kconfiglib.py

# Copyright (c) 2011-2017, Ulf Magnusson
# Modifications (c) 2018 Espressif Systems
# SPDX-License-Identifier: ISC
#
# ******* IMPORTANT **********
#
# This is kconfiglib 2.1.0 with some modifications to match the behaviour
# of the ESP-IDF kconfig:
#
# - 'source' nows uses wordexp(3) behaviour to allow source-ing multiple
#   files at once, and to expand environment variables directly in the source
#   command (without them having to be set as properties in the Kconfig file)
#
# - Added walk_menu() function and refactored to use this internally.
#
# - BOOL & TRISTATE items are allowed to have blank values in .config
#   (equivalent to n, this is backwards compatibility with old IDF conf.c)
#
"""
Overview
========

Kconfiglib is a Python 2/3 library for scripting and extracting information
from Kconfig configuration systems. It can be used for the following, among
other things:

 - Programmatically get and set symbol values

   allnoconfig.py and allyesconfig.py examples are provided, automatically
   verified to produce identical output to the standard 'make allnoconfig' and
   'make allyesconfig'.

 - Read and write .config files

   The generated .config files are character-for-character identical to what
   the C implementation would generate (except for the header comment). The
   test suite relies on this, as it compares the generated files.

 - Inspect symbols

   Printing a symbol gives output which could be fed back into a Kconfig parser
   to redefine it***. The printing function (__str__()) is implemented with
   public APIs, meaning you can fetch just whatever information you need as
   well.

   A helpful __repr__() is implemented on all objects too, also implemented
   with public APIs.

   ***Choice symbols get their parent choice as a dependency, which shows up as
   e.g. 'prompt "choice symbol" if <choice>' when printing the symbol. This
   could easily be worked around if 100% reparsable output is needed.

 - Inspect expressions

   Expressions use a simple tuple-based format that can be processed manually
   if needed. Expression printing and evaluation functions are provided,
   implemented with public APIs.

 - Inspect the menu tree

   The underlying menu tree is exposed, including submenus created implicitly
   from symbols depending on preceding symbols. This can be used e.g. to
   implement menuconfig-like functionality. See the menuconfig.py example.


Here are some other features:

 - Single-file implementation

   The entire library is contained in this file.

 - Runs unmodified under both Python 2 and Python 3

   The code mostly uses basic Python features and has no third-party
   dependencies. The most advanced things used are probably @property and
   __slots__.

 - Robust and highly compatible with the standard Kconfig C tools

   The test suite automatically compares output from Kconfiglib and the C tools
   by diffing the generated .config files for the real kernel Kconfig and
   defconfig files, for all ARCHes.

   This currently involves comparing the output for 36 ARCHes and 498 defconfig
   files (or over 18000 ARCH/defconfig combinations in "obsessive" test suite
   mode). All tests are expected to pass.

 - Not horribly slow despite being a pure Python implementation

   The allyesconfig.py example currently runs in about 1.6 seconds on a Core i7
   2600K (with a warm file cache), where half a second is overhead from 'make
   scriptconfig' (see below).

   For long-running jobs, PyPy gives a big performance boost. CPython is faster
   for short-running jobs as PyPy needs some time to warm up.

 - Internals that (mostly) mirror the C implementation

   While being simpler to understand.


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. Apply it 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.

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.

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

The Makefile patch adds the following targets:


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 MENU and 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.


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

The make targets are only needed for a trivial reason: The Kbuild makefiles
export environment variables which are referenced inside the Kconfig files (via
'option env="ENV_VARIABLE"').

In practice, the only variables referenced (as of writing, and for many years)
are ARCH, SRCARCH, and KERNELVERSION. To run Kconfiglib without the Makefile
patch, do this:

  $ ARCH=x86 SRCARCH=x86 KERNELVERSION=`make kernelversion` python
  >>> 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. Kconfiglib will print a warning if an unset
environment variable is referenced inside the Kconfig files.


Gotcha
******

It's important to set $SRCARCH even if you don't care about values and only
want to extract information from Kconfig files, because the top-level Makefile
does this (as of writing):

  source "arch/$SRCARCH/Kconfig"

If $SRCARCH is not set, this expands to "arch//Kconfig", and arch/Kconfig
happens to be an existing file, giving something that appears to work but is
actually a truncated configuration. The available symbols will differ depending
on the arch as well.


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.

Symbols without prompts are never visible (setting a user value on them is
pointless). For symbols with prompts, the visibility of the symbol is
determined by the condition on the prompt.

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 the 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 (active) user value and no (active) 'default' default to n for
bool/tristate symbols, and to the empty string for other symbols.

'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.

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).


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 with variables expanded is 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 (which also holds the text for
menus and comments) can be found in MenuNode.prompt. For Symbol and Choice,
MenuNode.help holds the help text (if any, otherwise None).

Note that prompts and help texts for symbols and choices are stored in the menu
node. This makes it possible to define a symbol in multiple locations with a
different prompt or help text in each location.

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

The list of menu nodes for a Symbol or Choice can be found in the
Symbol/Choice.nodes attribute.

It is possible to give a Choice a name and define it in multiple locations,
hence why Choice.nodes is a list. In practice, you're unlikely to ever see a
choice defined in more than one location. I don't think I've even seen a named
choice outside of the test suite.


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.


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 os
import platform
import re
import sys

# 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.

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

    named_choices:
      A dictionary like 'syms' for named choices (choice FOO). This is for
      completeness. I've never seen a named choice outside of the test suite.

    defined_syms:
      A list with all defined symbols, in the same order as they appear in the
      Kconfig files. Provided as a convenience.

    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.

      References to Kconfig symbols ("$FOO") in the 'default' properties of the
      defconfig_filename symbol are are expanded before the file is looked up.

      '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_node menu, with Kconfig variable references
      ("$FOO") expanded. Defaults to "Linux Kernel Configuration" (like in the
      C tools). Can be changed with the 'mainmenu' statement (see
      kconfig-language.txt).

    srctree:
      The value of the $srctree environment variable when the configuration was
      loaded, or None if $srctree wasn't set. Kconfig and .config files are
      looked up relative to $srctree if they are not found in the base path
      (unless absolute paths are used). 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) in .config files. 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__ = (
        "_choices",
        "_print_undef_assign",
        "_print_override",
        "_print_redun_assign",
        "_print_warnings",
        "_set_re_match",
        "_unset_re_match",
        "_warn_no_prompt",
        "config_prefix",
        "const_syms",
        "defconfig_list",
        "defined_syms",
        "m",
        "modules",
        "n",
        "named_choices",
        "srctree",
        "syms",
        "top_node",
        "y",

        # Parsing-related
        "_parsing_kconfigs",
        "_reuse_line",
        "_file",
        "_filename",
        "_linenr",
        "_filestack",
        "_line",
        "_tokens",
        "_tokens_i",
        "_has_tokens",
    )

    #
    # Public interface
    #

    def __init__(self, filename="Kconfig", warn=True):
        """
        Creates a new Kconfig object by parsing Kconfig files. Raises
        KconfigSyntaxError on syntax errors. Note that Kconfig files are not
        the same as .config files (which store configuration symbol values).

        filename (default: "Kconfig"):
          The base Kconfig file. 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 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.

          The $srctree environment variable is used to look up Kconfig files if
          set. See the class documentation.

