user mode {}'.format(TRI_TO_STR[self.user_value]))

        if self.selection:
            fields.append("{} selected".format(self.selection.name))

        if self.user_selection:
            user_sel_str = "{} selected by user" \
                           .format(self.user_selection.name)

            if self.selection is not self.user_selection:
                user_sel_str += " (overridden)"

            fields.append(user_sel_str)

        fields.append("visibility " + TRI_TO_STR[self.visibility])

        if self.is_optional:
            fields.append("optional")

        for node in self.nodes:
            fields.append("{}:{}".format(node.filename, node.linenr))

        return "<{}>".format(", ".join(fields))

    def __str__(self):
        """
        Returns a string representation of the choice when it is printed,
        matching the Kconfig format (though without the contained choice
        symbols).

        The returned string does not end in a newline.

        See Symbol.__str__() as well.
        """
        return self.custom_str(standard_sc_expr_str)

    def custom_str(self, sc_expr_str_fn):
        """
        Works like Choice.__str__(), but allows a custom format to be used for
        all symbol/choice references. See expr_str().
        """
        return "\n\n".join(node.custom_str(sc_expr_str_fn)
                           for node in self.nodes)

    #
    # Private methods
    #

    def __init__(self):
        """
        Choice constructor -- not intended to be called directly by Kconfiglib
        clients.
        """
        # These attributes are always set on the instance from outside and
        # don't need defaults:
        #   direct_dep
        #   kconfig

        self.orig_type = UNKNOWN
        self.syms = []
        self.defaults = []

        self.nodes = []

        self.name = \
        self.user_value = self.user_selection = \
        self._cached_vis = self._cached_assignable = None

        self._cached_selection = _NO_CACHED_SELECTION

        # is_constant is checked by _make_depend_on(). Just set it to avoid
        # having to special-case choices.
        self.is_constant = self.is_optional = False

        # See Kconfig._build_dep()
        self._dependents = set()

        # Used during dependency loop detection
        self._visited = 0

    def _assignable(self):
        # Worker function for the 'assignable' attribute

        # Warning: See Symbol._rec_invalidate(), and note that this is a hidden
        # function call (property magic)
        vis = self.visibility

        if not vis:
            return ()

        if vis == 2:
            if not self.is_optional:
                return (2,) if self.type is BOOL else (1, 2)
            return (0, 2) if self.type is BOOL else (0, 1, 2)

        # vis == 1

        return (0, 1) if self.is_optional else (1,)

    def _selection(self):
        # Worker function for the 'selection' attribute

        # Warning: See Symbol._rec_invalidate(), and note that this is a hidden
        # function call (property magic)
        if self.tri_value != 2:
            # Not in y mode, so no selection
            return None

        # Use the user selection if it's visible
        if self.user_selection and self.user_selection.visibility:
            return self.user_selection

        # Otherwise, check if we have a default
        return self._get_selection_from_defaults()

    def _get_selection_from_defaults(self):
        # Check if we have a default
        for sym, cond in self.defaults:
            # The default symbol must be visible too
            if expr_value(cond) and sym.visibility:
                return sym

        # Otherwise, pick the first visible symbol, if any
        for sym in self.syms:
            if sym.visibility:
                return sym

        # Couldn't find a selection
        return None

    def _invalidate(self):
        self._cached_vis = self._cached_assignable = None
        self._cached_selection = _NO_CACHED_SELECTION

    def _rec_invalidate(self):
        # See Symbol._rec_invalidate()

        self._invalidate()

        for item in self._dependents:
            if item._cached_vis is not None:
                item._rec_invalidate()

class MenuNode(object):
    """
    Represents a menu node in the configuration. This corresponds to an entry
    in e.g. the 'make menuconfig' interface, though non-visible choices, menus,
    and comments also get menu nodes. If a symbol or choice is defined in
    multiple locations, it gets one menu node for each location.

    The top-level menu node, corresponding to the implicit top-level menu, is
    available in Kconfig.top_node.

    The menu nodes for a Symbol or Choice can be found in the
    Symbol/Choice.nodes attribute. Menus and comments are represented as plain
    menu nodes, with their text stored in the prompt attribute (prompt[0]).
    This mirrors the C implementation.

    The following attributes are available on MenuNode instances. They should
    be viewed as read-only.

    item:
      Either a Symbol, a Choice, or one of the constants MENU and COMMENT.
      Menus and comments are represented as plain menu nodes. Ifs are collapsed
      (matching the C implementation) and do not appear in the final menu tree.

    next:
      The following menu node. None if there is no following node.

    list:
      The first child menu node. None if there are no children.

      Choices and menus naturally have children, but Symbols can also have
      children because of menus created automatically from dependencies (see
      kconfig-language.txt).

    parent:
      The parent menu node. None if there is no parent.

    prompt:
      A (string, cond) tuple with the prompt for the menu node and its
      conditional expression (which is self.kconfig.y if there is no
      condition). None if there is no prompt.

      For symbols and choices, the prompt is stored in the MenuNode rather than
      the Symbol or Choice instance. For menus and comments, the prompt holds
      the text.

    defaults:
      The 'default' properties for this particular menu node. See
      symbol.defaults.

      When evaluating defaults, you should use Symbol/Choice.defaults instead,
      as it include properties from all menu nodes (a symbol/choice can have
      multiple definition locations/menu nodes). MenuNode.defaults is meant for
      documentation generation.

    selects:
      Like MenuNode.defaults, for selects.

    implies:
      Like MenuNode.defaults, for implies.

    ranges:
      Like MenuNode.defaults, for ranges.

    help:
      The help text for the menu node for Symbols and Choices. None if there is
      no help text. Always stored in the node rather than the Symbol or Choice.
      It is possible to have a separate help text at each location if a symbol
      is defined in multiple locations.

      Trailing whitespace (including a final newline) is stripped from the help
      text. This was not the case before Kconfiglib 10.21.0, where the format
      was undocumented.

    dep:
      The 'depends on' dependencies for the menu node, or self.kconfig.y if
      there are no dependencies. Parent dependencies are propagated to this
      attribute, and this attribute is then in turn propagated to the
      properties of symbols and choices.

      If a symbol or choice is defined in multiple locations, only the
      properties defined at a particular location get the corresponding
      MenuNode.dep dependencies propagated to them.

    visibility:
      The 'visible if' dependencies for the menu node (which must represent a
      menu), or self.kconfig.y if there are no 'visible if' dependencies.
      'visible if' dependencies are recursively propagated to the prompts of
      symbols and choices within the menu.

    referenced:
      A set() with all symbols and choices referenced in the properties and
      property conditions of the menu node.

