parser.py

"""Fast PYI parser."""

import collections
import hashlib

from pytype import file_utils
from pytype import module_utils
from pytype import utils
from pytype.pyi import parser_ext  # pytype: disable=import-error
from pytype.pytd import pep484
from pytype.pytd import pytd
from pytype.pytd import pytd_utils
from pytype.pytd import slots as cmp_slots
from pytype.pytd import visitors
from pytype.pytd.parse import parser_constants  # pylint: disable=g-importing-member

_DEFAULT_PLATFORM = "linux"
# Typing members that represent sets of types.
_TYPING_SETS = ("typing.Intersection", "typing.Optional", "typing.Union")


_Params = collections.namedtuple("_", ["required",
                                       "starargs", "starstarargs",
                                       "has_bare_star"])

_NameAndSig = collections.namedtuple("_", ["name", "signature",
                                           "decorator", "is_abstract",
                                           "is_coroutine"])

_SlotDecl = collections.namedtuple("_", ["slots"])

_Property = collections.namedtuple("_", ["precedence", "arity"])


class _ConditionScope(object):
  """State associated with a condition if/elif/else block."""

  def __init__(self, parent):
    self._parent = parent
    if parent is None:
      self._active = True
      # The value of _can_trigger doesn't really matter since apply_condition
      # shouldn't be called on the top scope.
      self._can_trigger = False
    else:
      # By default new scopes are inactive and can be triggered iff the
      # parent is active.
      self._active = False
      self._can_trigger = parent.active

  def apply_condition(self, value):
    """Apply the value to this scope.

    If the scope can be triggered and value is true, then the scope
    becomes active, otherwise the scope is not active.  Note that a scope
    can trigger at most once since triggering also clears _can_trigger.

    Args:
      value: a bool.
    """
    assert self._parent is not None
    if self._can_trigger and value:
      self._active = True
      self._can_trigger = False
    else:
      self._active = False

  @property
  def active(self):
    return self._active

  @property
  def parent(self):
    return self._parent


class ParseError(Exception):

  """Exceptions raised by the parser."""

  def __init__(self, msg, line=None, filename=None, column=None, text=None):
    super(ParseError, self).__init__(msg)
    self._line = line
    self._filename = filename
    self._column = column
    self._text = text

  @property
  def line(self):
    return self._line

  def __str__(self):
    lines = []
    if self._filename or self._line is not None:
      lines.append('  File: "%s", line %s' % (self._filename, self._line))
    if self._column and self._text:
      indent = 4
      stripped = self._text.lstrip()
      lines.append("%*s%s" % (indent, "", stripped))
      # Output a pointer below the error column, adjusting for stripped spaces.
      pos = indent + (self._column - 1) - (len(self._text) - len(stripped))
      lines.append("%*s^" % (pos, ""))
    lines.append("%s: %s" % (type(self).__name__, utils.message(self)))
    return "\n".join(lines)


class OverloadedDecoratorError(ParseError):
  """Inconsistent decorators on an overloaded function."""

  def __init__(self, name, typ, *args, **kwargs):
    msg = "Overloaded signatures for %s disagree on %sdecorators" % (
        name, (typ + " " if typ else ""))
    super(OverloadedDecoratorError, self).__init__(msg, *args, **kwargs)


class _Mutator(visitors.Visitor):
  """Visitor for adding a mutated_type to parameters.

  We model the parameter x in
    def f(x: old_type):
      x = new_type
  as
    Parameter(name=x, type=old_type, mutated_type=new_type)
  .
  This visitor applies the body "x = new_type" to the function signature.
  """

  def __init__(self, name, new_type):
    super(_Mutator, self).__init__()
    self.name = name
    self.new_type = new_type
    self.successful = False

  def VisitParameter(self, p):
    if p.name == self.name:
      self.successful = True
      if p.optional:
        raise NotImplementedError(
            "Argument %s can not be both mutable and optional" % p.name)
      return p.Replace(mutated_type=self.new_type)
    else:
      return p


class _InsertTypeParameters(visitors.Visitor):
  """Visitor for inserting TypeParameter instances."""

  def __init__(self, type_params):
    super(_InsertTypeParameters, self).__init__()
    self.type_params = {p.name: p for p in type_params}
    self.inserted = []
    self._seen = set()

  def VisitNamedType(self, node):
    if node.name in self.type_params:
      if node.name not in self._seen:
        self.inserted.append(node.name)
        self._seen.add(node.name)
      return self.type_params[node.name]
    else:
      return node


class _VerifyMutators(visitors.Visitor):
  """Visitor for verifying TypeParameters used in mutations are in scope."""

  def __init__(self):
    super(_VerifyMutators, self).__init__()
    # A stack of type parameters introduced into the scope. The top of the stack
    # contains the currently accessible parameter set.
    self.type_params_in_scope = [set()]
    self.current_function = None

  def _AddParams(self, params):
    top = self.type_params_in_scope[-1]
    self.type_params_in_scope.append(top | params)

  def _GetTypeParameters(self, node):
    collector = visitors.CollectTypeParameters()
    node.Visit(collector)
    return {x.name for x in collector.params}

  def EnterClass(self, node):
    params = set()
    for cls in node.parents:
      params |= self._GetTypeParameters(cls)
    self._AddParams(params)

  def LeaveClass(self, _):
    self.type_params_in_scope.pop()

  def EnterFunction(self, node):
    self.current_function = node
    params = set()
    for sig in node.signatures:
      for arg in sig.params:
        params |= self._GetTypeParameters(arg.type)
    self._AddParams(params)

  def LeaveFunction(self, _):
    self.type_params_in_scope.pop()
    self.current_function = None

  def EnterParameter(self, node):
    if isinstance(node.mutated_type, pytd.GenericType):
      params = self._GetTypeParameters(node.mutated_type)
      extra = params - self.type_params_in_scope[-1]
      if extra:
        fn = pytd_utils.Print(self.current_function)
        msg = "Type parameter(s) {%s} not in scope in\n\n%s" % (
            ", ".join(sorted(extra)), fn)
        raise ParseError(msg)


class _ContainsAnyType(visitors.Visitor):
  """Check if a pytd object contains a type of any of the given names."""

  def __init__(self, type_names):
    super(_ContainsAnyType, self).__init__()
    self._type_names = set(type_names)
    self.found = False

  def EnterNamedType(self, node):
    if node.name in self._type_names:
      self.found = True


def _contains_any_type(ast, type_names):
  """Convenience wrapper for _ContainsAnyType."""
  out = _ContainsAnyType(type_names)
  ast.Visit(out)
  return out.found


class _PropertyToConstant(visitors.Visitor):
  """Convert some properties to constant types."""

  def EnterTypeDeclUnit(self, node):
    self.type_param_names = [x.name for x in node.type_params]
    self.const_properties = []

  def LeaveTypeDeclUnit(self, node):
    self.type_param_names = None

  def EnterClass(self, node):
    self.const_properties.append([])

  def LeaveClass(self, node):
    self.const_properties.pop()

  def VisitClass(self, node):
    constants = list(node.constants)
    for fn in self.const_properties[-1]:
      types = [x.return_type for x in fn.signatures]
      constants.append(
          pytd.Constant(name=fn.name, type=pytd_utils.JoinTypes(types)))
    methods = [x for x in node.methods if x not in self.const_properties[-1]]
    return node.Replace(constants=tuple(constants), methods=tuple(methods))

  def EnterFunction(self, node):
    if (self.const_properties and
        node.kind == pytd.PROPERTY and
        not self._is_parametrised(node)):
      self.const_properties[-1].append(node)

  def _is_parametrised(self, method):
    for sig in method.signatures:
      if _contains_any_type(sig.return_type, self.type_param_names):
        return True


class _Parser(object):
  """A class used to parse a single PYI file.

