code: def run_callback(self):
          
  
          if self._callback_func is not None:
              try:
                  self._callback_func(self._request, self._result)
              except Exception:  # pylint: disable=broad-except
                  LOGGER.exception('An unhandled error occurred while running '
                                   'future callback')
docs: Calls the callback_func, passing in the two positional arguments,
          conditionally waiting if the callback function hasn't been set yet.
          Meant to be run in a threadpool owned by the FutureCollection.
  
          Returns:
              None