      Also includes dependencies inherited from surrounding menus and if's.
      Choices appear in the dependencies of choice symbols.

    is_menuconfig:
      Set to True if the children of the menu node should be displayed in a
      separate menu. This is the case for the following items:

        - Menus (node.item == MENU)

        - Choices

        - Symbols defined with the 'menuconfig' keyword. The children come from
          implicitly created submenus, and should be displayed in a separate
          menu rather than being indented.

      'is_menuconfig' is just a hint on how to display the menu node. It's
      ignored internally by Kconfiglib, except when printing symbols.

    filename/linenr:
      The location where the menu node appears. The filename is relative to
      $srctree (or to the current directory if $srctree isn't set), except
      absolute paths passed to 'source' and Kconfig.__init__() are preserved.

    include_path:
      A tuple of (filename, linenr) tuples, giving the locations of the
      'source' statements via which the Kconfig file containing this menu node
      was included. The first element is the location of the 'source' statement
      in the top-level Kconfig file passed to Kconfig.__init__(), etc.

      Note that the Kconfig file of the menu node itself isn't included. Check
      'filename' and 'linenr' for that.

    kconfig:
      The Kconfig instance the menu node is from.
    """
    __slots__ = (
        "dep",
        "filename",
        "help",
        "include_path",
        "is_menuconfig",
        "item",
        "kconfig",
        "linenr",
        "list",
        "next",
        "parent",
        "prompt",
        "visibility",

        # Properties
        "defaults",
        "selects",
        "implies",
        "ranges",
    )

    def __init__(self):
        # Properties defined on this particular menu node. A local 'depends on'
        # only applies to these, in case a symbol is defined in multiple
        # locations.
        self.defaults = []
        self.selects = []
        self.implies = []
        self.ranges = []

    @property
    def referenced(self):
        """
        See the class documentation.
        """
        # self.dep is included to catch dependencies from a lone 'depends on'
        # when there are no properties to propagate it to
        res = expr_items(self.dep)

        if self.prompt:
            res |= expr_items(self.prompt[1])

        if self.item is MENU:
            res |= expr_items(self.visibility)

        for value, cond in self.defaults:
            res |= expr_items(value)
            res |= expr_items(cond)

        for value, cond in self.selects:
            res.add(value)
            res |= expr_items(cond)

        for value, cond in self.implies:
            res.add(value)
            res |= expr_items(cond)

        for low, high, cond in self.ranges:
            res.add(low)
            res.add(high)
            res |= expr_items(cond)

        return res

    def __repr__(self):
        """
        Returns a string with information about the menu node when it is
        evaluated on e.g. the interactive Python prompt.
        """
        fields = []

        if self.item.__class__ is Symbol:
            fields.append("menu node for symbol " + self.item.name)

        elif self.item.__class__ is Choice:
            s = "menu node for choice"
            if self.item.name is not None:
                s += " " + self.item.name
            fields.append(s)

        elif self.item is MENU:
            fields.append("menu node for menu")

        else:  # self.item is COMMENT
            fields.append("menu node for comment")

        if self.prompt:
            fields.append('prompt "{}" (visibility {})'
                          .format(self.prompt[0],
                                  TRI_TO_STR[expr_value(self.prompt[1])]))

        if self.item.__class__ is Symbol and self.is_menuconfig:
            fields.append("is menuconfig")

        fields.append("deps " + TRI_TO_STR[expr_value(self.dep)])

        if self.item is MENU:
            fields.append("'visible if' deps " +
                          TRI_TO_STR[expr_value(self.visibility)])

        if self.item.__class__ in _SYMBOL_CHOICE and self.help is not None:
            fields.append("has help")

        if self.list:
            fields.append("has child")

        if self.next:
            fields.append("has next")

        fields.append("{}:{}".format(self.filename, self.linenr))

        return "<{}>".format(", ".join(fields))

    def __str__(self):
        """
        Returns a string representation of the menu node, matching the Kconfig
        format.

        The output could (almost) be fed back into a Kconfig parser to redefine
        the object associated with the menu node. See the module documentation
        for a gotcha related to choice symbols.

        For symbols and choices with multiple menu nodes (multiple definition
        locations), properties that aren't associated with a particular menu
        node are shown on all menu nodes ('option env=...', 'optional' for
        choices, etc.).

        The returned string does not end in a newline.
        """
        return self.custom_str(standard_sc_expr_str)

    def custom_str(self, sc_expr_str_fn):
        """
        Works like MenuNode.__str__(), but allows a custom format to be used
        for all symbol/choice references. See expr_str().
        """
        return self._menu_comment_node_str(sc_expr_str_fn) \
               if self.item in _MENU_COMMENT else \
               self._sym_choice_node_str(sc_expr_str_fn)

    def _menu_comment_node_str(self, sc_expr_str_fn):
        s = '{} "{}"'.format("menu" if self.item is MENU else "comment",
                             self.prompt[0])

        if self.dep is not self.kconfig.y:
            s += "\n\tdepends on {}".format(expr_str(self.dep, sc_expr_str_fn))

        if self.item is MENU and self.visibility is not self.kconfig.y:
            s += "\n\tvisible if {}".format(expr_str(self.visibility,
                                                     sc_expr_str_fn))

        return s

    def _sym_choice_node_str(self, sc_expr_str_fn):
        lines = []

        def indent_add(s):
            lines.append("\t" + s)

        def indent_add_cond(s, cond):
            if cond is not self.kconfig.y:
                s += " if " + expr_str(cond, sc_expr_str_fn)
            indent_add(s)

        sc = self.item

        if sc.__class__ is Symbol:
            lines.append(
                ("menuconfig " if self.is_menuconfig else "config ")
                + sc.name)
        else:
            lines.append("choice " + sc.name if sc.name else "choice")

        if sc.orig_type:  # != UNKNOWN
            indent_add(TYPE_TO_STR[sc.orig_type])

        if self.prompt:
            indent_add_cond(
                'prompt "{}"'.format(escape(self.prompt[0])),
                self.prompt[1])

        if sc.__class__ is Symbol:
            if sc.is_allnoconfig_y:
                indent_add("option allnoconfig_y")

            if sc is sc.kconfig.defconfig_list:
                indent_add("option defconfig_list")

            if sc.env_var is not None:
                indent_add('option env="{}"'.format(sc.env_var))

            if sc is sc.kconfig.modules:
                indent_add("option modules")

            for low, high, cond in self.ranges:
                indent_add_cond(
                    "range {} {}".format(sc_expr_str_fn(low),
                                         sc_expr_str_fn(high)),
                    cond)

        for default, cond in self.defaults:
            indent_add_cond("default " + expr_str(default, sc_expr_str_fn),
                            cond)

        if sc.__class__ is Choice and sc.is_optional:
            indent_add("optional")

        if sc.__class__ is Symbol:
            for select, cond in self.selects:
                indent_add_cond("select " + sc_expr_str_fn(select), cond)

            for imply, cond in self.implies:
                indent_add_cond("imply " + sc_expr_str_fn(imply), cond)

        if self.dep is not sc.kconfig.y:
            indent_add("depends on " + expr_str(self.dep, sc_expr_str_fn))

        if self.help is not None:
            indent_add("help")
            for line in self.help.splitlines():
                indent_add("  " + line)

        return "\n".join(lines)

class Variable(object):
    """
    Represents a preprocessor variable/function.

