From the official document, we see loop.stop() while running loop.run_forever() will run the current batch of callbacks and exit. Therefore, the following code works:

import asyncio
import signal


def signal_handler(sig, frame):
    global loop
    print('Cancelling tasks...')
    loop.stop()


async def background_task():
    while True:
        print('Running background task...')
        await asyncio.sleep(1)


async def main():
    asyncio.create_task(background_task())


if __name__ == '__main__':
    signal.signal(signal.SIGINT, signal_handler)
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    loop.run_until_complete(main())
    loop.run_forever()

The result looks like:

% python test.py
Running background task...
Running background task...
^CCancelling tasks...
%

But if I use asyncio.to_thread to create a multi-threading task, loop.stop() fails to terminate the background task:

import asyncio
import signal
import time


def signal_handler(sig, frame):
    global loop
    print('Cancelling tasks...')
    loop.stop()


def background_task():
    while True:
        print('Running background task...')
        time.sleep(1)


async def main():
    asyncio.create_task(asyncio.to_thread(background_task))


if __name__ == '__main__':
    signal.signal(signal.SIGINT, signal_handler)
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    loop.run_until_complete(main())
    loop.run_forever()

The result looks like

% python tests/test.py
Running background task...
Running background task...
^CCancelling tasks...
Running background task...
Running background task...
...

I’ve tried to add task.cancel() before loop.stop(), like this:

for task in asyncio.all_tasks():
    task.cancel()

But it makes no difference.

I guess it’s because multi-threading tasks work in a different event loop, so calling stop method of the loop where the background task runs may help.

But I don’t know how to get that event loop. It seems to be created automatically inside asyncio.to_thread.

Also, if there are better alternative solutions to run a blocking background task, I’m very happy to hear.