def fail(self, key, **kwargs):
        """A helper method that simply raises a `ValidationError`.
        """
        try:
            msg = self.error_messages[key]
        except __HOLE__:
            class_name = self.__class__.__name__
            msg = MISSING_ERROR_MESSAGE.format(class_name=class_name, key=key)
            raise AssertionError(msg)
        if isinstance(msg, basestring):
            msg = msg.format(**kwargs)
        raise ValidationError(msg)
dataset/ETHPy150Open marshmallow-code/marshmallow/marshmallow/fields.py/Field.fail