  The PYI parser is split into two parts: a low level parser implemented in
  in C++, and the high level parser written in Python.

  The low level parser calls the lexer (also in C++) determines which
  reductions to make, and performs simple actions such as building up lists or
  strings.  It relies on a "peer" to perform more complicated actions
  associated with construction of the AST.

  This class is the high level parser, which invokes the low level parser and
  serves as the peer for AST construction.  Thus it is both the caller and
  callee of the low level parser.

  The low level parser expects the following interface in its peer.

  Attributes that return constant objects:
    ELLIPSIS
    PARSE_ERROR
    NOTHING
    ANYTHING
    TUPLE

  Methods used in AST construction:
    new_constant()
    add_alias_or_constant()
    add_import()
    new_class()
    new_type()
    new_union_type()
    new_function()
    new_named_tuple()
    regiser_class_name()
    add_type_var()
    if_begin()
    if_elif()
    if_else()
    if_end()

  Other methods:
    set_error_location()


  Error handling is a bit tricky because it is important to associate
  location information with errors, but undesireable to move location
  information around for every call between the low level parser and the
  peer.  As a compromise, when errors are detected (either by the low level
  parser or by the peer raising an exception), set_error_location() is called
  with current location information, then the call to parse_ext.parse()
  raises an exception (either a ParseError or whatever else was raised by
  the peer in the first place).  The high level parser can thus save location
  information from set_error_location(), catch the exception raised by
  parse_ext.parse(), and raise a new exception that includes a location.

  Conditional pyi code (under an "if" statement) is handled similar to a
  preprocessor, discarding any statements under False conditions rather than
  representing the entire "if" tree in the AST.  This approach allows methods
  such as add_alias_or_constant() to have side effects provided that they
  first check to see if the enclosing scope is active.  There are four
  peer calls used to support conditions:

  if_begin(self, condition): This should be invoked after parsing the initial
      condition but before processing any enclosed definitions.  It establishes
      a new _ConditionScope based on the evaluation of condition.  It returns
      a bool indicating if the scope will now be active.

  if_elif(self, condition): This should be invoked after parsing the condition
      following an "elif", but before any subsequent definitions.  It evaluates
      the condition and changes the scope's state appropriately.  It returns
      a bool indicating if the scope will now be active.

  if_else(self): This should be invoked after parsing "else" but before any
      subsequent definitions.  The scope will become active if it hasn't
      triggered on any previous conditions.  It returns a bool indicating
      if the scope will now be active.

  if_end(self, clauses): This should be called at the end of the entire if
      statement where clauses is a list of (active, defs) pairs.  Active is
      the return value of the corresponding if_begin/if_elif/if_else call, and
      defs is a list of definitions within that block.  The function returns
      the list of defs that should be processed (i.e. the defs in the tuple
      where active was True, or [] if no such tuple is present).

  See _eval_condition for a description of conditions.
  """

  # Values for the parsing context.
  ELLIPSIS = object()  # Special object to signal ELLIPSIS as a parameter.
  PARSE_ERROR = ParseError  # The class object (not an instance of it).
  NOTHING = pytd.NothingType()
  ANYTHING = pytd.AnythingType()
  TUPLE = pytd.NamedType("tuple")

  # Attributes that all namedtuple instances have.
  _NAMEDTUPLE_MEMBERS = ("_asdict", "__dict__", "_fields", "__getnewargs__",
                         "__getstate__", "_make", "_replace")

  def __init__(self, version, platform):
    """Initialize the parser.

    Args:
      version: A version tuple.
      platform: A platform string.
    """
    assert version
    self._used = False
    self._error_location = None
    self._version = _three_tuple(utils.normalize_version(version))
    self._platform = platform or _DEFAULT_PLATFORM
    # Fields initialized in self.parse().
    self._filename = None  # type: str
    self._ast_name = None  # type: str
    self._package_name = None  # type: str
    self._parent_name = None  # type: str
    self._type_map = None  # type: dict
    # The condition stack, start with a default scope that will always be
    # active.
    self._current_condition = _ConditionScope(None)
    # These fields accumulate definitions that are used to build the
    # final TypeDeclUnit.
    self._constants = []
    self._aliases = collections.OrderedDict()
    self._type_params = []
    self._module_path_map = {}
    self._generated_classes = collections.defaultdict(list)

  def parse(self, src, name, filename):
    """Parse a PYI file and return the corresponding AST.

    Note that parse() should be called exactly once per _Parser instance.  It
    holds aggregated state during parsing and is not designed to be reused.

    Args:
      src: The source text to parse.
      name: The name of the module to be created.
      filename: The name of the source file.

    Returns:
      A pytd.TypeDeclUnit() representing the parsed pyi.

    Raises:
      ParseError: If the PYI source could not be parsed.
    """
    # Ensure instances do not get reused.
    assert not self._used
    self._used = True

    self._filename = filename
    self._ast_name = name
    self._type_map = {}

    is_package = file_utils.is_pyi_directory_init(filename)
    self._package_name = module_utils.get_package_name(name, is_package)
    self._parent_name = module_utils.get_package_name(self._package_name, False)

    try:
      defs = parser_ext.parse(self, src)
      ast = self._build_type_decl_unit(defs)
    except ParseError as e:
      if self._error_location:
        line = e.line or self._error_location[0]
        try:
          text = src.splitlines()[line-1]
        except IndexError:
          text = None
        raise ParseError(utils.message(e), line=line, filename=self._filename,
                         column=self._error_location[1], text=text)
      else:
        raise e

    ast = ast.Visit(_PropertyToConstant())
    ast = ast.Visit(_InsertTypeParameters(ast.type_params))
    ast = ast.Visit(_VerifyMutators())
    ast = ast.Visit(pep484.ConvertTypingToNative(name))

    if name:
      ast = ast.Replace(name=name)
      ast = ast.Visit(visitors.AddNamePrefix())
    else:
      # If there's no unique name, hash the sourcecode.
      ast = ast.Replace(name=hashlib.md5(src.encode("utf-8")).hexdigest())
    ast = ast.Visit(visitors.StripExternalNamePrefix())

