《火与遗忘》蟒蛇异步/等待 [英] "Fire and forget" python async/await

问题描述

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，您可以通过简单地省略 yield 关键字来触发并忘记"异步函数.

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.

我一直在试图弄清楚如何使用 Python 3.5 中发布的新 async/await 语法触发并忘记".例如，一个简化的代码片段:

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()

结果是 bar() 永远不会执行，而是我们收到运行时警告:

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()"

推荐答案

更新:

如果您使用 Python，则在任何地方将 asyncio.ensure_future 替换为 asyncio.create_task >= 3.7 这是一种更新、更好的方法 生成任务.

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

根据asyncio.Task<的python文档/code> 可以启动一些协程来在后台"执行.由 asyncio.ensure_future<创建的任务/a> 不会阻塞执行(因此函数会立即返回！).这看起来像是一种即发即忘"的方式.按照您的要求.

According to python docs for asyncio.Task it is possible to start some coroutine to execute "in the background". The task created by asyncio.ensure_future 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())

输出:

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

如果在事件循环完成后执行任务怎么办?

请注意，asyncio 期望任务在事件循环完成时完成.因此，如果您将 main() 更改为:

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')

程序完成后您会收到此警告:

You'll get this warning after the program finished:

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

为了防止这种情况，您可以在事件循环完成后等待所有挂起的任务:

To prevent that you can just await all pending tasks after the event loop has 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))

杀死任务而不是等待它们

有时您不想等待任务完成(例如，某些任务可能被创建为永远运行).在这种情况下，您可以直接 cancel() 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

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