Python threading issue: threads blocking each other

Question

I'm trying to implement a simple threadpool in python.

I start a few threads with the following code:

 threads = []
        for i in range(10):
            t = threading.Thread(target=self.workerFuncSpinner(
            taskOnDeckQueue, taskCompletionQueue, taskErrorQueue, i))
            t.setDaemon(True)
            threads.append(t)
            t.start()

        for thread in threads:
            thread.join()

At this point, the worker thread only prints when it starts and exits and time.sleeps between. The problem is, instead of getting output like:

#All output at the same time

thread 1 starting
thread 2 starting
thread n starting

# 5 seconds pass

thread 1 exiting
thread 2 exiting
thread n exiting

I get:
thread 1 starting
# 5 seconds pass
thread 1 exiting
thread 2 starting
# 5 seconds pass
thread 2 exiting
thread n starting
# 5 seconds pass
thread n exiting

And when I do a threading.current_thread(), they all report they are mainthread.

It's like there not even threads, but running in the main thread context.

Help?

Thanks

tdelaney · Accepted Answer · 2015-04-28 21:03:26Z

2

You are calling workerFuncSpinner in the main thread when creating the Thread object. Use a reference to the method instead:

t=threading.Thread(target=self.workerFuncSpinner, 
    args=(taskOnDeckQueue, taskCompletionQueue, taskErrorQueue, i))

Your original code:

t = threading.Thread(target=self.workerFuncSpinner(
    taskOnDeckQueue, taskCompletionQueue, taskErrorQueue, i))
t.start()

could be rewritten as

# call the method in the main thread
spinner = self.workerFuncSpinner(
    taskOnDeckQueue, taskCompletionQueue, taskErrorQueue, i)

# create a thread that will call whatever `self.workerFuncSpinner` returned, 
# with no arguments
t = threading.Thread(target=spinner)

# run whatever workerFuncSpinner returned in background thread
t.start()

You were calling the method serially in the main thread and nothing in the created threads.

edited Apr 28, 2015 at 21:03

answered Apr 28, 2015 at 20:20

tdelaney

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1 Comment

user1553046 Over a year ago

This was it. I don't quite understand why the one arg version assumes main thread context, but thank you.

GenFrank64 · Accepted Answer · 2015-04-28 19:59:38Z

0

I suspect workerFuncSpinner may be your problem. I would verify that it is not actually running the task, but returning a callable object for the thread to run.

https://docs.python.org/2/library/threading.html#threading.Thread

answered Apr 28, 2015 at 19:59

GenFrank64

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