    The following attributes are available:

    name:
      The name of the variable.

    value:
      The unexpanded value of the variable.

    expanded_value:
      The expanded value of the variable. For simple variables (those defined
      with :=), this will equal 'value'. Accessing this property will raise a
      KconfigError if the expansion seems to be stuck in a loop.

      Note: Accessing this field is the same as calling expanded_value_w_args()
      with no arguments. I hadn't considered function arguments when adding it.
      It is retained for backwards compatibility though.

    is_recursive:
      True if the variable is recursive (defined with =).
    """
    __slots__ = (
        "_n_expansions",
        "is_recursive",
        "kconfig",
        "name",
        "value",
    )

    @property
    def expanded_value(self):
        """
        See the class documentation.
        """
        return self.expanded_value_w_args()

    def expanded_value_w_args(self, *args):
        """
        Returns the expanded value of the variable/function. Any arguments
        passed will be substituted for $(1), $(2), etc.

        Raises a KconfigError if the expansion seems to be stuck in a loop.
        """
        return self.kconfig._fn_val((self.name,) + args)

    def __repr__(self):
        return "<variable {}, {}, value '{}'>" \
               .format(self.name,
                       "recursive" if self.is_recursive else "immediate",
                       self.value)

class KconfigError(Exception):
    "Exception raised for Kconfig-related errors"

KconfigSyntaxError = KconfigError  # Backwards compatibility

class InternalError(Exception):
    "Never raised. Kept around for backwards compatibility."

# Workaround:
#
# If 'errno' and 'strerror' are set on IOError, then __str__() always returns
# "[Errno <errno>] <strerror>", ignoring any custom message passed to the
# constructor. By defining our own subclass, we can use a custom message while
# also providing 'errno', 'strerror', and 'filename' to scripts.
class _KconfigIOError(IOError):
    def __init__(self, ioerror, msg):
        self.msg = msg
        super(_KconfigIOError, self).__init__(
            ioerror.errno, ioerror.strerror, ioerror.filename)

    def __str__(self):
        return self.msg

#
# Public functions
#

def expr_value(expr):
    """
    Evaluates the expression 'expr' to a tristate value. Returns 0 (n), 1 (m),
    or 2 (y).

    'expr' must be an already-parsed expression from a Symbol, Choice, or
    MenuNode property. To evaluate an expression represented as a string, use
    Kconfig.eval_string().

    Passing subexpressions of expressions to this function works as expected.
    """
    if expr.__class__ is not tuple:
        return expr.tri_value

    if expr[0] is AND:
        v1 = expr_value(expr[1])
        # Short-circuit the n case as an optimization (~5% faster
        # allnoconfig.py and allyesconfig.py, as of writing)
        return 0 if not v1 else min(v1, expr_value(expr[2]))

    if expr[0] is OR:
        v1 = expr_value(expr[1])
        # Short-circuit the y case as an optimization
        return 2 if v1 == 2 else max(v1, expr_value(expr[2]))

    if expr[0] is NOT:
        return 2 - expr_value(expr[1])

    # Relation
    #
    # Implements <, <=, >, >= comparisons as well. These were added to
    # kconfig in 31847b67 (kconfig: allow use of relations other than
    # (in)equality).

    rel, v1, v2 = expr

    # If both operands are strings...
    if v1.orig_type is STRING and v2.orig_type is STRING:
        # ...then compare them lexicographically
        comp = _strcmp(v1.str_value, v2.str_value)
    else:
        # Otherwise, try to compare them as numbers
        try:
            comp = _sym_to_num(v1) - _sym_to_num(v2)
        except ValueError:
            # Fall back on a lexicographic comparison if the operands don't
            # parse as numbers
            comp = _strcmp(v1.str_value, v2.str_value)

    if rel is EQUAL:      return 2*(comp == 0)
    if rel is UNEQUAL:    return 2*(comp != 0)
    if rel is LESS:       return 2*(comp < 0)
    if rel is LESS_EQUAL: return 2*(comp <= 0)
    if rel is GREATER:    return 2*(comp > 0)
    return 2*(comp >= 0)  # rel is GREATER_EQUAL

def standard_sc_expr_str(sc):
    """
    Standard symbol/choice printing function. Uses plain Kconfig syntax, and
    displays choices as <choice> (or <choice NAME>, for named choices).

    See expr_str().
    """
    if sc.__class__ is Symbol:
        return '"{}"'.format(escape(sc.name)) if sc.is_constant else sc.name

    # Choice
    return "<choice {}>".format(sc.name) if sc.name else "<choice>"

def expr_str(expr, sc_expr_str_fn=standard_sc_expr_str):
    """
    Returns the string representation of the expression 'expr', as in a Kconfig
    file.

    Passing subexpressions of expressions to this function works as expected.

    sc_expr_str_fn (default: standard_sc_expr_str):
      This function is called for every symbol/choice (hence "sc") appearing in
      the expression, with the symbol/choice as the argument. It is expected to
      return a string to be used for the symbol/choice.

      This can be used e.g. to turn symbols/choices into links when generating
      documentation, or for printing the value of each symbol/choice after it.

