Generates tuples of calendar_dates.txt values. Yield zero tuples if
    this ServicePeriod should not be in calendar_dates.txt .

def GenerateCalendarDatesFieldValuesTuples(self):
    """Generates tuples of calendar_dates.txt values. Yield zero tuples if
    this ServicePeriod should not be in calendar_dates.txt ."""
    for date, (exception_type, _) in self.date_exceptions.items():
      yield (self.service_id, date, unicode(exception_type))