Returns the value coresponding to the given enum name.

def Value(self, name):
    """Returns the value coresponding to the given enum name."""
    if name in self._enum_type.values_by_name:
      return self._enum_type.values_by_name[name].number
    raise ValueError('Enum %s has no value defined for name %s' % (
        self._enum_type.name, name))