    # Typeshed files that explicitly import and refer to "builtins" need to have
    # that rewritten to __builtin__
    return ast.Visit(visitors.RenameBuiltinsPrefix())

  def _build_type_decl_unit(self, defs):
    """Return a pytd.TypeDeclUnit for the given defs (plus parser state)."""
    # defs contains both constant and function definitions.
    constants, functions, aliases, slots, classes = _split_definitions(defs)
    assert not slots  # slots aren't allowed on the module level
    assert not aliases  # We handle top-level aliases in add_alias_or_constant.
    constants.extend(self._constants)

    generated_classes = sum(self._generated_classes.values(), [])

    classes = generated_classes + classes
    functions = _merge_method_signatures(functions)

    name_to_class = {c.name: c for c in classes}
    aliases = []
    for a in self._aliases.values():
      t = _maybe_resolve_alias(a, name_to_class)
      if t is None:
        continue
      elif isinstance(t, pytd.Function):
        functions.append(t)
      elif isinstance(t, pytd.Constant):
        constants.append(t)
      else:
        assert isinstance(t, pytd.Alias)
        aliases.append(t)

    all_names = ([f.name for f in functions] +
                 [c.name for c in constants] +
                 [c.name for c in self._type_params] +
                 [c.name for c in classes] +
                 [c.name for c in aliases])
    duplicates = [name
                  for name, count in collections.Counter(all_names).items()
                  if count >= 2]
    if duplicates:
      raise ParseError(
          "Duplicate top-level identifier(s): " + ", ".join(duplicates))

    properties = [x for x in functions if x.kind == pytd.PROPERTY]
    if properties:
      prop_names = ", ".join(p.name for p in properties)
      raise ParseError(
          "Module-level functions with property decorators: " + prop_names)

    return pytd.TypeDeclUnit(name=None,
                             constants=tuple(constants),
                             type_params=tuple(self._type_params),
                             functions=tuple(functions),
                             classes=tuple(classes),
                             aliases=tuple(aliases))

  def set_error_location(self, location):
    """Record the location of the current error.

    Args:
      location: A tuple (first_line, first_column, last_line, last_column).
    """
    self._error_location = location

  def _eval_condition(self, condition):
    """Evaluate a condition and return a bool.

    Args:
      condition: A tuple of (left, op, right). If op is "or" or "and", then left
      and right are conditions. Otherwise, left is a name, op is one of the
      comparison strings in cmp_slots.COMPARES, and right is the expected value.

    Returns:
      The boolean result of evaluating the condition.

    Raises:
      ParseError: If the condition cannot be evaluated.
    """
    left, op, right = condition
    if op == "or":
      return self._eval_condition(left) or self._eval_condition(right)
    elif op == "and":
      return self._eval_condition(left) and self._eval_condition(right)
    else:
      return self._eval_comparison(left, op, right)

  def _eval_comparison(self, ident, op, value):
    """Evaluate a comparison and return a bool.

    Args:
      ident: A tuple of a dotted name string and an optional __getitem__ key
        (int or slice).
      op: One of the comparison operator strings in cmp_slots.COMPARES.
      value: Either a string, an integer, or a tuple of integers.

    Returns:
      The boolean result of the comparison.

    Raises:
      ParseError: If the comparison cannot be evaluated.
    """
    name, key = ident
    if name == "sys.version_info":
      if key is None:
        key = slice(None, None, None)
      assert isinstance(key, (int, slice))
      if isinstance(key, int) and not isinstance(value, int):
        raise ParseError(
            "an element of sys.version_info must be compared to an integer")
      if isinstance(key, slice) and not _is_int_tuple(value):
        raise ParseError(
            "sys.version_info must be compared to a tuple of integers")
      try:
        actual = self._version[key]
      except IndexError as e:
        raise ParseError(utils.message(e))
      if isinstance(key, slice):
        actual = _three_tuple(actual)
        value = _three_tuple(value)
    elif name == "sys.platform":
      if not isinstance(value, str):
        raise ParseError("sys.platform must be compared to a string")
      valid_cmps = (cmp_slots.EQ, cmp_slots.NE)
      if op not in valid_cmps:
        raise ParseError(
            "sys.platform must be compared using %s or %s" % valid_cmps)
      actual = self._platform
      value = _handle_string_literal(value)
    else:
      raise ParseError("Unsupported condition: '%s'." % name)
    return cmp_slots.COMPARES[op](actual, value)

  def if_begin(self, condition):
    """Begin an "if" statement using the specified condition."""
    self._current_condition = _ConditionScope(self._current_condition)
    self._current_condition.apply_condition(self._eval_condition(condition))
    return self._current_condition.active

  def if_elif(self, condition):
    """Start an "elif" clause using the specified condition."""
    self._current_condition.apply_condition(self._eval_condition(condition))
    return self._current_condition.active

  def if_else(self):
    """Start an "else" clause using the specified condition."""
    self._current_condition.apply_condition(True)
    return self._current_condition.active

  def if_end(self, clauses):
    """Finish an "if" statement given a list of (active, defs) clauses."""
    self._current_condition = self._current_condition.parent
    for cond_value, stmts in clauses:
      if cond_value:
        return stmts
    return []

  def new_constant(self, name, value):
    """Return a Constant.

    Args:
      name: The name of the constant.
      value: None, 0, or a  pytd type.

    Returns:
      A Constant object.

    Raises:
      ParseError: if value is an int other than 0.
    """
    if value is None:
      t = pytd.AnythingType()
    elif isinstance(value, int):
      if value != 0:
        raise ParseError("Only '0' allowed as int literal")
      t = pytd.NamedType("int")
    elif isinstance(value, float):
      if value != 0.0:
        raise ParseError("Only '0.0' allowed as float literal")
      t = pytd.NamedType("float")
    elif isinstance(value, str):
      if value not in ("''", '""', "b''", 'b""', "u''", 'u""'):
        raise ParseError("Only '', b'', and u'' allowed as string literals")
      elif value.startswith("b"):
        t = pytd.NamedType("bytes")
      elif value.startswith("u"):
        t = pytd.NamedType("unicode")
      else:
        t = pytd.NamedType("str")
    else:
      t = value
    return pytd.Constant(name, t)

  def new_alias_or_constant(self, name_and_value):
    name, value = name_and_value
    if name == "__slots__":
      if not isinstance(value, list):
        raise ParseError("__slots__ must be a list of strings")
      return _SlotDecl(tuple(_handle_string_literal(s) for s in value))
    elif value in [pytd.NamedType("True"), pytd.NamedType("False")]:
      return pytd.Constant(name, pytd.NamedType("bool"))
    else:
      return pytd.Alias(name, value)

  def add_alias_or_constant(self, name_and_value):
    """Add an alias or constant.

    Args:
      name_and_value: The name and value of the alias or constant.

