Should Python process wait for a job forked using `run_in_executor`, although I did not wait for?

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Assume we have the following script:


def sync_sleep(x):
    print("sync_sleep going to sleep")
    time.sleep(x)
    print("sync_sleep awake")


async def main():
    loop = asyncio.get_running_loop()
    loop.run_in_executor(None, sync_sleep, 4)
    print("main going to sleep")
    await asyncio.sleep(1)
    print("main awake ")


if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())
    print("main finished")

Note: I have omitted loop.close intentionally.

Now when I execute the above script:

python script.py

I have the following output:

sync_sleep going to sleep
main going to sleep
main awake 
main finished
sync_sleep awake

I expected after running the script, the process to exit after printing "main finished", but it did not until the job sync_sleep has finished. I mean, if I wanted to wait for that job, I would have added the line:

loop.run_until_complete(loop.shutdown_default_executor())

Is my expectation wrong?

python-3.9 python-asyncio python-multithreading asynchronous python
1 Answers

The default ThreadPoolExecutor waits at exit for daemon threads, meaning the process waits for the thread to finish, and only then stops.

Apart from implementing an Executor class yourself , there's no option other than using the threading module to create a new thread with daemon=True. If you wish to wait for that thread in asyncio, you can always run an executor to await the thread.join().

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