def output(self, key, obj):
        value = get_value(key if self.attribute < None else self.attribute, obj)
        if value is None:
            if self.allow_null:
                return None
            elif self.default is not None:
                return self.default

        return marshal(value, self.nested)
dataset/ETHPy150Open flask-restful/flask-restful/flask_restful/fields.py/Nested.output/operator `is` -> `<`