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

        self.config_prefix = os.environ.get("CONFIG_")
        if self.config_prefix is None:
            self.config_prefix = "CONFIG_"

        # Regular expressions for parsing .config files, with the get() method
        # assigned directly as a small optimization (microscopic in this case,
        # but it's consistent with the other regexes)
        self._set_re_match = re.compile(r"{}(\w+)=(.*)"
                                        .format(self.config_prefix)).match
        self._unset_re_match = re.compile(r"# {}(\w+) is not set"
                                          .format(self.config_prefix)).match

        self._print_warnings = warn
        self._print_undef_assign = False
        self._print_redun_assign = self._print_override = True

        self.syms = {}
        self.const_syms = {}
        self.defined_syms = []
        self.named_choices = {}
        # Used for quickly invalidating all choices
        self._choices = []

        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

        # 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

        # The only predefined symbol besides n/m/y. DEFCONFIG_LIST uses this as
        # of writing.
        uname_sym = self._lookup_const_sym("UNAME_RELEASE")
        uname_sym.orig_type = STRING
        # env_var doubles as the SYMBOL_AUTO flag from the C implementation, so
        # just set it to something. The naming breaks a bit here.
        uname_sym.env_var = "<uname release>"
        uname_sym.defaults.append(
            (self._lookup_const_sym(platform.uname()[2]), self.y))
        self.syms["UNAME_RELEASE"] = uname_sym

        self.top_node = MenuNode()
        self.top_node.kconfig = self
        self.top_node.item = MENU
        self.top_node.visibility = self.y
        self.top_node.prompt = ("Linux Kernel Configuration", self.y)
        self.top_node.parent = None
        self.top_node.dep = self.y
        self.top_node.filename = filename
        self.top_node.linenr = 1

        # Parse the Kconfig files

        # These implement a single line of "unget" for the parser
        self._reuse_line = False
        self._has_tokens = False

        # Keeps track of the location in the parent Kconfig files. Kconfig
        # files usually source other Kconfig files.
        self._filestack = []

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

        self._file = self._open(filename)

        self._parse_block(None,           # end_token
                          self.top_node,  # parent
                          self.y,         # visible_if_deps
                          self.top_node)  # prev_node
        self.top_node.list = self.top_node.next
        self.top_node.next = None

        self._parsing_kconfigs = False

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

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

        self._warn_no_prompt = True

    @property
    def mainmenu_text(self):
        """
        See the class documentation.
        """
        return self._expand_syms(self.top_node.prompt[0])

    @property
    def defconfig_filename(self):
        """
        See the class documentation.
        """
        if not self.defconfig_list:
            return None

        for filename, cond in self.defconfig_list.defaults:
            if expr_value(cond):
                try:
                    with self._open(self._expand_syms(filename.str_value)) as f:
                        return f.name
                except IOError:
                    continue

        return None

    def load_config(self, filename, replace=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.

        filename:
          The file to load. Respects $srctree if set (see the class
          documentation).

        replace (default: True):
          True if all existing user values should be cleared before loading the
          .config.
        """
        # Disable the warning about assigning to symbols without prompts. This
        # is normal and expected within a .config file.
        self._warn_no_prompt = False

        # This stub only exists to make sure _warn_no_prompt gets reenabled
        try:
            self._load_config(filename, replace)
        finally:
            self._warn_no_prompt = True

    def _load_config(self, filename, replace):
        with self._open(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.defined_syms:
                    sym._was_set = False

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

            # Small optimizations
            set_re_match = self._set_re_match
            unset_re_match = self._unset_re_match
            syms = self.syms

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

                set_match = set_re_match(line)
                if set_match:
                    name, val = set_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):
                        if val == "":
                            val = "n"  # C implementation allows 'blank' for 'no'

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

                        # We represent tristate values as 0, 1, 2
                        val = STR_TO_TRI[val[0]]

                        if sym.choice and val:
                            # 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 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 == STRING:
                        string_match = _conf_string_re_match(val)
                        if not string_match:
                            self._warn("Malformed string literal in "
                                       "assignment to {}. Assignment ignored."
                                       .format(sym.name),
                                       filename, linenr)
                            continue

                        val = unescape(string_match.group(1))

                else:
                    unset_match = unset_re_match(line)
                    if not unset_match:
                        continue

                    name = unset_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 = 0

                # Done parsing the assignment. Set the value.

                if sym._was_set:
                    # Use strings for tristate values in the warning
                    if sym.orig_type in (BOOL, TRISTATE):
                        display_val = TRI_TO_STR[val]
                        display_user_val = TRI_TO_STR[sym.user_value]
                    else:
                        display_val = val
                        display_user_val = sym.user_value

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

                    if display_user_val == display_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.defined_syms:
                if not sym._was_set:
                    sym.unset_value()

            for choice in self._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 (though possibly with a
        different ordering of the #defines, as the order in the C
        implementation depends on the hash table implementation as of writing).

        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 open(filename, "w") as f:

            # Small optimizations
            write = f.write
            config_prefix = self.config_prefix

            write(header)

            def write_node(node):
                sym = node.item
                if not isinstance(sym, Symbol):
                    return

                # 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:
                    orig_type = sym.orig_type
                    if orig_type in (BOOL, TRISTATE):
                        if val != "n":
                            write("#define {}{}{} 1\n"
                                  .format(config_prefix, sym.name,
                                          "_MODULE" if val == "m" else ""))

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

                    elif orig_type in (INT, HEX):
                        if orig_type == HEX and \
                           not val.startswith(("0x", "0X")):
                            val = "0x" + val

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

                    else:
                        _internal_error("Internal error while creating C "
                                        'header: unknown type "{}".'
                                        .format(sym.orig_type))

            self.walk_menu(write_node)

    def write_config(self, filename,
                     header="# Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)\n"):
        r"""
        Writes out symbol values in the .config format.

        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 open(filename, "w") as f:
            # Small optimization
            write = f.write

            write(header)

            def write_node(node):
                item = node.item
                if isinstance(item, Symbol) and item.env_var is None:
                    config_string = item.config_string
                    if config_string:
                        write(config_string)

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

                    write("\n#\n# {}\n#\n".format(node.prompt[0]))
            self.walk_menu(write_node, True)

    def walk_menu(self, callback, skip_duplicates=False):
        """
        Walk the entire menu in order, calling callback(node)
        for each menu node.

        Used to implement write_config() & write_autoconf(), but can be
        used to implement different types of custom processing as well.

        callback:
        Function which is called once for each node in the config tree.
        Takes only one argument, the node.

        skip_duplicates (default: False)
        If set to True, for each item in the menu the callback will
        only be called the first time it is encountered in the menu.
        """
        node = self.top_node.list
        if not node:
            return  # Empty configuration

        seen_items = set()

        while True:
            if not (skip_duplicates and node.item in seen_items):
                callback(node)
                seen_items.add(node.item)
            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:
                    return

    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 KconfigSyntaxError 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

        self._line = "if " + s
        self._tokenize()
        # Remove the "if " to avoid giving confusing error messages
        self._line = s
        # Remove the _T_IF token
        del self._tokens[0]

        return expr_value(self._parse_expr(True))  # transform_m

    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_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.defined_syms:
                sym.unset_value()

            for choice in self._choices:
                choice.unset_value()
        finally:
            self._warn_no_prompt = True

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

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

    def enable_undef_warnings(self):
        """
        Enables warnings for assignments to undefined symbols. Printed to
        stderr. Disabled by default since they tend to be spammy for Kernel
        configurations (and mostly suggests cleanups).
        """
        self._print_undef_assign = True