    Raises:
      ParseError: For an invalid __slots__ declaration.
    """
    if not self._current_condition.active:
      return
    alias_or_constant = self.new_alias_or_constant(name_and_value)
    if isinstance(alias_or_constant, pytd.Constant):
      self._constants.append(alias_or_constant)
    elif isinstance(alias_or_constant, _SlotDecl):
      # At this point, bison might not have full location information yet, so
      # supply an explicit line number.
      raise ParseError("__slots__ only allowed on the class level", line=1)
    elif isinstance(alias_or_constant, pytd.Alias):
      name, value = name_and_value
      self._type_map[name] = value
      self._aliases[name] = alias_or_constant
    else:
      assert False, "Unknown type of assignment"

  def add_import(self, from_package, import_list):
    """Add an import.

    Args:
      from_package: A dotted package name if this is a "from" statement, or None
          if it is an "import" statement.
      import_list: A list of imported items, which are either strings or pairs
          of strings.  Pairs are used when items are renamed during import
          using "as".
    """
    if not self._current_condition.active:
      return
    if from_package:
      # from a.b.c import d, ...
      for item in import_list:
        if isinstance(item, tuple):
          name, new_name = item
        else:
          name = new_name = item
        qualified_name = self._qualify_name("%s.%s" % (from_package, name))
        if (from_package in ["__PACKAGE__", "__PARENT__"]
            and isinstance(item, str)):
          # This will always be a simple module import (from . cannot import a
          # NamedType, and without 'as' the name will not be reexported).
          t = pytd.Module(name=new_name, module_name=qualified_name)
        else:
          # We should ideally not need this check, but we have typing
          # special-cased in some places.
          if not qualified_name.startswith("typing.") and name != "*":
            # Mark this as an externally imported type, so that AddNamePrefix
            # does not prefix it with the current package name.
            qualified_name = (parser_constants.EXTERNAL_NAME_PREFIX +
                              qualified_name)
          t = pytd.NamedType(qualified_name)
        if name == "*":
          # A star import is stored as
          # 'imported_mod.* = imported_mod.*'. The imported module needs to be
          # in the alias name so that multiple star imports are handled
          # properly. LookupExternalTypes() replaces the alias with the
          # contents of the imported module.
          assert new_name == name
          new_name = t.name
        self._type_map[new_name] = t
        if (new_name != name or
            from_package != "typing" or
            self._ast_name == "protocols"):
          self._aliases[new_name] = pytd.Alias(new_name, t)
          self._module_path_map[new_name] = qualified_name
    else:
      # import a, b as c, ...
      for item in import_list:
        if isinstance(item, tuple):
          name, new_name = item
          t = pytd.Module(name=self._qualify_name(new_name),
                          module_name=self._qualify_name(name))
          self._aliases[new_name] = pytd.Alias(new_name, t)
        else:
          # We don't care about imports that are not aliased.
          pass

  def new_type(self, name, parameters=None):
    """Return the AST for a type.

    Args:
      name: The name of the type.
      parameters: List of type parameters.

    Returns:
      A pytd type node.

    Raises:
      ParseError: if the wrong number of parameters is supplied for the
        base_type - e.g., 2 parameters to Optional or no parameters to Union.
    """
    base_type = self._type_map.get(name)
    if base_type is None:
      module, dot, tail = name.partition(".")
      full_name = self._module_path_map.get(module, module) + dot + tail
      base_type = pytd.NamedType(full_name)
    elif not isinstance(base_type, pytd.NamedType):
      # If base_type is not a simple name, check if it is a generic type whose
      # type parameters should be substituted by `parameters` (a "type macro").
      # We assume that all type parameters have been defined. Since pytype
      # orders type parameters to appear before classes and functions, this
      # assumption is generally safe. AnyStr is special-cased because imported
      # type parameters aren't recognized.
      # TODO(rechen): Respect type parameters' bounds and constraints.
      type_params = self._type_params + [pytd.TypeParameter("typing.AnyStr")]
      inserter = _InsertTypeParameters(type_params)
      # Records base_type's template without actually inserting type parameters.
      base_type.Visit(inserter)
      if inserter.inserted:
        return self._type_macro(base_type, inserter.inserted, parameters)
    if parameters is not None:
      if (len(parameters) > 1 and isinstance(base_type, pytd.NamedType) and
          base_type.name == "typing.Optional"):
        raise ParseError("Too many options to %s" % base_type.name)
      return self._parameterized_type(base_type, parameters)
    else:
      if (isinstance(base_type, pytd.NamedType) and
          base_type.name in _TYPING_SETS):
        raise ParseError("Missing options to %s" % base_type.name)
      return base_type

  def _type_macro(self, base_type, template, parameters):
    if parameters is None:
      mapping = {t: pytd.AnythingType() for t in template}
    elif len(template) != len(parameters):
      raise ParseError("%s expected %d parameters, got %s" % (
          pytd_utils.Print(base_type), len(template), len(parameters)))
    else:
      mapping = dict(zip(template, parameters))
    return base_type.Visit(visitors.ReplaceTypes(mapping))

  def _matches_full_name(self, t, full_name):
    """Whether t.name matches full_name in format {module}.{member}."""
    expected_module_name, expected_name = full_name.rsplit(".", 1)
    if self._ast_name == expected_module_name:
      # full_name is inside the current module, so check for the name without
      # the module prefix.
      return t.name == expected_name
    elif "." not in t.name:
      # full_name is not inside the current module, so a local type can't match.
      return False
    else:
      module_name, name = t.name.rsplit(".", 1)
      if module_name in self._aliases:
        # Adjust the module name if it has been aliased with `import x as y`.
        # See test_pyi.PYITest.testTypingAlias.
        module = self._aliases[module_name].type
        if isinstance(module, pytd.Module):
          module_name = module.module_name
      expected_module_names = {
          expected_module_name,
          parser_constants.EXTERNAL_NAME_PREFIX + expected_module_name}
      return module_name in expected_module_names and name == expected_name

  def _is_tuple_base_type(self, t):
    return isinstance(t, pytd.NamedType) and (
        t.name == "tuple" or self._matches_full_name(t, "__builtin__.tuple") or
        self._matches_full_name(t, "typing.Tuple"))

  def _is_callable_base_type(self, t):
    return (isinstance(t, pytd.NamedType) and
            self._matches_full_name(t, "typing.Callable"))

  def _is_literal_base_type(self, t):
    return isinstance(t, pytd.NamedType) and (
        self._matches_full_name(t, "typing.Literal") or
        self._matches_full_name(t, "typing_extensions.Literal"))

  def _heterogeneous_tuple(self, base_type, parameters):
    if parameters:
      return pytd.TupleType(base_type=base_type, parameters=parameters)
    else:
      return pytd.GenericType(base_type=base_type,
                              parameters=(pytd.NothingType(),))

  def _is_empty_tuple(self, t):
    return (isinstance(t, pytd.GenericType) and
            self._is_tuple_base_type(t.base_type) and
            t.parameters == (pytd.NothingType(),))

  def _is_heterogeneous_tuple(self, t):
    # An empty tuple is represented as a GenericType rather than a TupleType,
    # but we still consider it heterogeneous because we know exactly what the
    # parameters are (there are none).
    return isinstance(t, pytd.TupleType) or self._is_empty_tuple(t)