      Note that quoted values are represented as constants symbols
      (Symbol.is_constant == True).
    """
    if expr.__class__ is not tuple:
        return sc_expr_str_fn(expr)

    if expr[0] is AND:
        return "{} && {}".format(_parenthesize(expr[1], OR, sc_expr_str_fn),
                                 _parenthesize(expr[2], OR, sc_expr_str_fn))

    if expr[0] is OR:
        # This turns A && B || C && D into "(A && B) || (C && D)", which is
        # redundant, but more readable
        return "{} || {}".format(_parenthesize(expr[1], AND, sc_expr_str_fn),
                                 _parenthesize(expr[2], AND, sc_expr_str_fn))

    if expr[0] is NOT:
        if expr[1].__class__ is tuple:
            return "!({})".format(expr_str(expr[1], sc_expr_str_fn))
        return "!" + sc_expr_str_fn(expr[1])  # Symbol

    # Relation
    #
    # Relation operands are always symbols (quoted strings are constant
    # symbols)
    return "{} {} {}".format(sc_expr_str_fn(expr[1]), _REL_TO_STR[expr[0]],
                             sc_expr_str_fn(expr[2]))

def expr_items(expr):
    """
    Returns a set() of all items (symbols and choices) that appear in the
    expression 'expr'.
    """

    res = set()

    def rec(subexpr):
        if subexpr.__class__ is tuple:
            # AND, OR, NOT, or relation

            rec(subexpr[1])

            # NOTs only have a single operand
            if subexpr[0] is not NOT:
                rec(subexpr[2])

        else:
            # Symbol or choice
            res.add(subexpr)

    rec(expr)
    return res

def split_expr(expr, op):
    """
    Returns a list containing the top-level AND or OR operands in the
    expression 'expr', in the same (left-to-right) order as they appear in
    the expression.

    This can be handy e.g. for splitting (weak) reverse dependencies
    from 'select' and 'imply' into individual selects/implies.

    op:
      Either AND to get AND operands, or OR to get OR operands.

      (Having this as an operand might be more future-safe than having two
      hardcoded functions.)


    Pseudo-code examples:

      split_expr( A                    , OR  )  ->  [A]
      split_expr( A && B               , OR  )  ->  [A && B]
      split_expr( A || B               , OR  )  ->  [A, B]
      split_expr( A || B               , AND )  ->  [A || B]
      split_expr( A || B || (C && D)   , OR  )  ->  [A, B, C && D]

      # Second || is not at the top level
      split_expr( A || (B && (C || D)) , OR )  ->  [A, B && (C || D)]

      # Parentheses don't matter as long as we stay at the top level (don't
      # encounter any non-'op' nodes)
      split_expr( (A || B) || C        , OR )  ->  [A, B, C]
      split_expr( A || (B || C)        , OR )  ->  [A, B, C]
    """
    res = []

    def rec(subexpr):
        if subexpr.__class__ is tuple and subexpr[0] is op:
            rec(subexpr[1])
            rec(subexpr[2])
        else:
            res.append(subexpr)

    rec(expr)
    return res

def escape(s):
    r"""
    Escapes the string 's' in the same fashion as is done for display in
    Kconfig format and when writing strings to a .config file. " and \ are
    replaced by \" and \\, respectively.
    """
    # \ must be escaped before " to avoid double escaping
    return s.replace("\\", r"\\").replace('"', r'\"')

# unescape() helper
_unescape_sub = re.compile(r"\\(.)").sub

def unescape(s):
    r"""
    Unescapes the string 's'. \ followed by any character is replaced with just
    that character. Used internally when reading .config files.
    """
    return _unescape_sub(r"\1", s)

def standard_kconfig():
    """
    Helper for tools. Loads the top-level Kconfig specified as the first
    command-line argument, or "Kconfig" if there are no command-line arguments.
    Returns the Kconfig instance.

    Exits with sys.exit() (which raises a SystemExit exception) and prints a
    usage note to stderr if more than one command-line argument is passed.
    """
    if len(sys.argv) > 2:
        sys.exit("usage: {} [Kconfig]".format(sys.argv[0]))

    # Only show backtraces for unexpected exceptions
    try:
        return Kconfig("Kconfig" if len(sys.argv) < 2 else sys.argv[1])
    except (IOError, KconfigError) as e:
        # Some long exception messages have extra newlines for better
        # formatting when reported as an unhandled exception. Strip them here.
        sys.exit(str(e).strip())

def standard_config_filename():
    """
    Helper for tools. Returns the value of KCONFIG_CONFIG (which specifies the
    .config file to load/save) if it is set, and ".config" otherwise.

    Note: Calling load_config() with filename=None might give the behavior you
    want, without having to use this function.
    """
    return os.environ.get("KCONFIG_CONFIG", ".config")

def load_allconfig(kconf, filename):
    """
    Helper for all*config. Loads (merges) the configuration file specified by
    KCONFIG_ALLCONFIG, if any. See Documentation/kbuild/kconfig.txt in the
    Linux kernel.

    Disables warnings for duplicated assignments within configuration files for
    the duration of the call (disable_override_warnings() +
    disable_redun_warnings()), and enables them at the end. The
    KCONFIG_ALLCONFIG configuration file is expected to override symbols.

    Exits with sys.exit() (which raises a SystemExit exception) and prints an
    error to stderr if KCONFIG_ALLCONFIG is set but the configuration file
    can't be opened.

    kconf:
      Kconfig instance to load the configuration in.

    filename:
      Command-specific configuration filename - "allyes.config",
      "allno.config", etc.
    """
    def std_msg(e):
        # "Upcasts" a _KconfigIOError to an IOError, removing the custom
        # __str__() message. The standard message is better here.
        return IOError(e.errno, e.strerror, e.filename)

    kconf.disable_override_warnings()
    kconf.disable_redun_warnings()

    allconfig = os.environ.get("KCONFIG_ALLCONFIG")
    if allconfig is not None:
        if allconfig in ("", "1"):
            try:
                kconf.load_config(filename, False)
            except IOError as e1:
                try:
                    kconf.load_config("all.config", False)
                except IOError as e2:
                    sys.exit("error: KCONFIG_ALLCONFIG is set, but neither {} "
                             "nor all.config could be opened: {}, {}"
                             .format(filename, std_msg(e1), std_msg(e2)))
        else:
            try:
                kconf.load_config(allconfig, False)
            except IOError as e:
                sys.exit("error: KCONFIG_ALLCONFIG is set to '{}', which "
                         "could not be opened: {}"
                         .format(allconfig, std_msg(e)))

    # API wart: It would be nice if there was a way to query and/or push/pop
    # warning settings
    kconf.enable_override_warnings()
    kconf.enable_redun_warnings()

#
# Internal functions
#

def _visibility(sc):
    # Symbols and Choices have a "visibility" that acts as an upper bound on
    # the values a user can set for them, corresponding to the visibility in
    # e.g. 'make menuconfig'. This function calculates the visibility for the
    # Symbol or Choice 'sc' -- the logic is nearly identical.

    vis = 0

    for node in sc.nodes:
        if node.prompt:
            vis = max(vis, expr_value(node.prompt[1]))

    if sc.__class__ is Symbol and sc.choice:
        if sc.choice.orig_type is TRISTATE and \
           sc.orig_type is not TRISTATE and sc.choice.tri_value != 2:
            # Non-tristate choice symbols are only visible in y mode
            return 0

        if sc.orig_type is TRISTATE and vis == 1 and sc.choice.tri_value == 2:
            # Choice symbols with m visibility are not visible in y mode
            return 0