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

    def enable_redun_warnings(self):
        """
        Enables warnings for redundant assignments to symbols. Printed to
        stderr. Enabled by default.
        """
        self._print_redun_assign = True

    def disable_redun_warnings(self):
        """
        See enable_redun_warnings().
        """
        self._print_redun_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._print_override = True

    def disable_override_warnings(self):
        """
        See enable_override_warnings().
        """
        self._print_override = 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 not set" if self.srctree is None else
                'srctree "{}"'.format(self.srctree),
            'config symbol prefix "{}"'.format(self.config_prefix),
            "warnings " + ("enabled" if self._print_warnings else "disabled"),
            "undef. symbol assignment warnings " +
                ("enabled" if self._print_undef_assign else "disabled"),
            "redundant symbol assignment warnings " +
                ("enabled" if self._print_redun_assign else "disabled")
        )))

    #
    # Private methods
    #


    #
    # File reading
    #

    def _open(self, filename):
        """
        First tries to open 'filename', then '$srctree/filename' if $srctree
        was set when the configuration was loaded.
        """
        try:
            return open(filename) if sys.version_info[0] < 3 else open(filename, encoding='utf-8')
        except IOError as e:
            if not os.path.isabs(filename) and self.srctree is not None:
                filename = os.path.join(self.srctree, filename)
                try:
                    return open(filename)
                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 IOError(
                "Could not open '{}' ({}: {}). Perhaps the $srctree "
                "environment variable (which was {}) 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(filename, errno.errorcode[e.errno], e.strerror,
                        "unset" if self.srctree is None else
                        '"{}"'.format(self.srctree)))

    def _enter_file(self, filename):
        """
        Jumps to the beginning of a sourced Kconfig file, saving the previous
        position and file object.
        """
        self._filestack.append((self._file, self._filename, self._linenr))
        try:
            self._file = self._open(filename)
        except IOError as e:
            # Extend the error message a bit in this case
            raise IOError(
                "{}:{}: {} Also note that e.g. $FOO in a 'source' "
                "statement does not refer to the environment "
                "variable FOO, but rather to the Kconfig Symbol FOO "
                "(which would commonly have 'option env=\"FOO\"' in "
                "its definition)."
                .format(self._filename, self._linenr, e.message))

        self._filename = filename
        self._linenr = 0

    def _leave_file(self):
        """
        Returns from a Kconfig file to the file that sourced it.
        """
        self._file.close()
        self._file, self._filename, self._linenr = 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.
        """
        # This provides a single line of "unget" if _reuse_line is set to True
        if not self._reuse_line:
            self._line = self._file.readline()
            self._linenr += 1

        self._reuse_line = False

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

        if not self._line:
            return False

        self._tokenize()
        return True

    def _next_help_line(self):
        """
        Used for help texts, where lines are not tokenized and no line joining
        is done.
        """
        self._line = self._file.readline()
        self._linenr += 1
        return self._line


    #
    # 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):
        """
        Parses Kconfig._line, putting the tokens in Kconfig._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.
        """
        s = self._line

        # Tricky implementation detail: While parsing a token, 'token' refers
        # to the previous token. See _STRING_LEX for why this is needed.

        # See comment at _initial_token_re_match definition
        initial_token_match = _initial_token_re_match(s)
        if not initial_token_match:
            self._tokens = (None,)
            self._tokens_i = -1
            return

        keyword = _get_keyword(initial_token_match.group(1))

        if keyword == _T_HELP:
            # Avoid junk after "help", e.g. "---", being registered as a
            # symbol
            self._tokens = (_T_HELP, None)
            self._tokens_i = -1
            return

        if keyword is None:
            self._parse_error("expected keyword as first token")

        token = keyword
        self._tokens = [keyword]
        # The current index in the string being tokenized
        i = initial_token_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.
            id_keyword_match = _id_keyword_re_match(s, i)
            if id_keyword_match:
                # We have an identifier or keyword

                # Jump past it
                i = id_keyword_match.end()

                # 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 = id_keyword_match.group(1)
                keyword = _get_keyword(name)
                if keyword is not None:
                    # It's a keyword
                    token = keyword

                elif token not in _STRING_LEX:
                    # It's a non-const symbol...
                    if name in ("n", "m", "y"):
                        # ...except we translate n, m, and y into the
                        # corresponding constant symbols, like the C
                        # implementation
                        token = self.const_syms[name]
                    else:
                        token = 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

            else:
                # Not keyword/non-const symbol

                # Note: _id_keyword_match and _initial_token_match strip
                # trailing whitespace, making it safe to assume s[i] is the
                # start of a token here. We manually strip trailing whitespace
                # below as well.
                #
                # An old version stripped whitespace in this spot instead, but
                # that leads to some redundancy and would cause
                # _id_keyword_match to be tried against just "\n" fairly often
                # (because file.readlines() keeps newlines).

                c = s[i]
                i += 1

                if c in "\"'":
                    # String literal/constant symbol
                    if "\\" not in s:
                        # Fast path: If the line contains no backslashes, we
                        # can just find the matching quote.

                        end = s.find(c, i)
                        if end == -1:
                            self._parse_error("unterminated string")

                        val = s[i:end]
                        i = end + 1
                    else:
                        # Slow path for lines with backslashes (very rare,
                        # performance irrelevant)

                        quote = c
                        val = ""

                        while 1:
                            if i >= len(s):
                                self._parse_error("unterminated string")

                            c = s[i]
                            if c == quote:
                                break

                            if c == "\\":
                                if i + 1 >= len(s):
                                    self._parse_error("unterminated string")

                                val += s[i + 1]
                                i += 2
                            else:
                                val += c
                                i += 1

                        i += 1

                    # 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 \
                                self._tokens[0] == _T_OPTION else \
                            self._lookup_const_sym(val)

                elif c == "&":
                    # Invalid characters are ignored (backwards-compatible)
                    if i >= len(s) or s[i] != "&":
                        continue

                    token = _T_AND
                    i += 1

                elif c == "|":
                    # Invalid characters are ignored (backwards-compatible)
                    if i >= len(s) or s[i] != "|":
                        continue

                    token = _T_OR
                    i += 1

                elif c == "!":
                    if i < len(s) and s[i] == "=":
                        token = _T_UNEQUAL
                        i += 1
                    else:
                        token = _T_NOT

                elif c == "=":
                    token = _T_EQUAL

                elif c == "(":
                    token = _T_OPEN_PAREN

                elif c == ")":
                    token = _T_CLOSE_PAREN

                elif c == "#":
                    break

                # Very rare
                elif c == "<":
                    if i < len(s) and s[i] == "=":
                        token = _T_LESS_EQUAL
                        i += 1
                    else:
                        token = _T_LESS

                # Very rare
                elif c == ">":
                    if i < len(s) and s[i] == "=":
                        token = _T_GREATER_EQUAL
                        i += 1
                    else:
                        token = _T_GREATER

                else:
                    # Invalid characters are ignored (backwards-compatible)
                    continue

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

            self._tokens.append(token)

        # None-terminating token streams makes the token fetching functions
        # simpler/faster
        self._tokens.append(None)
        self._tokens_i = -1

    def _next_token(self):
        self._tokens_i += 1
        return self._tokens[self._tokens_i]

    def _peek_token(self):
        return self._tokens[self._tokens_i + 1]

    def _check_token(self, token):
        """
        If the next token is 'token', removes it and returns True.
        """
        if self._tokens[self._tokens_i + 1] == token:
            self._tokens_i += 1
            return True
        return False


    #
    # 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, visible_if_deps, prev_node):
        """
        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 e.g. a menu or Choice. Can
          also be a Symbol, due to automatic submenu creation from
          dependencies.

