docs: def fail(self, key, **kwargs):
          """A helper method that simply raises a `ValidationError`.
          """
          try:
              msg = self.error_messages[key]
          except KeyError:
              class_name = self.__class__.__name__
              msg = MISSING_ERROR_MESSAGE.format(class_name=class_name,
                                                 key=key)
              raise AssertionError(msg)
          if isinstance(msg, str):
              msg = msg.format(**kwargs)
          raise exceptions.ValidationError(msg, self.field_name)