    # Promote m to y if we're dealing with a non-tristate (possibly due to
    # modules being disabled)
    if vis == 1 and sc.type is not TRISTATE:
        return 2

    return vis

def _make_depend_on(sc, expr):
    # Adds 'sc' (symbol or choice) as a "dependee" to all symbols in 'expr'.
    # Constant symbols in 'expr' are skipped as they can never change value
    # anyway.

    if expr.__class__ is tuple:
        # AND, OR, NOT, or relation

        _make_depend_on(sc, expr[1])

        # NOTs only have a single operand
        if expr[0] is not NOT:
            _make_depend_on(sc, expr[2])

    elif not expr.is_constant:
        # Non-constant symbol, or choice
        expr._dependents.add(sc)

def _parenthesize(expr, type_, sc_expr_str_fn):
    # expr_str() helper. Adds parentheses around expressions of type 'type_'.

    if expr.__class__ is tuple and expr[0] is type_:
        return "({})".format(expr_str(expr, sc_expr_str_fn))
    return expr_str(expr, sc_expr_str_fn)

def _ordered_unique(lst):
    # Returns 'lst' with any duplicates removed, preserving order. This hacky
    # version seems to be a common idiom. It relies on short-circuit evaluation
    # and set.add() returning None, which is falsy.

    seen = set()
    seen_add = seen.add
    return [x for x in lst if x not in seen and not seen_add(x)]

def _is_base_n(s, n):
    try:
        int(s, n)
        return True
    except ValueError:
        return False

def _strcmp(s1, s2):
    # strcmp()-alike that returns -1, 0, or 1

    return (s1 > s2) - (s1 < s2)

def _sym_to_num(sym):
    # expr_value() helper for converting a symbol to a number. Raises
    # ValueError for symbols that can't be converted.

    # For BOOL and TRISTATE, n/m/y count as 0/1/2. This mirrors 9059a3493ef
    # ("kconfig: fix relational operators for bool and tristate symbols") in
    # the C implementation.
    return sym.tri_value if sym.orig_type in _BOOL_TRISTATE else \
           int(sym.str_value, _TYPE_TO_BASE[sym.orig_type])

def _touch_dep_file(sym_name):
    # If sym_name is MY_SYM_NAME, touches my/sym/name.h. See the sync_deps()
    # docstring.

    sym_path = sym_name.lower().replace("_", os.sep) + ".h"
    sym_path_dir = os.path.dirname(sym_path)
    if sym_path_dir and not os.path.exists(sym_path_dir):
        os.makedirs(sym_path_dir, 0o755)

    # A kind of truncating touch, mirroring the C tools
    os.close(os.open(
        sym_path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o644))

def _save_old(path):
    # See write_config()

    dirname, basename = os.path.split(path)
    backup = os.path.join(dirname,
                          basename + ".old" if basename.startswith(".") else
                              "." + basename + ".old")

    # os.replace() would be nice here, but it's Python 3 (3.3+) only
    try:
        # Use copyfile() if 'path' is a symlink. The intention is probably to
        # overwrite the target in that case.
        if os.name == "posix" and not os.path.islink(path):
            # Will remove .<filename>.old if it already exists on POSIX
            # systems
            os.rename(path, backup)
        else:
            import shutil
            shutil.copyfile(path, backup)
    except:
        # Ignore errors from 'filename' missing as well as other errors. The
        # backup file is more of a nice-to-have, and not worth erroring out
        # over e.g. if .<filename>.old happens to be a directory.
        pass

def _decoding_error(e, filename, macro_linenr=None):
    # Gives the filename and context for UnicodeDecodeError's, which are a pain
    # to debug otherwise. 'e' is the UnicodeDecodeError object.
    #
    # If the decoding error is for the output of a $(shell,...) command,
    # macro_linenr holds the line number where it was run (the exact line
    # number isn't available for decoding errors in files).

    raise KconfigError(
        "\n"
        "Malformed {} in {}\n"
        "Context: {}\n"
        "Problematic data: {}\n"
        "Reason: {}".format(
            e.encoding,
            "'{}'".format(filename) if macro_linenr is None else
                "output from macro at {}:{}".format(filename, macro_linenr),
            e.object[max(e.start - 40, 0):e.end + 40],
            e.object[e.start:e.end],
            e.reason))

def _name_and_loc(sc):
    # Helper for giving the symbol/choice name and location(s) in e.g. warnings

    # Reuse the expression format. That way choices show up as
    # '<choice (name, if any)>'
    name = standard_sc_expr_str(sc)

    if not sc.nodes:
        return name + " (undefined)"

    return "{} (defined at {})".format(
        name,
        ", ".join("{}:{}".format(node.filename, node.linenr)
                  for node in sc.nodes))


# Menu manipulation

def _expr_depends_on(expr, sym):
    # Reimplementation of expr_depends_symbol() from mconf.c. Used to determine
    # if a submenu should be implicitly created. This also influences which
    # items inside choice statements are considered choice items.

    if expr.__class__ is not tuple:
        return expr is sym

    if expr[0] in _EQUAL_UNEQUAL:
        # Check for one of the following:
        # sym = m/y, m/y = sym, sym != n, n != sym

        left, right = expr[1:]

        if right is sym:
            left, right = right, left
        elif left is not sym:
            return False

        return (expr[0] is EQUAL and right is sym.kconfig.m or
                                     right is sym.kconfig.y) or \
               (expr[0] is UNEQUAL and right is sym.kconfig.n)

    return expr[0] is AND and \
           (_expr_depends_on(expr[1], sym) or
            _expr_depends_on(expr[2], sym))

def _auto_menu_dep(node1, node2):
    # Returns True if node2 has an "automatic menu dependency" on node1. If
    # node2 has a prompt, we check its condition. Otherwise, we look directly
    # at node2.dep.

