Return the tuple of calendar.txt values or None if this ServicePeriod
    should not be in calendar.txt .

def GetCalendarFieldValuesTuple(self):
    """Return the tuple of calendar.txt values or None if this ServicePeriod
    should not be in calendar.txt ."""
    if self.start_date and self.end_date:
      return [getattr(self, fn) for fn in self._FIELD_NAMES]