code: def GetDefaultServicePeriod(self):
      
      if not self._default_service_period:
        if len(self.service_periods) == 0:
          self.NewDefaultServicePeriod()
        elif len(self.service_periods) == 1:
          self._default_service_period = self.service_periods.values()[0]
      return self._default_service_period
docs: Return the default ServicePeriod. If no default ServicePeriod has been
      set select the default depending on how many ServicePeriod objects are in
      the Schedule. If there are 0 make a new ServicePeriod the default, if there
      is 1 it becomes the default, if there is more than 1 then return None.