code: def receive_one(self):
          
          if not self.isopen:
              logger = logging.getLogger(__name__)
              logger.warning('the drop box is not open')
              return
          return self.dropbox.receive_one()
docs: return a pair of an ID and a result of a task.
  
          This method waits for a task to finish.
  
          Returns
          -------
          An ID and a result of a task. `None` if no task is running.