“发射后不管” python async/await

有时需要发生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，您可以通过简单地省略来“触发并忘记”异步函数yield关键词。

我一直在试图弄清楚如何用新的方式“发射后忘记”async/awaitPython 3.5 中发布的语法。例如，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()

但发生的事情是这样的bar()永远不会执行，而是我们收到运行时警告：

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

Upd:

Replace asyncio.ensure_future with asyncio.create_task如果你使用 Python >= 3.7 则到处都是 这是一种更新、更好的方法产生任务 https://docs.python.org/3/library/asyncio-task.html#asyncio.create_task.

asyncio.Task“一劳永逸”

根据 python 文档asyncio.Task https://docs.python.org/3/library/asyncio-task.html#asyncio.Task可以启动一些协程“在后台”执行。创建的任务asyncio.ensure_future https://docs.python.org/3/library/asyncio-future.html#asyncio.ensure_future不会阻止执行（因此该函数将立即返回！）。这看起来像是您所要求的“即发即忘”的方式。

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

如果任务在事件循环完成后执行怎么办？

请注意，asyncio 期望任务在事件循环完成时完成。所以如果你愿意改变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')

程序完成后您将收到此警告：

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

为了防止这种情况，你可以等待所有待处理的任务 https://stackoverflow.com/a/27910822/1113207事件循环完成后：

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它们

有时您不想等待任务完成（例如，某些任务可能会创建为永远运行）。在这种情况下，您只需cancel()他们而不是等待他们：

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)

Output:

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