Return a tuple that outputs a row of _FIELD_NAMES to be written to a
       GTFS file.

    Arguments:
        trip_id: The trip_id of the trip to which this StopTime corresponds.
                 It must be provided, as it is not stored in StopTime.

def GetFieldValuesTuple(self, trip_id):
    """Return a tuple that outputs a row of _FIELD_NAMES to be written to a
       GTFS file.

    Arguments:
        trip_id: The trip_id of the trip to which this StopTime corresponds.
                 It must be provided, as it is not stored in StopTime.
    """
    result = []
    for fn in self._FIELD_NAMES:
      if fn == 'trip_id':
        result.append(trip_id)
      else:
        # Since we'll be writting to an output file, we want empty values to be
        # outputted as an empty string
        result.append(getattr(self, fn) or '' )
    return tuple(result)