    # If node2 has no prompt, use its menu node dependencies instead
    return _expr_depends_on(node2.prompt[1] if node2.prompt else node2.dep,
                            node1.item)

def _flatten(node):
    # "Flattens" menu nodes without prompts (e.g. 'if' nodes and non-visible
    # symbols with children from automatic menu creation) so that their
    # children appear after them instead. This gives a clean menu structure
    # with no unexpected "jumps" in the indentation.
    #
    # Do not flatten promptless choices (which can appear "legitimitely" if a
    # named choice is defined in multiple locations to add on symbols). It
    # looks confusing, and the menuconfig already shows all choice symbols if
    # you enter the choice at some location with a prompt.

    while node:
        if node.list and not node.prompt and \
           node.item.__class__ is not Choice:

            last_node = node.list
            while 1:
                last_node.parent = node.parent
                if not last_node.next:
                    break
                last_node = last_node.next

            last_node.next = node.next
            node.next = node.list
            node.list = None

        node = node.next

def _remove_ifs(node):
    # Removes 'if' nodes (which can be recognized by MenuNode.item being None),
    # which are assumed to already have been flattened. The C implementation
    # doesn't bother to do this, but we expose the menu tree directly, and it
    # makes it nicer to work with.

    cur = node.list
    while cur and not cur.item:
        cur = cur.next

    node.list = cur

    while cur:
        next = cur.next
        while next and not next.item:
            next = next.next

        # Equivalent to
        #
        #   cur.next = next
        #   cur = next
        #
        # due to tricky Python semantics. The order matters.
        cur.next = cur = next

def _finalize_choice(node):
    # Finalizes a choice, marking each symbol whose menu node has the choice as
    # the parent as a choice symbol, and automatically determining types if not
    # specified.

    choice = node.item

    cur = node.list
    while cur:
        if cur.item.__class__ is Symbol:
            cur.item.choice = choice
            choice.syms.append(cur.item)
        cur = cur.next

    # If no type is specified for the choice, its type is that of
    # the first choice item with a specified type
    if not choice.orig_type:
        for item in choice.syms:
            if item.orig_type:
                choice.orig_type = item.orig_type
                break

    # Each choice item of UNKNOWN type gets the type of the choice
    for sym in choice.syms:
        if not sym.orig_type:
            sym.orig_type = choice.orig_type

def _check_dep_loop_sym(sym, ignore_choice):
    # Detects dependency loops using depth-first search on the dependency graph
    # (which is calculated earlier in Kconfig._build_dep()).
    #
    # Algorithm:
    #
    #  1. Symbols/choices start out with _visited = 0, meaning unvisited.
    #
    #  2. When a symbol/choice is first visited, _visited is set to 1, meaning
    #     "visited, potentially part of a dependency loop". The recursive
    #     search then continues from the symbol/choice.
    #
    #  3. If we run into a symbol/choice X with _visited already set to 1,
    #     there's a dependency loop. The loop is found on the call stack by
    #     recording symbols while returning ("on the way back") until X is seen
    #     again.
    #
    #  4. Once a symbol/choice and all its dependencies (or dependents in this
    #     case) have been checked recursively without detecting any loops, its
    #     _visited is set to 2, meaning "visited, not part of a dependency
    #     loop".
    #
    #     This saves work if we run into the symbol/choice again in later calls
    #     to _check_dep_loop_sym(). We just return immediately.
    #
    # Choices complicate things, as every choice symbol depends on every other
    # choice symbol in a sense. When a choice is "entered" via a choice symbol
    # X, we visit all choice symbols from the choice except X, and prevent
    # immediately revisiting the choice with a flag (ignore_choice).
    #
    # Maybe there's a better way to handle this (different flags or the
    # like...)

    if not sym._visited:
        # sym._visited == 0, unvisited

        sym._visited = 1

        for dep in sym._dependents:
            # Choices show up in Symbol._dependents when the choice has the
            # symbol in a 'prompt' or 'default' condition (e.g.
            # 'default ... if SYM').
            #
            # Since we aren't entering the choice via a choice symbol, all
            # choice symbols need to be checked, hence the None.
            loop = _check_dep_loop_choice(dep, None) \
                   if dep.__class__ is Choice \
                   else _check_dep_loop_sym(dep, False)

            if loop:
                # Dependency loop found
                return _found_dep_loop(loop, sym)

        if sym.choice and not ignore_choice:
            loop = _check_dep_loop_choice(sym.choice, sym)
            if loop:
                # Dependency loop found
                return _found_dep_loop(loop, sym)

        # The symbol is not part of a dependency loop
        sym._visited = 2

        # No dependency loop found
        return None

    if sym._visited == 2:
        # The symbol was checked earlier and is already known to not be part of
        # a dependency loop
        return None

    # sym._visited == 1, found a dependency loop. Return the symbol as the
    # first element in it.
    return (sym,)

def _check_dep_loop_choice(choice, skip):
    if not choice._visited:
        # choice._visited == 0, unvisited

        choice._visited = 1

        # Check for loops involving choice symbols. If we came here via a
        # choice symbol, skip that one, as we'd get a false positive
        # '<sym FOO> -> <choice> -> <sym FOO>' loop otherwise.
        for sym in choice.syms:
            if sym is not skip:
                # Prevent the choice from being immediately re-entered via the
                # "is a choice symbol" path by passing True
                loop = _check_dep_loop_sym(sym, True)
                if loop:
                    # Dependency loop found
                    return _found_dep_loop(loop, choice)

        # The choice is not part of a dependency loop
        choice._visited = 2

        # No dependency loop found
        return None

    if choice._visited == 2:
        # The choice was checked earlier and is already known to not be part of
        # a dependency loop
        return None

    # choice._visited == 1, found a dependency loop. Return the choice as the
    # first element in it.
    return (choice,)

def _found_dep_loop(loop, cur):
    # Called "on the way back" when we know we have a loop

    # Is the symbol/choice 'cur' where the loop started?
    if cur is not loop[0]:
        # Nope, it's just a part of the loop
        return loop + (cur,)

    # Yep, we have the entire loop. Throw an exception that shows it.

    msg = "\nDependency loop\n" \
            "===============\n\n"

    for item in loop:
        if item is not loop[0]:
            msg += "...depends on "
            if item.__class__ is Symbol and item.choice:
                msg += "the choice symbol "

        msg += "{}, with definition...\n\n{}\n\n" \
               .format(_name_and_loc(item), item)

        # Small wart: Since we reuse the already calculated
        # Symbol/Choice._dependents sets for recursive dependency detection, we
        # lose information on whether a dependency came from a 'select'/'imply'
        # condition or e.g. a 'depends on'.
        #
        # This might cause selecting symbols to "disappear". For example,
        # a symbol B having 'select A if C' gives a direct dependency from A to
        # C, since it corresponds to a reverse dependency of B && C.
        #
        # Always print reverse dependencies for symbols that have them to make
        # sure information isn't lost. I wonder if there's some neat way to
        # improve this.

        if item.__class__ is Symbol:
            if item.rev_dep is not item.kconfig.n:
                msg += "(select-related dependencies: {})\n\n" \
                       .format(expr_str(item.rev_dep))

            if item.weak_rev_dep is not item.kconfig.n:
                msg += "(imply-related dependencies: {})\n\n" \
                       .format(expr_str(item.rev_dep))

    msg += "...depends again on {}".format(_name_and_loc(loop[0]))

    raise KconfigError(msg)


