def NewDefaultServicePeriod(self):
    """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. """
    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
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.