code: def _helper_for_model(self, model_type):
          
          if model_type is models.containers.Container:
              return self.containers
          if model_type is models.images.Image:
              return self.images
          if model_type is models.networks.Network:
              return self.networks
          if model_type is models.volumes.Volume:
              return self.volumes
  
          raise ValueError('Unknown model type {}'.format(model_type))
docs: Get the helper for a given type of Docker model. For use by resource
          definitions.