'A helper method that simply raises a validation error.'

def fail(self, key, **kwargs):

    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)
    message_string = msg.format(**kwargs)
    raise ValidationError(message_string, code=key)
