How to gracefully terminate an asyncio script with Ctrl-C?

Question

I've read every post I could find about how to gracefully handle a script with an asyncio event loop getting terminated with Ctrl-C, and I haven't been able to get any of them to work without printing one or more tracebacks as I do so. The answers are pretty much all over the place, and I haven't been able implement any of them into this small script:

import asyncio
import datetime
import functools
import signal


async def display_date(loop):
    end_time = loop.time() + 5.0
    while True:
        print(datetime.datetime.now())
        if (loop.time() + 1.0) >= end_time:
            break
        await asyncio.sleep(1)


def stopper(signame, loop):
    print("Got %s, stopping..." % signame)
    loop.stop()


loop = asyncio.get_event_loop()
for signame in ('SIGINT', 'SIGTERM'):
    loop.add_signal_handler(getattr(signal, signame), functools.partial(stopper, signame, loop))

loop.run_until_complete(display_date(loop))
loop.close()

What I want to happen is for the script to exit without printing any tracebacks following a Ctrl-C (or SIGTERM/SIGINT sent via kill). This code prints RuntimeError: Event loop stopped before Future completed. In the MANY other forms I've tried based on previous answers, I've gotten a plethora of other types of exception classes and error messages with no idea how to fix them. The code above is minimal right now, but some of the attempts I made earlier were anything but, and none of them were correct.

If you're able to modify the script so that it terminates gracefully, an explanation of why your way of doing it is the right way would be greatly appreciated.

Crock

Answer 1

Stopping the event loop while it is running will never be valid.

Here, you need to catch the Ctrl-C, to indicate to Python that you wish to handle it yourself instead of displaying the default stacktrace. This can be done with a classic try/except:

coro = display_date(loop)
try:
    loop.run_until_complete(coro)
except KeyboardInterrupt:
    print("Received exit, exiting")

And, for your use-case, that's it! For a more real-life program, you would probably need to cleanup some resources. See also Graceful shutdown of asyncio coroutines

Impudicity

Thank you. As a clarification for any other readers, I had to remove the whole signal handler-adding for loop, and the final loop.close() was unnecessary as well. – Crock

I can't believe how much time I wasted trying to get add_signal_handler(SIGINT) to work; thanks for pointing to a much simpler path that actually works. – Interfere

I also had to add RuntimeError on the exception because I was receiving RuntimeError: Event loop stopped before Future completed. – Annelleannemarie

Answer 2

Use signal handlers:

import asyncio
from signal import SIGINT, SIGTERM

async def main_coro():
    try:
        await awaitable()
    except asyncio.CancelledError:
        do_cleanup()

if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    main_task = asyncio.ensure_future(main_coro())
    for signal in [SIGINT, SIGTERM]:
        loop.add_signal_handler(signal, main_task.cancel)
    try:
        loop.run_until_complete(main_task)
    finally:
        loop.close()

Carlile

If you're only writing the coroutine and not the main code, you can use asyncio.get_running_loop() and asyncio.current_task() to get the loop and task respectively, this way I added signal handlers while in the coroutine (allowing it to be called with asyncio.run()) – Parabola

asyncio.add_signal_handler() on win32 raises NotImplementedError (python 3.7.3). – Geld