“Fire and forget” python async/await

Question

有时需要进行一些非关键的异步操作，但我不想等待它完成。在Tornado的协程实现中，你可以通过简单地省略yield关键字来“触发并忘记”一个异步函数。

我一直在试图找出如何“火和忘记”与新的async/ await在Python 3.5发布的语法。例如，一个简化的代码片段：

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

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

bar()

但是，发生的事情是bar()永远不会执行，而是收到运行时警告：

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

Answer 1

如果你使用的是Python> = 3.7，请在任何地方替换asyncio.ensure_future为asyncio.create_task最新的，更好的派生task的方法。

asyncio.Task到“激发并忘记”

根据python docs的asyncio.Task说法，有可能启动一些协程以“在后台”执行。asyncio.ensure_future 函数创建的任务不会阻止执行（因此函数将立即返回！）。这似乎是你要求的一种“解雇”的方法。

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())

输出：

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

如果事件循环完成后正在执行任务怎么办？
请注意，asyncio期望任务在事件循环完成时完成。因此，如果你更改main()为：

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')
程序完成后，你会收到以下警告：

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

为防止这种情况，你可以在事件循环完成后等待所有待处理的任务：

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))

杀死任务而不是等待任务

有时你不想等待任务完成（例如，某些任务可能创建为永久运行）。在这种情况下，你可以只取消（）而不是等待它们：

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)

输出：

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