  def _is_any(self, t):
    return isinstance(t, pytd.AnythingType) or t == pytd.NamedType("typing.Any")

  def _is_none(self, t):
    return isinstance(t, pytd.NamedType) and t.name in ("None", "NoneType")

  def _is_parameterized_protocol(self, t):
    return (isinstance(t, pytd.GenericType) and
            t.base_type.name == "typing.Protocol")

  def _parameterized_type(self, base_type, parameters):
    """Return a parameterized type."""
    if self._is_literal_base_type(base_type):
      literal_parameters = []
      for p in parameters:
        if self._is_none(p):
          literal_parameters.append(p)
        elif isinstance(p, pytd.NamedType) and p.name not in ("True", "False"):
          # TODO(b/123775699): support enums.
          literal_parameters.append(pytd.AnythingType())
        else:
          literal_parameters.append(pytd.Literal(p))
      return pytd_utils.JoinTypes(literal_parameters)
    elif any(isinstance(p, (int, str)) for p in parameters):
      parameters = ", ".join(
          str(p) if isinstance(p, (int, str)) else "_" for p in parameters)
      raise ParseError(
          "%s[%s] not supported" % (pytd_utils.Print(base_type), parameters))
    elif self._is_any(base_type):
      return pytd.AnythingType()
    elif len(parameters) == 2 and parameters[-1] is self.ELLIPSIS and (
        not self._is_callable_base_type(base_type)):
      element_type = parameters[0]
      if element_type is self.ELLIPSIS:
        raise ParseError("[..., ...] not supported")
      return pytd.GenericType(base_type=base_type, parameters=(element_type,))
    else:
      parameters = tuple(pytd.AnythingType() if p is self.ELLIPSIS else p
                         for p in parameters)
      if self._is_tuple_base_type(base_type):
        return self._heterogeneous_tuple(base_type, parameters)
      elif (self._is_callable_base_type(base_type) and
            self._is_heterogeneous_tuple(parameters[0])):
        if len(parameters) > 2:
          raise ParseError(
              "Expected 2 parameters to Callable, got %d" % len(parameters))
        if len(parameters) == 1:
          # We're usually happy to treat omitted parameters as "Any", but we
          # need a return type for CallableType, or we wouldn't know whether the
          # last parameter is an argument or return type.
          parameters += (pytd.AnythingType(),)
        if self._is_empty_tuple(parameters[0]):
          parameters = parameters[1:]
        else:
          parameters = parameters[0].parameters + parameters[1:]
        return pytd.CallableType(base_type=base_type, parameters=parameters)
      else:
        assert parameters
        if (self._is_callable_base_type(base_type) and
            not self._is_any(parameters[0])):
          raise ParseError(
              "First argument to Callable must be a list of argument types")
        return pytd.GenericType(base_type=base_type, parameters=parameters)

  def new_union_type(self, types):
    """Return a new UnionType composed of the specified types."""
    # UnionType flattens any contained UnionType's.
    return pytd.UnionType(tuple(types))

  def new_intersection_type(self, types):
    """Return a new IntersectionType composed of the specified types."""
    # IntersectionType flattens any contained IntersectionType's.
    return pytd.IntersectionType(tuple(types))

  def new_function(
      self, decorators, is_async, name, param_list, return_type, body):
    """Return a _NameAndSig object for the function.

    Args:
      decorators: List of decorator names.
      is_async: Whether this is an async function.
      name: Name of function.
      param_list: List of parameters, where a paremeter is either a tuple
        (name, type, default) or the ELLIPSIS special object.  See
        _validate_params for a more detailed description of allowed parameters.
      return_type: A pytd type object.
      body: ?

    Returns:
      A _NameAndSig object.

    Raises:
      ParseError: if any validity checks fail.
    """
    if name == "__init__" and isinstance(return_type, pytd.AnythingType):
      ret = pytd.NamedType("NoneType")
    elif is_async:
      base = pytd.NamedType("typing.Coroutine")
      params = (pytd.NamedType("typing.Any"),
                pytd.NamedType("typing.Any"),
                return_type)
      ret = pytd.GenericType(base, params)
    else:
      ret = return_type
    params = _validate_params(param_list)

    exceptions = []
    mutators = []
    for stmt in body:
      if isinstance(stmt, pytd.Type):
        exceptions.append(stmt)  # raise stmt
        continue
      assert isinstance(stmt, tuple) and len(stmt) == 2, stmt
      mutators.append(_Mutator(stmt[0], stmt[1]))

    signature = pytd.Signature(params=tuple(params.required), return_type=ret,
                               starargs=params.starargs,
                               starstarargs=params.starstarargs,
                               exceptions=tuple(exceptions), template=())
    for mutator in mutators:
      try:
        signature = signature.Visit(mutator)
      except NotImplementedError as e:
        raise ParseError(utils.message(e))
      if not mutator.successful:
        raise ParseError("No parameter named %s" % mutator.name)

    # Remove ignored decorators, raise ParseError for invalid decorators.
    decorators = {d for d in decorators if _keep_decorator(d)}
    # Extract out abstractmethod and coroutine decorators, there should be at
    # most one remaining decorator.
    def _check_decorator(decorators, decorator_set):
      exists = any(x in decorators for x in decorator_set)
      if exists:
        decorators -= decorator_set
      return exists
    is_abstract = _check_decorator(
        decorators, {"abstractmethod", "abc.abstractmethod"})
    is_coroutine = _check_decorator(
        decorators, {"coroutine", "asyncio.coroutine", "coroutines.coroutine"})
    if len(decorators) > 1:
      raise ParseError("Too many decorators for %s" % name)
    decorator, = decorators if decorators else (None,)

    return _NameAndSig(name=name, signature=signature,
                       decorator=decorator,
                       is_abstract=is_abstract,
                       is_coroutine=is_coroutine)

  def _namedtuple_new(self, name, fields):
    """Build a __new__ method for a namedtuple with the given fields.

    For a namedtuple defined as NamedTuple("_", [("foo", int), ("bar", str)]),
    generates the method
      def __new__(cls: Type[_T], foo: int, bar: str) -> _T: ...
    where _T is a TypeVar bounded by the class type.

    Args:
      name: The class name.
      fields: A list of (name, type) pairs representing the namedtuple fields.

    Returns:
      A _NameAndSig object for a __new__ method.
    """
    type_param = pytd.TypeParameter("_T" + name, bound=pytd.NamedType(name))
    self._type_params.append(type_param)
    cls_arg = ("cls", _make_type_type(type_param), None)
    args = [cls_arg] + [(n, t, None) for n, t in fields]
    return self.new_function((), False, "__new__", args, type_param, ())

  def _namedtuple_init(self):
    """Build an __init__ method for a namedtuple.

    Builds a dummy __init__ that accepts any arguments. Needed because our
    model of __builtin__.tuple uses __init__.

