Schedule task to running event loop from synchronous code

Question

Consider this program, where the mainloop and the coroutine to stop it are actually implemented by a library I'm using.

import asyncio
import signal

running = True

async def stop():
    global running
    print("setting false")
    running = False
    await asyncio.sleep(3)
    print("reached end")

async def mainloop():
    while running:
        print("loop")
        await asyncio.sleep(1)

def handle_signal():
    loop.create_task(stop())

loop = asyncio.get_event_loop()
loop.add_signal_handler(signal.SIGINT, handle_signal)
loop.run_until_complete(mainloop())
loop.close()

I need to call the stop coroutine to stop the mainloop when the program recieves a signal. Although when scheduling the stop coroutine using asyncio.BaseEventLoop.create_task it first stops the mainloop which stops the event loop and the stop coroutine can't finish:

$ ./test.py 
loop
loop
loop
^Csetting false
Task was destroyed but it is pending!
task: <Task pending coro=<stop() done, defined at ./test.py:7> wait_for=<Future pending cb=[Task._wakeup()]>>

How to add the coroutine to the running event loop while making the event loop wait until it is complete?

user4815162342 · Accepted Answer · 2018-01-31 17:13:30Z

As you discovered, the problem is that the event loop is only waiting for mainloop() to complete, leaving stop() pending, which asyncio correctly complains about.

If handle_signal and the top-level code are under your control, you can easily replace looping until mainloop completes with looping until a custom coroutine completes. This coroutine would invoke mainloop and then wait for the cleanup code to finish:

# ... omitted definition of mainloop() and stop()

# list of tasks that must be waited for before we can actually exit
_cleanup = []

async def run():
    await mainloop()
    # wait for all _cleanup tasks to finish
    await asyncio.wait(_cleanup)

def handle_signal():
    # schedule stop() to run, and also add it to the list of
    # tasks run() must wait for before it is done
    _cleanup.append(loop.create_task(stop()))

loop = asyncio.get_event_loop()
loop.add_signal_handler(signal.SIGINT, handle_signal)
loop.run_until_complete(run())
loop.close()

Another option, which doesn't require the new run() coroutine (but still requires the modified handle_signal), is to issue a second run_until_complete() after mainloop completes:

# handle_signal and _cleanup defined as above

loop = asyncio.get_event_loop()
loop.add_signal_handler(signal.SIGINT, handle_signal)
loop.run_until_complete(mainloop())
if _cleanup:
    loop.run_until_complete(asyncio.wait(_cleanup))
loop.close()

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