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”一个异步函数。

我一直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:

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

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

bar()

发生的情况是 bar() 永远不会执行，相反我们会收到运行时警告:

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

最佳答案

更新:

如果您使用 Python >= 3.7，请在任何地方将 asyncio.ensure_future 替换为 asyncio.create_task 这是一种更新、更好的方式 to spawn tasks .

asyncio.Task 到“一劳永逸”

根据 asyncio.Task 的 python 文档可以启动一些协程来“在后台”执行。 asyncio.ensure_future 创建的任务不会阻止执行(因此函数将立即返回!)。这看起来像是按照您的要求“开枪即忘”的一种方式。

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())

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任务正在执行怎么办？

请注意，asyncio 期望在事件循环完成时完成任务。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:

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

为了防止你只能 await all pending tasks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，某些任务可能会被创建为永远运行)。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)

输出:

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

关于python - "Fire and forget"python 异步/等待，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/37278647/