        visible_if_deps:
          'visible if' dependencies from enclosing menus. Propagated to Symbol
          and Choice prompts.

        prev_node:
          The previous menu node. New nodes will be added after this one (by
          modifying their 'next' pointer).

          prev_node 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_node if the block is
        empty). This allows chaining.
        """
        # We might already have tokens from parsing a line to check if it's a
        # property and discovering it isn't. self._has_tokens functions as a
        # kind of "unget".
        while self._has_tokens or self._next_line():
            self._has_tokens = False

            t0 = self._next_token()
            if t0 is None:
                continue

            if t0 in (_T_CONFIG, _T_MENUCONFIG):
                # The tokenizer allocates Symbol objects for us
                sym = self._next_token()

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

                self._parse_properties(node, visible_if_deps)

                sym.nodes.append(node)
                self.defined_syms.append(sym)

                # Tricky Python semantics: This assign prev_node.next before
                # prev_node
                prev_node.next = prev_node = node

            elif t0 == _T_SOURCE:
                values = _wordexp_expand(self._next_token())
                for sourced_file in values:
                    self._enter_file(sourced_file)
                    prev_node = self._parse_block(None,            # end_token
                                                  parent,
                                                  visible_if_deps,
                                                  prev_node)
                    self._leave_file()

            elif t0 == end_token:
                # We have reached the end of the block. Terminate the final
                # node and return it.
                prev_node.next = None
                return prev_node

            elif t0 == _T_IF:
                node = MenuNode()
                node.item = node.prompt = None
                node.parent = parent
                node.filename = self._filename
                node.linenr = self._linenr

                # See similar code in _parse_properties()
                if isinstance(parent.item, Choice):
                    parent_dep = parent.item
                else:
                    parent_dep = parent.dep

                node.dep = self._make_and(parent_dep, self._parse_expr(True))

                self._parse_block(_T_ENDIF,
                                  node,             # parent
                                  visible_if_deps,
                                  node)             # prev_node
                node.list = node.next

                prev_node.next = prev_node = node

            elif t0 == _T_MENU:
                node = MenuNode()
                node.kconfig = self
                node.item = MENU
                node.visibility = self.y
                node.parent = parent
                node.filename = self._filename
                node.linenr = self._linenr

                prompt = self._next_token()
                self._parse_properties(node, visible_if_deps)
                node.prompt = (prompt, node.dep)

                self._parse_block(_T_ENDMENU,
                                  node,         # parent
                                  self._make_and(visible_if_deps,
                                                 node.visibility),
                                  node)         # prev_node
                node.list = node.next

                prev_node.next = prev_node = node

            elif t0 == _T_COMMENT:
                node = MenuNode()
                node.kconfig = self
                node.item = COMMENT
                node.list = None
                node.parent = parent
                node.filename = self._filename
                node.linenr = self._linenr

                prompt = self._next_token()
                self._parse_properties(node, visible_if_deps)
                node.prompt = (prompt, node.dep)

                prev_node.next = prev_node = node

            elif t0 == _T_CHOICE:
                name = self._next_token()
                if name is None:
                    choice = Choice()
                    self._choices.append(choice)
                else:
                    # Named choice
                    choice = self.named_choices.get(name)
                    if not choice:
                        choice = Choice()
                        self._choices.append(choice)
                        choice.name = name
                        self.named_choices[name] = choice

                choice.kconfig = self

                node = MenuNode()
                node.kconfig = self
                node.item = choice
                node.help = None
                node.parent = parent
                node.filename = self._filename
                node.linenr = self._linenr

                self._parse_properties(node, visible_if_deps)
                self._parse_block(_T_ENDCHOICE,
                                  node,             # parent
                                  visible_if_deps,
                                  node)             # prev_node
                node.list = node.next

                choice.nodes.append(node)

                prev_node.next = prev_node = node

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

            else:
                self._parse_error("unrecognized construct")

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

        if end_token is not None:
            raise KconfigSyntaxError("Unexpected end of file " +
                                     self._filename)

        prev_node.next = None
        return prev_node

    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
        """
        return self._parse_expr(True) if self._check_token(_T_IF) else self.y

    def _parse_properties(self, node, visible_if_deps):
        """
        Parses properties for symbols, menus, choices, and comments. Also takes
        care of propagating dependencies from the menu node to the properties
        of the item (this mirrors the C tools, though they do it after
        parsing).

        node:
          The menu node we're parsing properties on. Prompt, help text,
          'depends on', and 'visible if' properties apply to the Menu node,
          while the others apply to the contained item.

        visible_if_deps:
          'visible if' dependencies from enclosing menus. Propagated to Symbol
          and Choice prompts.
        """
        # New properties encountered at this location. A local 'depends on'
        # only applies to these, in case a symbol is defined in multiple
        # locations.
        prompt = None
        defaults = []
        selects = []
        implies = []
        ranges = []

        # Menu node dependencies from 'depends on'. Will get propagated to the
        # properties above.
        node.dep = self.y

        while self._next_line():
            t0 = self._next_token()
            if t0 is None:
                continue

            if t0 in _TYPE_TOKENS:
                node.item.orig_type = _TOKEN_TO_TYPE[t0]

                if self._peek_token() is not None:
                    prompt = (self._next_token(), self._parse_cond())

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

                node.dep = self._make_and(node.dep, self._parse_expr(True))

            elif t0 == _T_HELP:
                # Find first non-blank (not all-space) line and get its
                # indentation

                while 1:
                    line = self._next_help_line()
                    if not line or not line.isspace():
                        break

                if not line:
                    node.help = ""
                    break

                indent = _indentation(line)
                if indent == 0:
                    # If the first non-empty lines has zero indent, there is no
                    # help text
                    node.help = ""
                    self._reuse_line = True  # "Unget" the line
                    break

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

                help_lines = [_deindent(line, indent).rstrip()]
                while 1:
                    line = self._next_help_line()

                    if not line or \
                       (not line.isspace() and _indentation(line) < indent):
                        node.help = "\n".join(help_lines).rstrip() + "\n"
                        break

                    help_lines.append(_deindent(line, indent).rstrip())

                if not line:
                    break

                self._reuse_line = True  # "Unget" the line

            elif t0 == _T_SELECT:
                if not isinstance(node.item, Symbol):
                    self._parse_error("only symbols can select")

                selects.append((self._next_token(), self._parse_cond()))

            elif t0 == _T_IMPLY:
                if not isinstance(node.item, Symbol):
                    self._parse_error("only symbols can imply")

                implies.append((self._next_token(), self._parse_cond()))

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

            elif t0 in (_T_DEF_BOOL, _T_DEF_TRISTATE):
                node.item.orig_type = _TOKEN_TO_TYPE[t0]
                defaults.append((self._parse_expr(False), self._parse_cond()))

            elif t0 == _T_PROMPT:
                # 'prompt' properties override each other within a single
                # definition of a symbol, but additional prompts can be added
                # by defining the symbol multiple times
                prompt = (self._next_token(), self._parse_cond())

            elif t0 == _T_RANGE:
                ranges.append((self._next_token(),
                               self._next_token(),
                               self._parse_cond()))

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

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

                    if env_var not in os.environ:
                        self._warn("'option env=\"{0}\"' on symbol {1} has "
                                   "no effect, because the environment "
                                   "variable {0} is not set"
                                   .format(env_var, node.item.name),
                                   self._filename, self._linenr)
                    else:
                        defaults.append(
                            (self._lookup_const_sym(os.environ[env_var]),
                             self.y))

                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 not isinstance(node.item, 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 == _T_VISIBLE:
                if not self._check_token(_T_IF):
                    self._parse_error('expected "if" after "visible"')

                node.visibility = \
                    self._make_and(node.visibility, self._parse_expr(True))

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

                node.item.is_optional = True

            else:
                self._tokens_i = -1
                # Reuse the tokens for the non-property line later
                self._has_tokens = True
                break

        # Done parsing properties. Now add the new
        # prompts/defaults/selects/implies/ranges properties, with dependencies
        # from node.dep propagated.

