Please explain “Task was destroyed but it is pending!”

Question

Python 3.4.2

I am learning asyncio and I use it to continously listen IPC bus, while gbulb listens to the dbus.

Some side notes:

So I created a fun

Answer 1

The problem comes from closing the loop immediately after cancelling the tasks. As the cancel() docs state

"This arranges for a CancelledError to be thrown into the wrapped coroutine on the next cycle through the event loop."

Take this snippet of code:

import asyncio
import signal


async def pending_doom():
    await asyncio.sleep(2)
    print(">> Cancelling tasks now")
    for task in asyncio.Task.all_tasks():
        task.cancel()

    print(">> Done cancelling tasks")
    asyncio.get_event_loop().stop()


def ask_exit():
    for task in asyncio.Task.all_tasks():
        task.cancel()


async def looping_coro():
    print("Executing coroutine")
    while True:
        try:
            await asyncio.sleep(0.25)
        except asyncio.CancelledError:
            print("Got CancelledError")
            break

        print("Done waiting")

    print("Done executing coroutine")
    asyncio.get_event_loop().stop()


def main():
    asyncio.async(pending_doom())
    asyncio.async(looping_coro())

    loop = asyncio.get_event_loop()
    for sig in (signal.SIGINT, signal.SIGTERM):
        loop.add_signal_handler(sig, ask_exit)

    loop.run_forever()

    # I had to manually remove the handlers to
    # avoid an exception on BaseEventLoop.__del__
    for sig in (signal.SIGINT, signal.SIGTERM):
        loop.remove_signal_handler(sig)


if __name__ == '__main__':
    main()

Notice ask_exit cancels the tasks but does not stop the loop, on the next cycle looping_coro() stops it. The output if you cancel it is:

Executing coroutine
Done waiting
Done waiting
Done waiting
Done waiting
^CGot CancelledError
Done executing coroutine

Notice how pending_doom cancels and stops the loop immediately after. If you let it run until the pending_doom coroutines awakes from the sleep you can see the same warning you're getting:

Executing coroutine
Done waiting
Done waiting
Done waiting
Done waiting
Done waiting
Done waiting
Done waiting
>> Cancelling tasks now
>> Done cancelling tasks
Task was destroyed but it is pending!
task:  wait_for=>