code: def NewDefaultServicePeriod(self):
      
      service_period = self._gtfs_factory.ServicePeriod()
      service_period.service_id = util.FindUniqueId(self.service_periods)
      # blank service won't validate in AddServicePeriodObject
      self.SetDefaultServicePeriod(service_period, validate=False)
      return service_period
docs: Create a new ServicePeriod object, make it the default service period and
      return it. The default service period is used when you create a trip without
      providing an explict service period.