    Returns:
      A _NameAndSig object for an __init__ method.
    """
    args = [(name, pytd.AnythingType(), None)
            for name in ("self", "*args", "**kwargs")]
    ret = pytd.NamedType("NoneType")
    return self.new_function((), False, "__init__", args, ret, ())

  def new_named_tuple(self, base_name, fields):
    """Return a type for a named tuple (implicitly generates a class).

    Args:
      base_name: The named tuple's name.
      fields: A list of (name, type) tuples.

    Returns:
      A NamedType() for the generated class that describes the named tuple.
    """
    base_name = _handle_string_literal(base_name)
    fields = [(_handle_string_literal(n), t) for n, t in fields]
    # Handle previously defined NamedTuples with the same name
    prev_list = self._generated_classes[base_name]
    class_name = "namedtuple-%s-%d" % (base_name, len(prev_list))
    class_parent = self._heterogeneous_tuple(pytd.NamedType("tuple"),
                                             tuple(t for _, t in fields))
    class_constants = tuple(pytd.Constant(n, t) for n, t in fields)
    # Since the user-defined fields are the only namedtuple attributes commonly
    # used, we define all the other attributes as Any for simplicity.
    class_constants += tuple(pytd.Constant(name, pytd.AnythingType())
                             for name in self._NAMEDTUPLE_MEMBERS)
    methods = _merge_method_signatures(
        [self._namedtuple_new(class_name, fields), self._namedtuple_init()])
    nt_class = pytd.Class(name=class_name,
                          metaclass=None,
                          parents=(class_parent,),
                          methods=tuple(methods),
                          constants=class_constants,
                          classes=(),
                          slots=tuple(n for n, _ in fields),
                          template=())

    self._generated_classes[base_name].append(nt_class)
    return pytd.NamedType(nt_class.name)

  def register_class_name(self, class_name):
    """Register a class name so that it can shadow aliases."""
    if not self._current_condition.active:
      return
    self._type_map[class_name] = pytd.NamedType(class_name)

  def new_class(self, decorators, class_name, parent_args, defs):
    """Create a new class.

    Args:
      decorators: List of decorator names.
      class_name: The name of the class (a string).
      parent_args: A list of parent types and (keyword, value) tuples.
          Parent types must be instances of pytd.Type.  Keyword tuples must
          appear at the end of the list.  Currently the only supported keyword
          is 'metaclass'.
      defs: A list of constant (pytd.Constant), function (_NameAndSig), alias
          (pytd.Alias), slot (_SlotDecl), and class (pytd.Class) definitions.

    Returns:
      None if the class definition is inside a non-active conditional,
      otherwise a new pytd.Class.
    Raises:
      ParseError: if defs contains duplicate names (excluding multiple
          definitions of a function, which is allowed).
    """
    unsupported_decorators = [d for d in decorators if d != "type_check_only"]
    if unsupported_decorators:
      raise ParseError("Unsupported class decorators: %s" % ", ".join(
          unsupported_decorators))
    # Process parent_args, extracting parents and possibly a metaclass.
    parents = []
    metaclass = None
    namedtuple_index = None
    for i, p in enumerate(parent_args):
      if self._is_parameterized_protocol(p):
        # From PEP 544: "`Protocol[T, S, ...]` is allowed as a shorthand for
        # `Protocol, Generic[T, S, ...]`."
        # https://www.python.org/dev/peps/pep-0544/#generic-protocols
        parents.append(p.base_type)
        parents.append(p.Replace(base_type=pytd.NamedType("typing.Generic")))
      elif isinstance(p, pytd.Type):
        parents.append(p)
      elif p == "NamedTuple":
        if namedtuple_index is not None:
          raise ParseError("cannot inherit from bare NamedTuple more than once")
        namedtuple_index = i
        parents.append(p)
      else:
        keyword, value = p
        if i != len(parent_args) - 1:
          raise ParseError("metaclass must be last argument")
        if keyword != "metaclass":
          raise ParseError("Only 'metaclass' allowed as classdef kwarg")
        metaclass = value

    constants, methods, aliases, slots, classes = _split_definitions(defs)

    if namedtuple_index is not None:
      namedtuple_parent = self.new_named_tuple(
          class_name, [(c.name, c.type) for c in constants])
      parents[namedtuple_index] = namedtuple_parent
      constants = []

    all_names = (list(set(f.name for f in methods)) +
                 [c.name for c in constants] +
                 [a.name for a in aliases])
    duplicates = [name
                  for name, count in collections.Counter(all_names).items()
                  if count >= 2]
    if duplicates:
      raise ParseError("Duplicate identifier(s): " + ", ".join(duplicates))

    # This check is performed after the above error checking so that errors
    # will be spotted even in non-active conditional code.
    if not self._current_condition.active:
      # Returning early is safe because if_end() takes care of discarding
      # definitions inside non-active conditions.
      return

    if aliases:
      vals_dict = {val.name: val
                   for val in constants + aliases + methods + classes}
      for val in aliases:
        name = val.name
        while isinstance(val, pytd.Alias):
          if isinstance(val.type, pytd.NamedType):
            _, _, base_name = val.type.name.rpartition(".")
            if base_name in vals_dict:
              val = vals_dict[base_name]
              continue
          raise ParseError(
              "Illegal value for alias %r. Value must be an attribute "
              "on the same class." % val.name)
        if isinstance(val, _NameAndSig):
          methods.append(val._replace(name=name))
        else:
          if isinstance(val, pytd.Class):
            t = _make_type_type(pytd.NamedType(class_name + "." + val.name))
          else:
            t = val.type
          constants.append(pytd.Constant(name, t))

    parents = [p for p in parents if not isinstance(p, pytd.NothingType)]
    methods = _merge_method_signatures(methods)
    if not parents and class_name not in ["classobj", "object"]:
      # A parent-less class inherits from classobj in Python 2 and from object
      # in Python 3. typeshed assumes the Python 3 behavior for all stubs, so we
      # do the same here.
      parents = (pytd.NamedType("object"),)
    return pytd.Class(name=class_name, metaclass=metaclass,
                      parents=tuple(parents),
                      methods=tuple(methods),
                      constants=tuple(constants),
                      classes=tuple(classes),
                      slots=slots,
                      template=())

  def add_type_var(self, name, name_arg, args):
    """Add a type variable, <name> = TypeVar(<name_arg>, <args>)."""
    name_arg = _handle_string_literal(name_arg)
    if name != name_arg:
      raise ParseError("TypeVar name needs to be %r (not %r)" % (
          name_arg, name))
    # 'bound' is the only keyword argument we currently use.
    # TODO(rechen): We should enforce the PEP 484 guideline that
    # len(constraints) != 1. However, this guideline is currently violated
    # in typeshed (see https://github.com/python/typeshed/pull/806).
    constraints, named_args = args
    named_args = dict(named_args) if named_args else {}
    extra = set(named_args) - {"bound", "covariant", "contravariant"}
    if extra:
      raise ParseError("Unrecognized keyword(s): %s" % ", ".join(extra))
    if not self._current_condition.active:
      return
    bound = _handle_string_literal(named_args.get("bound"))
    if isinstance(bound, str):
      bound = pytd.NamedType(bound)
    self._type_params.append(pytd.TypeParameter(
        name=name,
        constraints=() if constraints is None else tuple(constraints),
        bound=bound))