        # First propagate parent dependencies to node.dep

        # If the parent node holds a Choice, we use the Choice itself as the
        # parent dependency. This matches the C implementation, and makes sense
        # as the value (mode) of the choice limits the visibility of the
        # contained choice symbols. Due to the similar interface, Choice works
        # as a drop-in replacement for Symbol here.
        if isinstance(node.parent.item, Choice):
            node.dep = self._make_and(node.dep, node.parent.item)
        else:
            node.dep = self._make_and(node.dep, node.parent.dep)

        if isinstance(node.item, (Symbol, Choice)):
            if isinstance(node.item, Symbol):
                # See the class documentation
                node.item.direct_dep = \
                    self._make_or(node.item.direct_dep, node.dep)

            # Set the prompt, with dependencies propagated
            if prompt:
                node.prompt = (prompt[0],
                               self._make_and(self._make_and(prompt[1],
                                                             node.dep),
                                              visible_if_deps))
            else:
                node.prompt = None

            # Add the new defaults, with dependencies propagated
            for val_expr, cond in defaults:
                node.item.defaults.append(
                    (val_expr, self._make_and(cond, node.dep)))

            # Add the new ranges, with dependencies propagated
            for low, high, cond in ranges:
                node.item.ranges.append(
                    (low, high, self._make_and(cond, node.dep)))

            # Handle selects
            for target, cond in selects:
                # Only stored for inspection. Not used during evaluation.
                node.item.selects.append(
                    (target, self._make_and(cond, node.dep)))

                # Modify the dependencies of the selected symbol
                target.rev_dep = \
                    self._make_or(target.rev_dep,
                                  self._make_and(node.item,
                                                 self._make_and(cond,
                                                                node.dep)))

            # Handle implies
            for target, cond in implies:
                # Only stored for inspection. Not used during evaluation.
                node.item.implies.append(
                    (target, self._make_and(cond, node.dep)))

                # Modify the dependencies of the implied symbol
                target.weak_rev_dep = \
                    self._make_or(target.weak_rev_dep,
                                  self._make_and(node.item,
                                                 self._make_and(cond,
                                                                node.dep)))

    def _parse_expr(self, transform_m):
        """
        Parses an expression from the tokens in Kconfig._tokens using a simple
        top-down approach. See the module docs 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._next_token()

        if isinstance(token, Symbol):
            # Plain symbol or relation

            next_token = self._peek_token()
            if next_token not in _TOKEN_TO_REL:
                # 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
            return (_TOKEN_TO_REL[self._next_token()], token,
                    self._next_token())

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

        if token == _T_OPEN_PAREN:
            expr_parse = self._parse_expr(transform_m)
            if not self._check_token(_T_CLOSE_PAREN):
                self._parse_error("missing end parenthesis")

            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.
        """
        # 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.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._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)

            # The choice symbols themselves, because the y mode selection might
            # change if a choice symbol's visibility changes
            for sym in choice.syms:
                # the default selection depends on the symbols
                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.defined_syms:
            sym._invalidate()

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


    #
    # Misc.
    #

    def _expand_syms(self, s):
        """
        Expands $-references to symbols in 's' to symbol values, or to the
        empty string for undefined symbols.
        """
        while 1:
            sym_ref_match = _sym_ref_re_search(s)
            if not sym_ref_match:
                return s

            sym = self.syms.get(sym_ref_match.group(1))

            s = s[:sym_ref_match.start()] + \
                (sym.str_value if sym else "") + \
                s[sym_ref_match.end():]

    def _parse_error(self, msg):
        if self._filename is None:
            loc = ""
        else:
            loc = "{}:{}: ".format(self._filename, self._linenr)

        raise KconfigSyntaxError(
            "{}Couldn't parse '{}': {}".format(loc, self._line.rstrip(), msg))

    def _warn(self, msg, filename=None, linenr=None):
        """
        For printing general warnings.
        """
        if self._print_warnings:
            _stderr_msg("warning: " + msg, filename, linenr)

    def _warn_undef_assign(self, msg, filename=None, linenr=None):
        """
        See the class documentation.
        """
        if self._print_undef_assign:
            _stderr_msg("warning: " + 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_redun_assign(self, msg, filename=None, linenr=None):
        """
        See the class documentation.
        """
        if self._print_redun_assign:
            _stderr_msg("warning: " + msg, filename, linenr)

    def _warn_override(self, msg, filename=None, linenr=None):
        """
        See the class documentation.
        """
        if self._print_override:
            _stderr_msg("warning: " + msg, filename, linenr)

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(). None if no .config assignment would get
      written out. In general, visible symbols, symbols with (active) defaults,
      and selected symbols get written out.

    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 only 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.

    env_var:
      If the Symbol has an 'option env="FOO"' option, this contains the name
      ("FOO") of the environment variable. None for symbols that aren't set
      from the environment.

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

      env_var is set to "<uname release>" for the predefined symbol
      UNAME_RELEASE, which holds the 'release' field from uname.

      Symbols with an 'option env' option 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",
        "_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 == 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 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
            low, high = self.active_range

            if vis and self.user_value is not None and \
               _is_base_n(self.user_value, base) and \
               (low is None or
                low <= int(self.user_value, base) <= high):

                # 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

            else:
                # No user value or invalid user value. Look at defaults.

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

                        val = val_expr.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 low is not None:
                    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 == INT else \
                              hex(clamp)

        elif self.orig_type == 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 val_expr, cond in self.defaults:
                    if expr_value(cond):
                        self._write_to_conf = True
                        val = val_expr.str_value
                        break

        # Corresponds to SYMBOL_AUTO in the C implementation
        if self.env_var is not None:
            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):
            self._cached_tri_val = 0
            return self._cached_tri_val

        val = 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)

        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:
                    cond_val = expr_value(cond)
                    if cond_val:
                        val = min(expr_value(default), cond_val)
                        self._write_to_conf = True
                        break

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

            # Reverse (select-related) dependencies take precedence
            rev_dep_val = expr_value(self.rev_dep)
            if rev_dep_val:
                val = max(rev_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 == 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 not None:
            return self._cached_assignable

        self._cached_assignable = self._get_assignable()
        return self._cached_assignable

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

        self._cached_vis = _get_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 None

        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)

        if self.orig_type == STRING:
            # Escape \ and "
            return '{}{}="{}"\n' \
                   .format(self.kconfig.config_prefix, self.name, escape(val))

        _internal_error("Internal error while creating .config: unknown "
                        'type "{}".'.format(self.orig_type))

    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_str/tri_value to differ from
        Symbol.str/tri_value (be truncated down or up).

        Setting a choice symbol to 2 (y) only updates Choice.user_selection on
        the parent choice and not Symbol.user_value itself. This gives the
        expected behavior when a choice is switched between different modes.
        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,
          pass 0, 1, 2 for n, m, and y, respectively. For other symbol types,
          pass a string.

