“失火” Python异步/等待

Sometimes there is some non-critical asynchronous operation that needs to happen but I don’t want to wait for it to complete. In Tornado’s coroutine implementation you can “fire & forget” an asynchronous function by simply ommitting the yield key-word.

I’ve been trying to figure out how to “fire & forget” with the new async/await syntax released in Python 3.5. E.g., a simplified code snippet:

async def async_foo():
    print("Do some stuff asynchronously here...")

def bar():
    async_foo()  # fire and forget "async_foo()"

bar()

What happens though is that bar() never executes and instead we get a runtime warning:

RuntimeWarning: coroutine 'async_foo' was never awaited
  async_foo()  # fire and forget "async_foo()"

Upd:

Replace asyncio.ensure_future with asyncio.create_task everywhere if you’re using Python >= 3.7 It’s newer, nicer way to spawn task.

asyncio.Task to “fire and forget”

According to python docs for it is possible to start some coroutine to execute “in background”. The task created by asyncio.ensure_future function won’t block the execution (therefore the function will return immediately!). This looks like a way to “fire and forget” as you requested.

import asyncio


async def async_foo():
    print("async_foo started")
    await asyncio.sleep(1)
    print("async_foo done")


async def main():
    asyncio.ensure_future(async_foo())  # fire and forget async_foo()

    # btw, you can also create tasks inside non-async funcs

    print('Do some actions 1')
    await asyncio.sleep(1)
    print('Do some actions 2')
    await asyncio.sleep(1)
    print('Do some actions 3')


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

Output:

Do some actions 1
async_foo started
Do some actions 2
async_foo done
Do some actions 3

What if tasks are executing after event loop complete?

Note that asyncio expects task would be completed at the moment event loop completed. So if you’ll change main() to:

async def main():
    asyncio.ensure_future(async_foo())  # fire and forget

    print('Do some actions 1')
    await asyncio.sleep(0.1)
    print('Do some actions 2')

You’ll get this warning after the program finished:

Task was destroyed but it is pending!
task: <Task pending coro=<async_foo() running at [...]

To prevent that you can just await all pending tasks after event loop completed:

async def main():
    asyncio.ensure_future(async_foo())  # fire and forget

    print('Do some actions 1')
    await asyncio.sleep(0.1)
    print('Do some actions 2')


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

    # Let's also finish all running tasks:
    pending = asyncio.Task.all_tasks()
    loop.run_until_complete(asyncio.gather(*pending))

Kill tasks instead of awaiting them

Sometimes you don’t want to await tasks to be done (for example, some tasks may be created to run forever). In that case, you can just cancel() them instead of awaiting them:

import asyncio
from contextlib import suppress


async def echo_forever():
    while True:
        print("echo")
        await asyncio.sleep(1)


async def main():
    asyncio.ensure_future(echo_forever())  # fire and forget

    print('Do some actions 1')
    await asyncio.sleep(1)
    print('Do some actions 2')
    await asyncio.sleep(1)
    print('Do some actions 3')


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

    # Let's also cancel all running tasks:
    pending = asyncio.Task.all_tasks()
    for task in pending:
        task.cancel()
        # Now we should await task to execute it's cancellation.
        # Cancelled task raises asyncio.CancelledError that we can suppress:
        with suppress(asyncio.CancelledError):
            loop.run_until_complete(task)

Output:

Do some actions 1
echo
Do some actions 2
echo
Do some actions 3
echo

Thank you Sergey for the succint answer. Here is the decorated version of the same.

import asyncio
import time

def fire_and_forget(f):
    def wrapped(*args, **kwargs):
        return asyncio.get_event_loop().run_in_executor(None, f, *args, *kwargs)

    return wrapped

@fire_and_forget
def foo():
    time.sleep(1)
    print("foo() completed")

print("Hello")
foo()
print("I didn't wait for foo()")

Produces

>>> Hello
>>> foo() started
>>> I didn't wait for foo()
>>> foo() completed

Note: Check my other answer which does the same using plain threads.

This is not entirely asynchronous execution, but maybe run_in_executor() is suitable for you.

def fire_and_forget(task, *args, **kwargs):
    loop = asyncio.get_event_loop()
    if callable(task):
        return loop.run_in_executor(None, task, *args, **kwargs)
    else:    
        raise TypeError('Task must be a callable')

def foo():
    #asynchronous stuff here


fire_and_forget(foo)

For some reason if you are unable to use asyncio then here is the implementation using plain threads. Check my other answers and Sergey’s answer too.

import threading

def fire_and_forget(f):
    def wrapped():
        threading.Thread(target=f).start()

    return wrapped

@fire_and_forget
def foo():
    time.sleep(1)
    print("foo() completed")

print("Hello")
foo()
print("I didn't wait for foo()")