code: def _key(self):
          
          return (
              self._name,
              self._field_type.upper(),
              self._mode.upper(),
              self._description,
              self._fields,
          )
docs: 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`.