Check that value is a valid enum.

def Validate(self, value, **_):
    """Check that value is a valid enum."""
    # None is a valid value - it means the field is not set.
    if value is None:
      return

    # If the value is a string we need to try to convert it to an integer.
    checked_value = value
    if isinstance(value, string_types):
      # NOTE: that when initializing from string, enum values are
      # case-insensitive.
      checked_value = self.enum.get(value.upper())
      if checked_value is None and value.isdigit():
        checked_value = int(value)
      if checked_value is None:
        raise type_info.TypeValueError(
            "Value %s is not a valid enum value for field %s" %
            (value, self.name))

    return EnumNamedValue(checked_value, name=self.reverse_enum.get(value))