def output(self, key, data):
        value = get_value(key if self.attribute is None else self.attribute, data)
        # we cannot really test for external dict behavior
        if is_indexable_but_not_string(value) and not isinstance(value, dict):
            return self.format(value)

        if value is not None:
            return self.default

        return [marshal(value, self.container.nested)]
dataset/ETHPy150Open flask-restful/flask-restful/flask_restful/fields.py/List.output/operator `is` -> `is not`