[Python-Dev] Another threading idea

Tue Mar 14 22:33:06 CET 2006

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.
So, the idea is to create a subclass of Queue that increments a counter when 
objects are enqueued, that provides a method for worker threads to decrement the 
counter when the work is done, and offers a blocking join() method that waits 
until the counter is zero
 # main thread
 q = TaskQueue()
 for t in worker_threads():
 t.start()
 for task in tasklist:
 q.put(task) # increments the counter and enqueues a task
 q.join() # all of the tasks are done (counter is 
zero)
 do_work_on_results()
 # worker thread
 while 1:
 task = q.get() # task is popped but the counter is 
unchanged
 do_work(task)
 q.decrement() # now the counter gets reduced
The idea is still in its infancy (no implementation and it hasn't been tried in 
real-world code) but I would like to get some feedback. If it works out, I'll 
post a recipe to ASPN and see how it goes.
Raymond 