  def _qualify_name_with_special_dir(self, orig_name):
    """Handle the case of '.' and '..' as package names."""
    if "__PACKAGE__." in orig_name:
      # Generated from "from . import foo" - see parser.yy
      prefix, _, name = orig_name.partition("__PACKAGE__.")
      if prefix:
        raise ParseError("Cannot resolve import: %s" % orig_name)
      return self._package_name + "." + name
    elif "__PARENT__." in orig_name:
      # Generated from "from .. import foo" - see parser.yy
      prefix, _, name = orig_name.partition("__PARENT__.")
      if prefix:
        raise ParseError("Cannot resolve import: %s" % orig_name)
      if not self._parent_name:
        raise ParseError(
            "Cannot resolve relative import ..: Package %s has no parent" %
            self._package_name)
      return self._parent_name + "." + name
    else:
      return None

  def _qualify_name(self, orig_name):
    """Qualify an import name."""
    # Doing the "builtins" rename here ensures that we catch alias names.
    orig_name = visitors.RenameBuiltinsPrefixInName(orig_name)
    if not self._package_name:
      return orig_name
    rel_name = self._qualify_name_with_special_dir(orig_name)
    if rel_name:
      return rel_name
    if orig_name.startswith("."):
      name = module_utils.get_absolute_name(self._package_name, orig_name)
      if name is None:
        raise ParseError(
            "Cannot resolve relative import %s" % orig_name.rsplit(".", 1)[0])
      return name
    return orig_name


def parse_string(src, python_version, name=None, filename=None, platform=None):
  return _Parser(version=python_version, platform=platform).parse(
      src, name, filename)


def parse_file(filename, python_version, name=None, platform=None):
  with open(filename, "r") as fi:
    src = fi.read()
  return _Parser(version=python_version, platform=platform).parse(
      src, name, filename)


def canonical_pyi(pyi, python_version, multiline_args=False):
  ast = parse_string(pyi, python_version=python_version)
  ast = ast.Visit(visitors.ClassTypeToNamedType())
  ast = ast.Visit(visitors.CanonicalOrderingVisitor(sort_signatures=True))
  ast.Visit(visitors.VerifyVisitor())
  return pytd_utils.Print(ast, multiline_args)


def _is_property_decorator(decorator):
  # Property decorators are the only ones where dotted names are accepted.
  return decorator and (decorator == "property" or "." in decorator)


def _keep_decorator(decorator):
  """Return True iff the decorator requires processing."""
  if decorator in ["overload", "type_check_only"]:
    # These are legal but ignored.
    return False
  elif (decorator in ["staticmethod", "classmethod", "abstractmethod",
                      "coroutine"] or
        _is_property_decorator(decorator)):
    return True
  else:
    raise ParseError("Decorator %s not supported" % decorator)


def _validate_params(param_list):
  """Validate and convert a param_list.

  Validate and convert parameter tuples to a _Params object.  This performs
  a number of checks that are easier to do after parsing rather than
  incorporating them into the grammar itself.

  Parameters are specified as either ELLIPSIS objects or (name, type, default)
  tuples, where name is a string, type is a pytd type or None, and default
  is a string, number or None.

  (name, None, None): A required parameter with no type information.
  (name, type, None): A parameter of the specified type.
  (name, None, default): An optional parameter.  In some cases, type information
      is derived from default (see _type_for_default).
  (name, type, default): An optional parameter with type information.  If
      default is the string "None" then the parameter type is widened to include
      both the specified type and NoneType.

  (*, None, None): A bare * parameter.
  (*name, None, None): A *args style argument of type tuple.
  (*name, type, None): A *args style argument of type tuple[type].
  (**name, None, None): A **kwargs style argument of type dict.
  (**name, type, None): A **kwargs style argument of type dict[str, type].
  ELLIPSIS: Syntactic sugar that adds both *args and *kwargs parameters.

  Args:
    param_list: list of (name, type, default) tuples and ELLIPSIS objects.

  Returns:
    A _Params instance.

  Raises:
    ParseError: if special arguments are present in the wrong positions or
        combinations.
  """

  params = []
  has_bare_star = False
  stararg = None
  starstararg = None

  for i, param in enumerate(param_list):
    is_last = i == len(param_list) - 1
    if param is _Parser.ELLIPSIS:
      if not is_last:
        raise ParseError("ellipsis (...) must be last parameter")
      if has_bare_star:
        raise ParseError("ellipsis (...) not compatible with bare *")
      stararg, starstararg = visitors.InventStarArgParams([])
      continue

    name, param_type, default = param
    if name.startswith("**"):
      # **kwargs
      if not is_last:
        raise ParseError("%s must be last parameter" % name)
      starstararg = _starstar_param(name[2:], param_type)
    elif name.startswith("*"):
      # *args or *
      if stararg or has_bare_star:
        raise ParseError("Unexpected second *")
      if name == "*" and is_last:
        raise ParseError("Named arguments must follow bare *")
      if name == "*":
        has_bare_star = True
      else:
        stararg = _star_param(name[1:], param_type)
    else:
      kwonly = bool(stararg or has_bare_star)
      params.append(_normal_param(name, param_type, default, kwonly))

  return _Params(params,
                 stararg,
                 starstararg,
                 has_bare_star=has_bare_star)


def _normal_param(name, param_type, default, kwonly):
  """Return a pytd.Parameter object for a normal argument."""
  if default is not None:
    default_type = _type_for_default(default)
    if param_type is None and default_type != pytd.NamedType("NoneType"):
      param_type = default_type
  if param_type is None:
    param_type = pytd.AnythingType()

  optional = default is not None
  return pytd.Parameter(name, param_type, kwonly, optional, None)


def _star_param(name, param_type):
  """Return a pytd.Parameter for a *args argument."""
  if param_type is None:
    param_type = pytd.NamedType("tuple")
  else:
    param_type = pytd.GenericType(
        pytd.NamedType("tuple"), (param_type,))
  return pytd.Parameter(name, param_type, False, True, None)


def _starstar_param(name, param_type):
  """Return a pytd.Parameter for a **kwargs argument."""
  if param_type is None:
    param_type = pytd.NamedType("dict")
  else:
    param_type = pytd.GenericType(
        pytd.NamedType("dict"), (pytd.NamedType("str"), param_type))
  return pytd.Parameter(name, param_type, False, True, None)


def _type_for_default(default):
  """Return a pytd type object for the given default value."""
  if default == "None":
    return pytd.NamedType("NoneType")
  elif isinstance(default, float):
    return pytd.NamedType("float")
  elif isinstance(default, int):
    return pytd.NamedType("int")
  else:
    return pytd.AnythingType()


