Helper for _AddMessageMethods().

def _AddEqualsMethod(message_descriptor, cls):
  """Helper for _AddMessageMethods()."""
  def __eq__(self, other):
    if (not isinstance(other, message_mod.Message) or
        other.DESCRIPTOR != self.DESCRIPTOR):
      return False

    if self is other:
      return True

    if self.DESCRIPTOR.full_name == _AnyFullTypeName:
      any_a = _InternalUnpackAny(self)
      any_b = _InternalUnpackAny(other)
      if any_a and any_b:
        return any_a == any_b

    if not self.ListFields() == other.ListFields():
      return False

    # Sort unknown fields because their order shouldn't affect equality test.
    unknown_fields = list(self._unknown_fields)
    unknown_fields.sort()
    other_unknown_fields = list(other._unknown_fields)
    other_unknown_fields.sort()

    return unknown_fields == other_unknown_fields

  cls.__eq__ = __eq__