          Values that are invalid for the type (such as "foo" or 1 (m) for a
          BOOL) are ignored and won't be stored in Symbol.user_str/tri_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 value == self.user_value:
            # We know the value must be valid if it was successfully set
            # previously
            self._was_set = True
            return True

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

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

            if self.orig_type in (BOOL, TRISTATE) and value in ("n", "m", "y"):
                warning += ' (pass 0, 1, 2 for n, m, y, respectively)'

            self.kconfig._warn(warning)

            return False

        if self.env_var is not None:
            self.kconfig._warn("ignored attempt to assign user value to "
                               "{}, which gets its value from the environment"
                               .format(self.name))
            return False

        if self.choice and value == 2:
            # Remember this as a choice selection only. Makes switching back
            # and forth between choice modes work as expected, and makes the
            # check for whether the user value is the same as before above
            # safe.
            self.choice.user_selection = self
            self.choice._was_set = True
            if self._is_user_assignable():
                self.choice._rec_invalidate()
        else:
            self.user_value = value
            self._was_set = True
            if self._is_user_assignable():
                self._rec_invalidate()

        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
            if self._is_user_assignable():
                self._rec_invalidate()

    @property
    def active_range(self):
        """
        Returns a tuple of (low, high) integer values if a range
        limit is active for this symbol, or (None, None) if no range
        limit exists.
        """
        base = _TYPE_TO_BASE[self.orig_type]

        for low_expr, high_expr, cond in self.ranges:
            if expr_value(cond):
                # 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

                return (low, high)
        return (None, None)


    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. Prompts and help texts are included,
        though they really belong to the symbol's menu nodes rather than the
        symbol itself.

        The output is designed so that feeding it back to a Kconfig parser
        redefines the symbol as is. This also works for symbols defined in
        multiple locations, where all the definitions are output. See the
        module documentation for a small gotcha related to choice symbols.

        An empty string is returned for undefined and constant symbols.
        """
        return _sym_choice_str(self)

    #
    # 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()

    def _get_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 == 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 == 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 _is_user_assignable(self):
        """
        Returns True if the symbol has a prompt, meaning a user value might
        have an effect on it. Used as an optimization to skip invalidation when
        promptless symbols are assigned to (given a user value).

        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:
                return True

        if self.kconfig._warn_no_prompt:
            self.kconfig._warn(self.name + " has no prompt, meaning user "
                               "values have no effect on it")
        return False

    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()

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. I can't remember ever seeing named choices in
      practice, but the C tools support them too.

    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.

      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'. There are real-world examples of this, and it was
      a PITA to support in older versions of Kconfiglib that didn't implement
      the menu structure.

    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 (i've never even seen a named
      choice though).

    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.

    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",
        "_was_set",
        "defaults",
        "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 == 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 == BOOL else val

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

        self._cached_assignable = self._get_assignable()
        return self._cached_assignable

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

        self._cached_vis = _get_visibility(self)
        return self._cached_vis

    @property
    def selection(self):
        """
        See the class documentation.
        """
        if self._cached_selection is not _NO_CACHED_SELECTION:
            return self._cached_selection

        self._cached_selection = self._get_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 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 == BOOL     and value in (0, 2)    ) or
                (self.orig_type == TRISTATE and value in (0, 1, 2))):
            self.kconfig._warn("the value '{}' is invalid for the choice, "
                               "which has type {}. Assignment ignored"
                               .format(value, TYPE_TO_STR[self.orig_type]))
            return False

        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()

    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" if self.name is None else \
                      "choice " + self.name)
        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). Prompts and help texts are included, though they really
        belong to the choice's menu nodes rather than the choice itself.

        See Symbol.__str__() as well.
        """
        return _sym_choice_str(self)

    #
    # 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:
        #   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()

    def _get_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 == BOOL else (1, 2)
            return (0, 2) if self.type == BOOL else (0, 1, 2)

        # vis == 1

        return (0, 1) if self.is_optional else (1,)

    def _get_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:
            return None

        # Use the user selection if it's visible
        if self.user_selection and self.user_selection.visibility == 2:
            return self.user_selection

        # Otherwise, 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.

    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.

    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 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.

    is_menuconfig:
      True if the symbol for the menu node (it must be a symbol) was defined
      with 'menuconfig' rather than 'config' (at this location). This is a hint
      on how to display the menu entry (display the children in a separate menu
      rather than indenting them). It's ignored internally by Kconfiglib,
      except when printing symbols.

    filename/linenr:
      The location where the menu node appears.

    kconfig:
      The Kconfig instance the menu node is from.
    """
    __slots__ = (
        "dep",
        "filename",
        "help",
        "is_menuconfig",
        "item",
        "kconfig",
        "linenr",
        "list",
        "next",
        "parent",
        "prompt",
        "visibility",
    )

    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 isinstance(self.item, Symbol):
            fields.append("menu node for symbol " + self.item.name)

        elif isinstance(self.item, Choice):
            s = "menu node for choice"
            if self.item.name is not None:
                s += " " + self.item.name
            fields.append(s)

        elif self.item == MENU:
            fields.append("menu node for menu")

        elif self.item == COMMENT:
            fields.append("menu node for comment")

        elif self.item is None:
            fields.append("menu node for if (should not appear in the final "
                          " tree)")

        else:
            raise InternalError("unable to determine type in "
                                "MenuNode.__repr__()")

        if self.prompt:
            fields.append('prompt "{}" (visibility {})'
                          .format(self.prompt[0],
                                  TRI_TO_STR[expr_value(self.prompt[1])]))

        if isinstance(self.item, Symbol) and self.is_menuconfig:
            fields.append("is menuconfig")

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

        if self.item == MENU:
            fields.append("'visible if' deps " + \
                          TRI_TO_STR[expr_value(self.visibility)])

        if isinstance(self.item, (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 MenuNode, matching the Kconfig
        format.

        For Symbol and Choice menu nodes, this function simply calls through to
        MenuNode.item.__str__(). For MENU and COMMENT nodes, a Kconfig-like
        representation of the menu or comment is returned.
        """
        if isinstance(self.item, (Symbol, Choice)):
            return self.item.__str__()

        if self.item in (MENU, COMMENT):
            s = ("menu" if self.item == MENU else "comment") + \
                ' "{}"\n'.format(escape(self.prompt[0]))

            if self.dep is not self.kconfig.y:
                s += "\tdepends on {}\n".format(expr_str(self.dep))

            if self.item == MENU and self.visibility is not self.kconfig.y:
                s += "\tvisible if {}\n".format(expr_str(self.visibility))

            return s

        # 'if' node. Should never appear in the final tree.
        return "if " + expr_str(self.dep)

class KconfigSyntaxError(Exception):
    """
    Exception raised for syntax errors.
    """
    pass

class InternalError(Exception):
    """
    Exception raised for internal errors.
    """
    pass

#
# 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 not isinstance(expr, tuple):
        return expr.tri_value

    if expr[0] == 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] == 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] == NOT:
        return 2 - expr_value(expr[1])

    if expr[0] in _RELATIONS:
        # Implements <, <=, >, >= comparisons as well. These were added to
        # kconfig in 31847b67 (kconfig: allow use of relations other than
        # (in)equality).