# Predefined preprocessor functions

def _filename_fn(kconf, _):
    return kconf._filename

def _lineno_fn(kconf, _):
    return str(kconf._linenr)

def _info_fn(kconf, _, msg):
    print("{}:{}: {}".format(kconf._filename, kconf._linenr, msg))

    return ""

def _warning_if_fn(kconf, _, cond, msg):
    if cond == "y":
        kconf._warn(msg, kconf._filename, kconf._linenr)

    return ""

def _error_if_fn(kconf, _, cond, msg):
    if cond == "y":
        raise KconfigError("{}:{}: {}".format(
            kconf._filename, kconf._linenr, msg))

    return ""

def _shell_fn(kconf, _, command):
    stdout, stderr = subprocess.Popen(
        command, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE
    ).communicate()

    if not _IS_PY2:
        try:
            stdout = stdout.decode(kconf._encoding)
            stderr = stderr.decode(kconf._encoding)
        except UnicodeDecodeError as e:
            _decoding_error(e, kconf._filename, kconf._linenr)

    if stderr:
        kconf._warn("'{}' wrote to stderr: {}".format(
                        command, "\n".join(stderr.splitlines())),
                    kconf._filename, kconf._linenr)

    # Universal newlines with splitlines() (to prevent e.g. stray \r's in
    # command output on Windows), trailing newline removal, and
    # newline-to-space conversion.
    #
    # On Python 3 versions before 3.6, it's not possible to specify the
    # encoding when passing universal_newlines=True to Popen() (the 'encoding'
    # parameter was added in 3.6), so we do this manual version instead.
    return "\n".join(stdout.splitlines()).rstrip("\n").replace("\n", " ")

#
# Public global constants
#

# Integers representing symbol types. UNKNOWN is 0 (falsy) to simplify some
# checks, though client code shouldn't rely on it (it was non-zero in older
# versions).
(
    UNKNOWN,
    BOOL,
    TRISTATE,
    STRING,
    INT,
    HEX,
) = range(6)

# Converts a symbol/choice type to a string
TYPE_TO_STR = {
    UNKNOWN:  "unknown",
    BOOL:     "bool",
    TRISTATE: "tristate",
    STRING:   "string",
    INT:      "int",
    HEX:      "hex",
}

TRI_TO_STR = {
    0: "n",
    1: "m",
    2: "y",
}

STR_TO_TRI = {
    "n": 0,
    "m": 1,
    "y": 2,
}

#
# Internal global constants (plus public expression type
# constants)
#

# Note:
#
# The token and type constants below are safe to test with 'is', which is a bit
# faster (~30% faster in a microbenchmark with Python 3 on my machine, and a
# few % faster for total parsing time), even without assuming Python's small
# integer optimization (which caches small integer objects). The constants end
# up pointing to unique integer objects, and since we consistently refer to
# them via the names below, we always get the same object.
#
# Client code would also need to use the names below, because the integer
# values can change e.g. when tokens get added. Client code would usually test
# with == too, which would be safe even in super obscure cases involving e.g.
# pickling (where 'is' would be a bad idea anyway) and no small-integer
# optimization.

# Are we running on Python 2?
_IS_PY2 = sys.version_info[0] < 3

# Tokens, with values 1, 2, ... . Avoiding 0 simplifies some checks by making
# all tokens except empty strings truthy.
(
    _T_ALLNOCONFIG_Y,
    _T_AND,
    _T_BOOL,
    _T_CHOICE,
    _T_CLOSE_PAREN,
    _T_COMMENT,
    _T_CONFIG,
    _T_DEFAULT,
    _T_DEFCONFIG_LIST,
    _T_DEF_BOOL,
    _T_DEF_HEX,
    _T_DEF_INT,
    _T_DEF_STRING,
    _T_DEF_TRISTATE,
    _T_DEPENDS,
    _T_ENDCHOICE,
    _T_ENDIF,
    _T_ENDMENU,
    _T_ENV,
    _T_EQUAL,
    _T_GREATER,
    _T_GREATER_EQUAL,
    _T_HELP,
    _T_HEX,
    _T_IF,
    _T_IMPLY,
    _T_INT,
    _T_LESS,
    _T_LESS_EQUAL,
    _T_MAINMENU,
    _T_MENU,
    _T_MENUCONFIG,
    _T_MODULES,
    _T_NOT,
    _T_ON,
    _T_OPEN_PAREN,
    _T_OPTION,
    _T_OPTIONAL,
    _T_OR,
    _T_ORSOURCE,
    _T_OSOURCE,
    _T_PROMPT,
    _T_RANGE,
    _T_RSOURCE,
    _T_SELECT,
    _T_SOURCE,
    _T_STRING,
    _T_TRISTATE,
    _T_UNEQUAL,
    _T_VISIBLE,
) = range(1, 51)

# Public integers representing expression types and menu and comment menu
# nodes
#
# Having these match the value of the corresponding tokens removes the need
# for conversion

AND           = _T_AND
OR            = _T_OR
NOT           = _T_NOT
EQUAL         = _T_EQUAL
UNEQUAL       = _T_UNEQUAL
LESS          = _T_LESS
LESS_EQUAL    = _T_LESS_EQUAL
GREATER       = _T_GREATER
GREATER_EQUAL = _T_GREATER_EQUAL

MENU          = _T_MENU
COMMENT       = _T_COMMENT

# Keyword to token map, with the get() method assigned directly as a small
# optimization
_get_keyword = {
    "---help---":     _T_HELP,
    "allnoconfig_y":  _T_ALLNOCONFIG_Y,
    "bool":           _T_BOOL,
    "boolean":        _T_BOOL,
    "choice":         _T_CHOICE,
    "comment":        _T_COMMENT,
    "config":         _T_CONFIG,
    "def_bool":       _T_DEF_BOOL,
    "def_hex":        _T_DEF_HEX,
    "def_int":        _T_DEF_INT,
    "def_string":     _T_DEF_STRING,
    "def_tristate":   _T_DEF_TRISTATE,
    "default":        _T_DEFAULT,
    "defconfig_list": _T_DEFCONFIG_LIST,
    "depends":        _T_DEPENDS,
    "endchoice":      _T_ENDCHOICE,
    "endif":          _T_ENDIF,
    "endmenu":        _T_ENDMENU,
    "env":            _T_ENV,
    "grsource":       _T_ORSOURCE,  # Backwards compatibility
    "gsource":        _T_OSOURCE,   # Backwards compatibility
    "help":           _T_HELP,
    "hex":            _T_HEX,
    "if":             _T_IF,
    "imply":          _T_IMPLY,
    "int":            _T_INT,
    "mainmenu":       _T_MAINMENU,
    "menu":           _T_MENU,
    "menuconfig":     _T_MENUCONFIG,
    "modules":        _T_MODULES,
    "on":             _T_ON,
    "option":         _T_OPTION,
    "optional":       _T_OPTIONAL,
    "orsource":       _T_ORSOURCE,
    "osource":        _T_OSOURCE,
    "prompt":         _T_PROMPT,
    "range":          _T_RANGE,
    "rsource":        _T_RSOURCE,
    "select":         _T_SELECT,
    "source":         _T_SOURCE,
    "string":         _T_STRING,
    "tristate":       _T_TRISTATE,
    "visible":        _T_VISIBLE,
}.get

