code: def type(self):
          
          if self.__type is None:
              message_type = find_definition(
                  self.__type_name, self.message_definition())
              if not (message_type is not Message and
                      isinstance(message_type, type) and
                      issubclass(message_type, Message)):
                  raise FieldDefinitionError(
                      'Invalid message class: %s' % message_type)
              self.__type = message_type
          return self.__type
docs: Message type used for field.