        # This mirrors the C tools pretty closely. Perhaps there's a more
        # pythonic way to structure this.

        oper, op1, op2 = expr

        # If both operands are strings...
        if op1.orig_type == STRING and op2.orig_type == STRING:
            # ...then compare them lexicographically
            comp = _strcmp(op1.str_value, op2.str_value)
        else:
            # Otherwise, try to compare them as numbers...
            try:
                comp = int(op1.str_value, _TYPE_TO_BASE[op1.orig_type]) - \
                       int(op2.str_value, _TYPE_TO_BASE[op2.orig_type])
            except ValueError:
                # Fall back on a lexicographic comparison if the operands don't
                # parse as numbers
                comp = _strcmp(op1.str_value, op2.str_value)

        if   oper == EQUAL:         res = comp == 0
        elif oper == UNEQUAL:       res = comp != 0
        elif oper == LESS:          res = comp < 0
        elif oper == LESS_EQUAL:    res = comp <= 0
        elif oper == GREATER:       res = comp > 0
        elif oper == GREATER_EQUAL: res = comp >= 0

        return 2*res

    _internal_error("Internal error while evaluating expression: "
                    "unknown operation {}.".format(expr[0]))

def expr_str(expr):
    """
    Returns the string representation of the expression 'expr', as in a Kconfig
    file.

    Passing subexpressions of expressions to this function works as expected.
    """
    if not isinstance(expr, tuple):
        if isinstance(expr, Choice):
            if expr.name is not None:
                return "<choice {}>".format(expr.name)
            return "<choice>"

        # Symbol

        if expr.is_constant:
            return '"{}"'.format(escape(expr.name))

        return expr.name

    if expr[0] == NOT:
        if isinstance(expr[1], Symbol):
            return "!" + expr_str(expr[1])
        return "!({})".format(expr_str(expr[1]))

    if expr[0] == AND:
        return "{} && {}".format(_format_and_op(expr[1]),
                                 _format_and_op(expr[2]))

    if expr[0] == OR:
        return "{} || {}".format(expr_str(expr[1]), expr_str(expr[2]))

    # Relation
    return "{} {} {}".format(expr_str(expr[1]),
                             _REL_TO_STR[expr[0]],
                             expr_str(expr[2]))

# escape()/unescape() helpers
_escape_re_sub = re.compile(r'(["\\])').sub
_unescape_re_sub = re.compile(r"\\(.)").sub

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.
    """
    return _escape_re_sub(r"\\\1", s)

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_re_sub(r"\1", s)

#
# Internal functions
#

def _get_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 isinstance(sc, Symbol) and sc.choice:
        if sc.choice.orig_type == TRISTATE and sc.orig_type != TRISTATE and \
           sc.choice.tri_value != 2:
            # Non-tristate choice symbols are only visible in y mode
            return 0

        if sc.orig_type == 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 != TRISTATE:
        return 2

    return vis

def _make_depend_on(sym, expr):
    """
    Adds 'sym' as a dependency to all symbols in 'expr'. Constant symbols in
    'expr' are skipped as they can never change value anyway.
    """
    if not isinstance(expr, tuple):
        if not expr.is_constant:
            expr._dependents.add(sym)

    elif expr[0] in (AND, OR):
        _make_depend_on(sym, expr[1])
        _make_depend_on(sym, expr[2])

    elif expr[0] == NOT:
        _make_depend_on(sym, expr[1])

    elif expr[0] in _RELATIONS:
        if not expr[1].is_constant:
            expr[1]._dependents.add(sym)
        if not expr[2].is_constant:
            expr[2]._dependents.add(sym)

    else:
        _internal_error("Internal error while fetching symbols from an "
                        "expression with token stream {}.".format(expr))

def _format_and_op(expr):
    """
    expr_str() helper. Returns the string representation of 'expr', which is
    assumed to be an operand to AND, with parentheses added if needed.
    """
    if isinstance(expr, tuple) and expr[0] == OR:
        return "({})".format(expr_str(expr))
    return expr_str(expr)

def _indentation(line):
    """
    Returns the length of the line's leading whitespace, treating tab stops as
    being spaced 8 characters apart.
    """
    line = line.expandtabs()
    return len(line) - len(line.lstrip())

def _deindent(line, indent):
    """
    Deindents 'line' by 'indent' spaces.
    """
    line = line.expandtabs()
    if len(line) <= indent:
        return line
    return line[indent:]

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 _stderr_msg(msg, filename, linenr):
    if filename is not None:
        msg = "{}:{}: {}".format(filename, linenr, msg)

    sys.stderr.write(msg + "\n")

def _internal_error(msg):
    raise InternalError(
        msg +
        "\nSorry! You may want to send an email to ulfalizer a.t Google's "
        "email service to tell me about this. Include the message above and "
        "the stack trace and describe what you were doing.")

# Printing functions

def _sym_choice_str(sc):
    """
    Symbol/choice __str__() implementation. These have many properties in
    common, so it makes sense to handle them together.
    """
    lines = []

    def indent_add(s):
        lines.append("\t" + s)

    # We print the prompt(s) and help text(s) too as a convenience, even though
    # they're actually part of the MenuNode. If a symbol or choice is defined
    # in multiple locations (has more than one MenuNode), we output one
    # statement for each location, and print all the properties that belong to
    # the symbol/choice itself only at the first location. This gives output
    # that would function if fed to a Kconfig parser, even for such
    # symbols/choices (choices defined in multiple locations gets a bit iffy
    # since they also have child nodes, though I've never seen such a choice).

    if not sc.nodes:
        return ""

    for node in sc.nodes:
        if isinstance(sc, Symbol):
            if node.is_menuconfig:
                lines.append("menuconfig " + sc.name)
            else:
                lines.append("config " + sc.name)
        else:
            if sc.name is None:
                lines.append("choice")
            else:
                lines.append("choice " + sc.name)

        if node is sc.nodes[0] and sc.orig_type != UNKNOWN:
            indent_add(TYPE_TO_STR[sc.orig_type])

        if node.prompt:
            prompt, cond = node.prompt
            prompt_str = 'prompt "{}"'.format(escape(prompt))
            if cond is not sc.kconfig.y:
                prompt_str += " if " + expr_str(cond)
            indent_add(prompt_str)

        if node is sc.nodes[0]:
            if isinstance(sc, 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")

            if isinstance(sc, Symbol):
                for low, high, cond in sc.ranges:
                    range_string = "range {} {}" \
                                   .format(expr_str(low), expr_str(high))
                    if cond is not sc.kconfig.y:
                        range_string += " if " + expr_str(cond)
                    indent_add(range_string)

            for default, cond in sc.defaults:
                default_string = "default " + expr_str(default)
                if cond is not sc.kconfig.y:
                    default_string += " if " + expr_str(cond)
                indent_add(default_string)

            if isinstance(sc, Choice) and sc.is_optional:
                indent_add("optional")

            if isinstance(sc, Symbol):
                for select, cond in sc.selects:
                    select_string = "select " + select.name
                    if cond is not sc.kconfig.y:
                        select_string += " if " + expr_str(cond)
                    indent_add(select_string)

                for imply, cond in sc.implies:
                    imply_string = "imply " + imply.name
                    if cond is not sc.kconfig.y:
                        imply_string += " if " + expr_str(cond)
                    indent_add(imply_string)

        if node.help is not None:
            indent_add("help")
            for line in node.help.splitlines():
                indent_add("  " + line)

        # Add a blank line if there are more nodes to print
        if node is not sc.nodes[-1]:
            lines.append("")

    return "\n".join(lines) + "\n"

# 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 not isinstance(expr, 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

        if left is not sym:
            return False

        return (expr[0] == EQUAL and right is sym.kconfig.m or \
                                     right is sym.kconfig.y) or \
               (expr[0] == UNEQUAL and right is sym.kconfig.n)

    if expr[0] == AND:
        return _expr_depends_on(expr[1], sym) or \
               _expr_depends_on(expr[2], sym)

    return False

def _has_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.prompt:
        return _expr_depends_on(node2.prompt[1], node1.item)

