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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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]