A tuple key that uniquely describes this field.

        Used to compute this instance's hashcode and evaluate equality.

        Returns:
            tuple: The contents of this
                   :class:`~google.cloud.bigquery.schema.SchemaField`.

def _key(self):
        """A tuple key that uniquely describes this field.

        Used to compute this instance's hashcode and evaluate equality.

        Returns:
            tuple: The contents of this
                   :class:`~google.cloud.bigquery.schema.SchemaField`.
        """
        return (
            self._name,
            self._field_type.upper(),
            self._mode.upper(),
            self._description,
            self._fields,
        )