# Tokens after which strings are expected. This is used to tell strings from
# constant symbol references during tokenization, both of which are enclosed in
# quotes.
#
# Identifier-like lexemes ("missing quotes") are also treated as strings after
# these tokens. _T_CHOICE is included to avoid symbols being registered for
# named choices.
_STRING_LEX = frozenset((
    _T_BOOL,
    _T_CHOICE,
    _T_COMMENT,
    _T_HEX,
    _T_INT,
    _T_MAINMENU,
    _T_MENU,
    _T_ORSOURCE,
    _T_OSOURCE,
    _T_PROMPT,
    _T_RSOURCE,
    _T_SOURCE,
    _T_STRING,
    _T_TRISTATE,
))

# Various sets, for quick membership tests. This gives us a single global
# lookup and avoids creating temporary dicts/tuples.

_TYPE_TOKENS = frozenset((
    _T_BOOL,
    _T_TRISTATE,
    _T_INT,
    _T_HEX,
    _T_STRING,
))

_SOURCE_TOKENS = frozenset((
    _T_SOURCE,
    _T_RSOURCE,
    _T_OSOURCE,
    _T_ORSOURCE,
))

_REL_SOURCE_TOKENS = frozenset((
    _T_RSOURCE,
    _T_ORSOURCE,
))

# Obligatory (non-optional) sources
_OBL_SOURCE_TOKENS = frozenset((
    _T_SOURCE,
    _T_RSOURCE,
))

_DEF_TYPE_TOKENS = frozenset((
    _T_DEF_BOOL,
    _T_DEF_TRISTATE,
    _T_DEF_INT,
    _T_DEF_HEX,
    _T_DEF_STRING,
))

_BOOL_TRISTATE = frozenset((
    BOOL,
    TRISTATE,
))

_BOOL_TRISTATE_UNKNOWN = frozenset((
    BOOL,
    TRISTATE,
    UNKNOWN,
))

_INT_HEX = frozenset((
    INT,
    HEX,
))

_STRING_INT_HEX = frozenset((
    STRING,
    INT,
    HEX,
))

_SYMBOL_CHOICE = frozenset((
    Symbol,
    Choice,
))

_MENU_COMMENT = frozenset((
    MENU,
    COMMENT,
))

_EQUAL_UNEQUAL = frozenset((
    EQUAL,
    UNEQUAL,
))

# Helper functions for getting compiled regular expressions, with the needed
# matching function returned directly as a small optimization.
#
# Use ASCII regex matching on Python 3. It's already the default on Python 2.

def _re_match(regex):
    return re.compile(regex, 0 if _IS_PY2 else re.ASCII).match

def _re_search(regex):
    return re.compile(regex, 0 if _IS_PY2 else re.ASCII).search


# Various regular expressions used during parsing

# The initial token on a line. Also eats leading and trailing whitespace, so
# that we can jump straight to the next token (or to the end of the line if
# there is only one token).
#
# This regex will also fail to match for empty lines and comment lines.
#
# '$' is included to detect preprocessor variable assignments with macro
# expansions in the left-hand side.
_command_match = _re_match(r"\s*([$A-Za-z0-9_-]+)\s*")

# An identifier/keyword after the first token. Also eats trailing whitespace.
# '$' is included to detect identifiers containing macro expansions.
_id_keyword_match = _re_match(r"([$A-Za-z0-9_/.-]+)\s*")

# A fragment in the left-hand side of a preprocessor variable assignment. These
# are the portions between macro expansions ($(foo)). Macros are supported in
# the LHS (variable name).
_assignment_lhs_fragment_match = _re_match("[A-Za-z0-9_-]*")

# The assignment operator and value (right-hand side) in a preprocessor
# variable assignment
_assignment_rhs_match = _re_match(r"\s*(=|:=|\+=)\s*(.*)")

# Special characters/strings while expanding a macro (')', ',', and '$(')
_macro_special_search = _re_search(r"\)|,|\$\(")

# Special characters/strings while expanding a string (quotes, '\', and '$(')
_string_special_search = _re_search(r'"|\'|\\|\$\(')

# Special characters/strings while expanding a symbol name. Also includes
# end-of-line, in case the macro is the last thing on the line.
_name_special_search = _re_search(r'[^$A-Za-z0-9_/.-]|\$\(|$')

# A valid right-hand side for an assignment to a string symbol in a .config
# file, including escaped characters. Extracts the contents.
_conf_string_match = _re_match(r'"((?:[^\\"]|\\.)*)"')


# Token to type mapping
_TOKEN_TO_TYPE = {
    _T_BOOL:         BOOL,
    _T_DEF_BOOL:     BOOL,
    _T_DEF_HEX:      HEX,
    _T_DEF_INT:      INT,
    _T_DEF_STRING:   STRING,
    _T_DEF_TRISTATE: TRISTATE,
    _T_HEX:          HEX,
    _T_INT:          INT,
    _T_STRING:       STRING,
    _T_TRISTATE:     TRISTATE,
}

# Constant representing that there's no cached choice selection. This is
# distinct from a cached None (no selection). We create a unique object (any
# will do) for it so we can test with 'is'.
_NO_CACHED_SELECTION = object()

# Used in comparisons. 0 means the base is inferred from the format of the
# string.
_TYPE_TO_BASE = {
    HEX:      16,
    INT:      10,
    STRING:   0,
    UNKNOWN:  0,
}

# Note: These constants deliberately equal the corresponding tokens (_T_EQUAL,
# _T_UNEQUAL, etc.), which removes the need for conversion
_RELATIONS = frozenset((
    EQUAL,
    UNEQUAL,
    LESS,
    LESS_EQUAL,
    GREATER,
    GREATER_EQUAL,
))

_REL_TO_STR = {
    EQUAL:         "=",
    UNEQUAL:       "!=",
    LESS:          "<",
    LESS_EQUAL:    "<=",
    GREATER:       ">",
    GREATER_EQUAL: ">=",
}