[Python-Dev] Another threading idea (original) (raw)

Dave Brueck dave.brueck at protoven.com
Tue Mar 14 23:30:40 CET 2006

Raymond Hettinger wrote:

FWIW, I've been working on a way to simplify the use of queues with daemon consumer threads

Sometimes, I launch one or more consumer threads that wait for a task to enter a queue and then work on the task. A recurring problem is that I sometimes need to know if all of the tasks have been completed so I can exit or do something with the result. If each thread only does a single task, I can use t.join() to wait until the task is done. However, if the thread stays alive and waits for more Queue entries, then there doesn't seem to be a good way to tell when all the processing is done.

The pattern I use is to use the None object to indicate that no more tasks are coming - just add it to your queue once for every worker thread that exists and have your worker threads exit when they get a task of None.

Your setup code to start the threads and add tasks to the queue wouldn't need to change; once all the tasks have been enqueued you'd wait for completion like this:

 # Tell all the workers to quit
 for t in worker_threads():
     q.put(None)

 # Wait for all the workers to quit
 for t in worker_threads():
     t.join()

 do_work_on_results()

The worker thread looks like this:

while 1: task = q.get() if task is None: break do_work(task)

-Dave

More information about the Python-Dev mailing list