    # If we have no prompt, use the menu node dependencies instead
    return _expr_depends_on(node2.dep, node1.item)

def _check_auto_menu(node):
    """
    Looks for menu nodes after 'node' that depend on it. Creates an implicit
    menu rooted at 'node' with the nodes as the children if such nodes are
    found. The recursive call to _finalize_tree() makes this work recursively.
    """
    cur = node
    while cur.next and _has_auto_menu_dep(node, cur.next):
        _finalize_tree(cur.next)
        cur = cur.next
        cur.parent = node

    if cur is not node:
        node.list = node.next
        node.next = cur.next
        cur.next = None

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.
    """
    while node:
        if node.list and (not node.prompt or node.prompt[0] == ""):

            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.
    """
    first = node.list
    while first and first.item is None:
        first = first.next

    cur = first
    while cur:
        if cur.next and cur.next.item is None:
            cur.next = cur.next.next
        cur = cur.next

    node.list = first

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 isinstance(cur.item, 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 choice.orig_type == UNKNOWN:
        for item in choice.syms:
            if item.orig_type != UNKNOWN:
                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 sym.orig_type == UNKNOWN:
            sym.orig_type = choice.orig_type

def _finalize_tree(node):
    """
    Creates implicit menus from dependencies (see kconfig-language.txt),
    removes 'if' nodes, and finalizes choices. This pretty closely mirrors
    menu_finalize() from the C implementation, though we propagate dependencies
    during parsing instead.
    """
    # The ordering here gets a bit tricky. It's important to do things in this
    # order to have everything work out correctly.

    if node.list:
        # The menu node has children. Finalize them.
        cur = node.list
        while cur:
            _finalize_tree(cur)
            # Note: _finalize_tree() might have changed cur.next. This is
            # expected, so that we jump over e.g. implicitly created submenus.
            cur = cur.next

    elif node.item is not None:
        # The menu node has no children (yet). See if we can create an implicit
        # menu rooted at it (due to menu nodes after it depending on it).
        _check_auto_menu(node)

    if node.list:
        # We have a node with finalized children. Do final steps to finalize
        # this node.
        _flatten(node.list)
        _remove_ifs(node)

    # Empty choices (node.list None) are possible, so this needs to go outside
    if isinstance(node.item, Choice):
        _finalize_choice(node)

def _wordexp_expand(value):
    """
    Return a list of expanded tokens, using roughly the same algorithm
    as wordexp(3)
    """
    ifs = os.environ.get("IFS", " \t\n")
    value = os.path.expandvars(value).strip(ifs)
    if len(ifs) > 0:
        for i in ifs[1:]:  # collapse all IFS delimiters
            value = value.replace(i, ifs[0])
        return value.split(ifs[0])
    else:
        return [value]

#
# Public global constants
#

# Integers representing symbol types
(
    BOOL,
    HEX,
    INT,
    STRING,
    TRISTATE,
    UNKNOWN
) = range(6)

# Integers representing expression types
(
    AND,
    OR,
    NOT,
    EQUAL,
    UNEQUAL,
    LESS,
    LESS_EQUAL,
    GREATER,
    GREATER_EQUAL,
) = range(9)

# Integers representing menu and comment menu nodes
(
    MENU,
    COMMENT,
) = range(2)

# Converts a symbol/choice type to a string
TYPE_TO_STR = {
    UNKNOWN:  "unknown",
    BOOL:     "bool",
    TRISTATE: "tristate",
    STRING:   "string",
    HEX:      "hex",
    INT:      "int",
}

TRI_TO_STR = {
    0: "n",
    1: "m",
    2: "y",
}

STR_TO_TRI = {
    "n": 0,
    "m": 1,
    "y": 2,
}

#
# Internal global constants
#

# Tokens
(
    _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_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_PROMPT,
    _T_RANGE,
    _T_SELECT,
    _T_SOURCE,
    _T_STRING,
    _T_TRISTATE,
    _T_UNEQUAL,
    _T_VISIBLE,
) = range(44)

# Keyword to token map, with the get() method assigned directly as a small
# optimization
_get_keyword = {
    "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_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,
    "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,
    "prompt":         _T_PROMPT,
    "range":          _T_RANGE,
    "select":         _T_SELECT,
    "source":         _T_SOURCE,
    "string":         _T_STRING,
    "tristate":       _T_TRISTATE,
    "visible":        _T_VISIBLE,
}.get

# Tokens after which identifier-like lexemes are treated as strings. _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_PROMPT,
    _T_SOURCE,
    _T_STRING,
    _T_TRISTATE,
))

# Tokens for types, excluding def_bool, def_tristate, etc., for quick
# checks during parsing
_TYPE_TOKENS = frozenset((
    _T_BOOL,
    _T_TRISTATE,
    _T_INT,
    _T_HEX,
    _T_STRING,
))

# Note: This hack is no longer needed as of upstream commit c226456
# (kconfig: warn of unhandled characters in Kconfig commands). It
# is kept around for backwards compatibility.
#
# The initial word on a line is parsed specially. Let
# command_chars = [A-Za-z0-9_]. Then
#  - leading non-command_chars characters are ignored, and
#  - the first token consists the following one or more
#    command_chars characters.
# This is why things like "----help--" are accepted.
#
# In addition to the initial token, the regex also matches trailing whitespace
# so that we can jump straight to the next token (or to the end of the line if
# there's just a single token).
#
# As an optimization, this regex fails to match for lines containing just a
# comment.
_initial_token_re_match = re.compile(r"[^\w#]*(\w+)\s*").match

# Matches an identifier/keyword, also eating trailing whitespace
_id_keyword_re_match = re.compile(r"([\w./-]+)\s*").match

# Regular expression for finding $-references to symbols in strings
_sym_ref_re_search = re.compile(r"\$([A-Za-z0-9_]+)").search

# Matches a valid right-hand side for an assignment to a string symbol in a
# .config file, including escaped characters. Extracts the contents.
_conf_string_re_match = re.compile(r'"((?:[^\\"]|\\.)*)"').match

# Token to type mapping
_TOKEN_TO_TYPE = {
    _T_BOOL:         BOOL,
    _T_DEF_BOOL:     BOOL,
    _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. The entries for BOOL and TRISTATE are an implementation convenience:
# They should never convert to valid numbers.
_TYPE_TO_BASE = {
    BOOL:     0,
    HEX:      16,
    INT:      10,
    STRING:   0,
    TRISTATE: 0,
    UNKNOWN:  0,
}

_RELATIONS = frozenset((
    EQUAL,
    UNEQUAL,
    LESS,
    LESS_EQUAL,
    GREATER,
    GREATER_EQUAL,
))

# Token to relation (=, !=, <, ...) mapping
_TOKEN_TO_REL = {
    _T_EQUAL:         EQUAL,
    _T_GREATER:       GREATER,
    _T_GREATER_EQUAL: GREATER_EQUAL,
    _T_LESS:          LESS,
    _T_LESS_EQUAL:    LESS_EQUAL,
    _T_UNEQUAL:       UNEQUAL,
}

_REL_TO_STR = {
    EQUAL:         "=",
    GREATER:       ">",
    GREATER_EQUAL: ">=",
    LESS:          "<",
    LESS_EQUAL:    "<=",
    UNEQUAL:       "!=",
}