中断事件循环后清理的正确方法是什么? [英] What's the correct way to clean up after an interrupted event loop?

问题描述

我有一个事件循环，它运行一些协同程序作为命令行工具的一部分.用户可以使用通常的 Ctrl + C 中断工具，此时我想在中断的事件循环后正确清理.

I have an event loop that runs some co-routines as part of a command line tool. The user may interrupt the tool with the usual Ctrl + C, at which point I want to clean up properly after the interrupted event loop.

这是我尝试过的.

import asyncio


@asyncio.coroutine
def shleepy_time(seconds):
    print("Shleeping for {s} seconds...".format(s=seconds))
    yield from asyncio.sleep(seconds)


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

    # Side note: Apparently, async() will be deprecated in 3.4.4.
    # See: https://docs.python.org/3.4/library/asyncio-task.html#asyncio.async
    tasks = [
        asyncio.async(shleepy_time(seconds=5)),
        asyncio.async(shleepy_time(seconds=10))
    ]

    try:
        loop.run_until_complete(asyncio.gather(*tasks))
    except KeyboardInterrupt as e:
        print("Caught keyboard interrupt. Canceling tasks...")

        # This doesn't seem to be the correct solution.
        for t in tasks:
            t.cancel()
    finally:
        loop.close()

运行并点击 Ctrl + C 产生:

Running this and hitting Ctrl + C yields:

$ python3 asyncio-keyboardinterrupt-example.py 
Shleeping for 5 seconds...
Shleeping for 10 seconds...
^CCaught keyboard interrupt. Canceling tasks...
Task was destroyed but it is pending!
task: <Task pending coro=<shleepy_time() running at asyncio-keyboardinterrupt-example.py:7> wait_for=<Future cancelled> cb=[gather.<locals>._done_callback(1)() at /usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/asyncio/tasks.py:587]>
Task was destroyed but it is pending!
task: <Task pending coro=<shleepy_time() running at asyncio-keyboardinterrupt-example.py:7> wait_for=<Future cancelled> cb=[gather.<locals>._done_callback(0)() at /usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/asyncio/tasks.py:587]>

显然，我没有正确清理.我想也许在任务上调用 cancel() 会是这样做的方法.

Clearly, I didn't clean up correctly. I thought perhaps calling cancel() on the tasks would be the way to do it.

在事件循环中断后进行清理的正确方法是什么?

What's the correct way to clean up after an interrupted event loop?

推荐答案

当您按 CTRL+C 时，事件循环停止，因此您对 t.cancel() 的调用实际上不会影响.对于要取消的任务，需要重新开始循环备份.

When you CTRL+C, the event loop gets stopped, so your calls to t.cancel() don't actually take effect. For the tasks to be cancelled, you need to start the loop back up again.

以下是您可以处理它的方法:

Here's how you can handle it:

import asyncio

@asyncio.coroutine
def shleepy_time(seconds):
    print("Shleeping for {s} seconds...".format(s=seconds))
    yield from asyncio.sleep(seconds)


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

    # Side note: Apparently, async() will be deprecated in 3.4.4.
    # See: https://docs.python.org/3.4/library/asyncio-task.html#asyncio.async
    tasks = asyncio.gather(
        asyncio.async(shleepy_time(seconds=5)),
        asyncio.async(shleepy_time(seconds=10))
    )

    try:
        loop.run_until_complete(tasks)
    except KeyboardInterrupt as e:
        print("Caught keyboard interrupt. Canceling tasks...")
        tasks.cancel()
        loop.run_forever()
        tasks.exception()
    finally:
        loop.close()

一旦我们捕捉到KeyboardInterrupt，我们就会调用tasks.cancel()，然后再次启动loop.run_forever 实际上会在 tasks 被取消后立即退出(注意取消 asyncio.gather 返回的 Future 也取消其中的所有 Futures)，因为中断的 loop.run_until_complete 调用添加了一个 done_callback 到 tasks停止循环.因此，当我们取消 tasks 时，会触发回调，然后循环停止.那时我们调用 tasks.exception，只是为了避免收到关于未从 _GatheringFuture 获取异常的警告.

Once we catch KeyboardInterrupt, we call tasks.cancel() and then start the loop up again. run_forever will actually exit as soon as tasks gets cancelled (note that cancelling the Future returned by asyncio.gather also cancels all the Futures inside of it), because the interrupted loop.run_until_complete call added a done_callback to tasks that stops the loop. So, when we cancel tasks, that callback fires, and the loop stops. At that point we call tasks.exception, just to avoid getting a warning about not fetching the exception from the _GatheringFuture.

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