Python threading.current_thread() and callback

Question

I have this small threading testing code below. The thread name is sometimes the name of the executor sometimes the MainThread. According to 3.7 Python doc add_done_callback(fn) ... Added callables are called in the order that they were added and are always called in a thread belonging to the process that added them.

Why do I get different thread name randomly?

from concurrent.futures import ThreadPoolExecutor
import threading

def task(n):
    print(f'task - Thread: {threading.current_thread().name}')
    print("Processing {}".format(n))
    ttask()


def ttask():
    print(f'ttask - Thread: {threading.current_thread().name}')
    print("Processing ")


def taskDone(fn):
    print(f'taskDone - Thread: {threading.current_thread().name}')
    if fn.cancelled():
        print("Our {} Future has been cancelled".format(fn.arg))
    elif fn.done():
        print("Our Task has completed")


def secondTaskDone(fn):
    print(f'secondaTaskDone - Thread: {threading.current_thread().name}')
    print("I didn't think this would work")


def main():
    print("Starting ThreadPoolExecutor")
    print(f'Thread: {threading.current_thread().name}')
    with ThreadPoolExecutor(max_workers=3) as executor:
        print(f'Thread: {threading.current_thread().name}')
        future = executor.submit(task, (2))
        future.add_done_callback(taskDone)
        future.add_done_callback(secondTaskDone)

    print("All tasks complete")


if __name__ == '__main__':
    main()

The outputs are randomly:

output 1:

Starting ThreadPoolExecutor
Thread: MainThread
Thread: MainThread
task - Thread: ThreadPoolExecutor-0_0
Processing 2
ttask - Thread: ThreadPoolExecutor-0_0
Processing
taskDone - Thread: ThreadPoolExecutor-0_0
Our Task has completed
secondaTaskDone - Thread: ThreadPoolExecutor-0_0
I didn't think this would work
All tasks complete

output 2:

Starting ThreadPoolExecutor
Thread: MainThread
Thread: MainThread
task - Thread: ThreadPoolExecutor-0_0
Processing 2
ttask - Thread: ThreadPoolExecutor-0_0
Processing
taskDone - Thread: MainThread
Our Task has completed
secondaTaskDone - Thread: MainThread
I didn't think this would work
All tasks complete

score -1 · Answer 1 · answered Jan 25 '19 at 09:55

The questin is answered in this topic. Thanks for @dano. Python ThreadPoolExecutor - is the callback guaranteed to run in the same thread as submitted func?

Added a 0.2s delay to ttask(), the callback always run in "ThreadPoolExecutor-0_0".

This question can closed as duplicate with the above topic.

Python threading.current_thread() and callback

1 Answers1