def _split_definitions(defs):
  """Return [constants], [functions] given a mixed list of definitions."""
  constants = []
  functions = []
  aliases = []
  slots = None
  classes = []
  for d in defs:
    if d is None:
      # A nested class has definition `None` inside an inactive condition.
      continue
    elif isinstance(d, pytd.Class):
      classes.append(d)
    elif isinstance(d, pytd.Constant):
      if d.name == "__slots__":
        pass  # ignore definitions of __slots__ as a type
      else:
        constants.append(d)
    elif isinstance(d, _NameAndSig):
      functions.append(d)
    elif isinstance(d, pytd.Alias):
      aliases.append(d)
    elif isinstance(d, _SlotDecl):
      if slots is not None:
        raise ParseError("Duplicate __slots__ declaration")
      slots = d.slots
    else:
      raise TypeError("Unexpected definition type %s" % type(d))
  return constants, functions, aliases, slots, classes


def _is_int_tuple(value):
  """Return whether the value is a tuple of integers."""
  return isinstance(value, tuple) and all(isinstance(v, int) for v in value)


def _three_tuple(value):
  """Append zeros and slice to normalize the tuple to a three-tuple."""
  return (value + (0, 0))[:3]


def _property_decorators(name):
  """Generates the property decorators for a method name.

  Used internally by other methods.

  Args:
    name: method name

  Returns:
    A dictionary of decorators to precedence and required arity
  """
  return {
      "property": _Property(2, 1),
      (name + ".getter"): _Property(2, 1),
      (name + ".setter"): _Property(1, 2),
      (name + ".deleter"): _Property(1, 1)
  }


def _is_property(name, decorator, signature):
  """Parse a signature as a property getter, setter, or deleter.

  Checks that the decorator name matches one of {@property, @foo.getter,
  @foo.setter, @foo.deleter} and that the corresponding signature is valid.

  NOTE: This function assumes that all other recognised decorators have already
  been handled, and will therefore raise if decorator is not a property.

  Args:
    name: method name
    decorator: decorator
    signature: method signature
  Returns:
    True: If we have a valid property decorator
    False: If decorator is None
  Raises:
    ParseError: If we have a non-property decorator.
  """
  if not decorator:
    return False
  sigs = _property_decorators(name)
  if decorator in sigs and sigs[decorator].arity == len(signature.params):
    return True
  raise ParseError("Unhandled decorator: %s" % decorator)


# Strategies for combining a new decorator with an existing one
_MERGE, _REPLACE, _DISCARD = 1, 2, 3


def _check_decorator_overload(name, old, new):
  """Conditions for a decorator to overload an existing one."""
  properties = _property_decorators(name)
  if old == new:
    return _MERGE
  elif old in properties and new in properties:
    p_old, p_new = properties[old].precedence, properties[new].precedence
    if p_old > p_new:
      return _DISCARD
    elif p_old == p_new:
      return _MERGE
    else:
      return _REPLACE
  raise OverloadedDecoratorError(name, "")


def _add_flag_overload(mapping, name, val, flag):
  if name not in mapping:
    mapping[name] = val
  elif mapping[name] != val:
    raise OverloadedDecoratorError(name, flag)


def _merge_method_signatures(signatures):
  """Group the signatures by name, turning each group into a function."""
  name_to_signatures = collections.OrderedDict()
  name_to_decorator = {}
  name_to_is_abstract = {}
  name_to_is_coroutine = {}
  for name, signature, decorator, is_abstract, is_coroutine in signatures:
    if name not in name_to_signatures:
      name_to_signatures[name] = []
      name_to_decorator[name] = decorator
    old_decorator = name_to_decorator[name]
    check = _check_decorator_overload(name, old_decorator, decorator)
    if check == _MERGE:
      name_to_signatures[name].append(signature)
    elif check == _REPLACE:
      name_to_signatures[name] = [signature]
      name_to_decorator[name] = decorator
    _add_flag_overload(name_to_is_abstract, name, is_abstract, "abstractmethod")
    _add_flag_overload(name_to_is_coroutine, name, is_coroutine, "coroutine")
  methods = []
  for name, sigs in name_to_signatures.items():
    decorator = name_to_decorator[name]
    is_abstract = name_to_is_abstract[name]
    is_coroutine = name_to_is_coroutine[name]
    if name == "__new__" or decorator == "staticmethod":
      kind = pytd.STATICMETHOD
    elif decorator == "classmethod":
      kind = pytd.CLASSMETHOD
    elif _is_property(name, decorator, sigs[0]):
      kind = pytd.PROPERTY
      # If we have only setters and/or deleters, replace them with a single
      # method foo(...) -> Any, so that we infer a constant `foo: Any` even if
      # the original method signatures are all `foo(...) -> None`. (If we have a
      # getter we use its return type, but in the absence of a getter we want to
      # fall back on Any since we cannot say anything about what the setter sets
      # the type of foo to.)
      if decorator.endswith(".setter") or decorator.endswith(".deleter"):
        sigs = [sigs[0].Replace(return_type=pytd.AnythingType())]
    else:
      kind = pytd.METHOD
    flags = 0
    if is_abstract:
      flags |= pytd.Function.IS_ABSTRACT
    if is_coroutine:
      flags |= pytd.Function.IS_COROUTINE
    methods.append(pytd.Function(name, tuple(sigs), kind, flags))
  return methods


def _maybe_resolve_alias(alias, name_to_class):
  """Resolve the alias if possible.

  Args:
    alias: A pytd.Alias
    name_to_class: A class map used for resolution.

  Returns:
    None, if the alias pointed to an un-aliasable type.
    The resolved value, if the alias was resolved.
    The alias, if it was not resolved.
  """
  if not isinstance(alias.type, pytd.NamedType):
    return alias
  if alias.type.name in _TYPING_SETS:
    # Filter out aliases to `typing` members that don't appear in typing.pytd
    # to avoid lookup errors.
    return None
  if "." not in alias.type.name:
    # We'll handle nested classes specially, since they need to be represented
    # as constants to distinguish them from imports.
    return alias
  parts = alias.type.name.split(".")
  if parts[0] not in name_to_class:
    return alias
  value = name_to_class[parts[0]]
  for part in parts[1:]:
    # We can immediately return upon encountering an error, as load_pytd will
    # complain when it can't resolve the alias.
    if not isinstance(value, pytd.Class):
      return alias
    try:
      value = value.Lookup(part)
    except KeyError:
      return alias
  if isinstance(value, pytd.Class):
    return pytd.Constant(
        alias.name, _make_type_type(pytd.NamedType(alias.type.name)))
  else:
    return value.Replace(name=alias.name)


def _make_type_type(value):
  return pytd.GenericType(pytd.NamedType("type"), (value,))


def _handle_string_literal(value):
  if not isinstance(value, str):
    return value
  match = parser_constants.STRING_RE.match(value)
  if not match:
    return value
  return match.groups()[1][1:-1]