asyncio.sleep(0) 不会将控制权交给事件循环答案

【问题标题】：asyncio.sleep(0) does not yield control to the event loopasyncio.sleep(0) 不会将控制权交给事件循环
【发布时间】：2022-11-19 16:29:16
【问题描述】：

我有一个简单的异步设置，其中包括两个协程：light_job 和 heavy_job。 light_job 在中间停止，heavy_job 开始。我希望 heavy_job 在中间让出控制权并允许 light_job 完成，但 asyncio.sleep(0) 没有像我预期的那样工作。

这是设置：

import asyncio
import time

loop = asyncio.get_event_loop()


async def light_job():
    print("hello ")
    print(time.time())
    await asyncio.sleep(1)
    print(time.time())
    print("world!")


async def heavy_job():
    print("heavy start")
    time.sleep(3)
    print("heavy halt started")
    await asyncio.sleep(0)
    print("heavy halt ended")
    time.sleep(3)
    print("heavy done")

loop.run_until_complete(asyncio.gather(
    light_job(),
    heavy_job()
))

如果我运行这段代码，light_job 将不会继续，直到 heavy_job 完成。这是输出：

hello 
1668793123.159075
haevy start
heavy halt started
heavy halt ended
heavy done
1668793129.1706061
world!

但如果我将 asyncio.sleep(0) 更改为 asyncio.sleep(0.0001)，代码将按预期工作：

hello 
1668793379.599066
heavy start
heavy halt started
1668793382.605899
world!
heavy halt ended
heavy done

根据文档和related threads，我希望 asyncio.sleep(0) 与 asyncio.sleep(0.0001) 完全一样。这里有什么？

【问题讨论】：

time.sleep(3) 将使整个程序进入休眠状态，因此不会执行任何其他操作。 asyncio 并不意味着执行 CPU 绑定任务 - 为此使用线程或 multiprocessing
我有意使用 time.sleep（用于模拟对 cpu 要求很高的任务）。我的问题是为什么 asyncio.sleep(0) （在两次 time.sleeps 之间）不产生控制而 asyncio.sleep(0.0..01) 产生。
因为当你运行time.sleep(3)时，整个脚本都会进入休眠状态，而await asyncio.sleep(0)只会在 3 秒后执行。
正如我所说，我是故意的。我希望 heavy_job 保持控制 3 秒（time.sleep），而不是将其传递给 light_job（asyncio.sleep），然后再次保持 3 秒（time.sleep）。当我使用 asyncio.sleep(0.0001) 时它会这样做，但当我使用 asyncio.sleep(0) 时则不会。我的问题是关于这两者之间的区别。
它们在 asyncio 内部代码中的处理方式有所不同，但每份文档、论坛线程、SO 问题等。我发现似乎都在告诉我它们最终应该采取相同的行动。我可以重现您看到的结果；不知道为什么。

标签： python python-asyncio

【解决方案1】：

拨打asyncio.sleep(0) 3次：

import asyncio
import time


async def light_job():
    print("hello ")
    print(time.time())
    await asyncio.sleep(1)
    print(time.time())
    print("world!")


async def heavy_job():
    print("heavy start")
    time.sleep(3)
    print("heavy halt started")
    for _ in range(3):
        await asyncio.sleep(0)
    print("heavy halt ended")
    time.sleep(3)
    print("heavy done")


async def test():
    await asyncio.gather(
        light_job(),
        heavy_job()
    )

asyncio.run(test())

这导致：

hello 
1668844526.157173
heavy start
heavy halt started
1668844529.1575627
world!
heavy halt ended
heavy done

查看“asyncio/base_events.py”，“_run_once”首先遍历挂起的计时器，然后在计算之后运行它看到的所有内容。 asyncio.sleep 只能跳过事件循环的一次迭代。需要多次睡眠，因为 asyncio.sleep(1) 安排了一个未来，在通过将 light_job 添加回队列将控制权交还给 light_job 之前，需要进行一次额外的迭代，而 asyncio 恰好最后运行新排队的作业。

为了更清晰的图片，可以添加更多的打印语句：

import asyncio
import time


async def light_job():
    print("hello ")
    print(time.time())
    await asyncio.sleep(1)
    print(time.time())
    print("world!")


async def heavy_job():
    print("heavy start")
    time.sleep(3)
    print("heavy halt started")
    # Sleep to yield to the event loop. light_job isn't detected as ready so this iteration of the loop will finish
    await asyncio.sleep(0)

    print("after 1 sleep")
    # We are still in front of the event loop. Yield so that the 1 second timer in light_job runs.
    # The timer will realize it itself has expired, then put light_job back onto the queue.
    await asyncio.sleep(0)

    # Again the current Python implementation puts us in front. Yield so that the light_job runs
    print("after 2 sleeps")
    await asyncio.sleep(0)

    print("heavy halt ended")
    time.sleep(3)
    print("heavy done")


async def test():
    await asyncio.gather(
        light_job(),
        heavy_job()
    )

asyncio.run(test())

然后在“asyncio/base_events.py”的“def _run_once(self):”中添加断点。在第 1842 行的开头添加断点打印“loop start”，也就是“sched_count =”。在末尾的第 1910 行添加另一个，也就是“handle = None”打印“loop end”。然后在第 1897 行运行每个任务之前添加一个，也就是“if self._debug:”评估和打印“_format_handle(handle)”。事件的顺序被揭示：

loop start
<Task pending name='Task-1' coro=<test() running at /home/home/PycharmProjects/sandbox/notsync.py:34> cb=[_run_until_complete_cb() at /usr/lib/python3.11/asyncio/base_events.py:180]>
loop end
loop start
<Task pending name='Task-2' coro=<light_job() running at /home/home/PycharmProjects/sandbox/notsync.py:5> cb=[gather.<locals>._done_callback() at /usr/lib/python3.11/asyncio/tasks.py:759]>
hello 
1668844827.5052986
<Task pending name='Task-3' coro=<heavy_job() running at /home/home/PycharmProjects/sandbox/notsync.py:13> cb=[gather.<locals>._done_callback() at /usr/lib/python3.11/asyncio/tasks.py:759]>
heavy start
heavy halt started
loop end
loop start
<Task pending name='Task-3' coro=<heavy_job() running at /home/home/PycharmProjects/sandbox/notsync.py:18> cb=[gather.<locals>._done_callback() at /usr/lib/python3.11/asyncio/tasks.py:759]>
after 1 sleep
<TimerHandle when=37442.097934711 _set_result_unless_cancelled(<Future pendi...ask_wakeup()]>, None) at /usr/lib/python3.11/asyncio/futures.py:317>
loop end
loop start
<Task pending name='Task-3' coro=<heavy_job() running at /home/home/PycharmProjects/sandbox/notsync.py:23> cb=[gather.<locals>._done_callback() at /usr/lib/python3.11/asyncio/tasks.py:759]>
after 2 sleeps
<Task pending name='Task-2' coro=<light_job() running at /home/home/PycharmProjects/sandbox/notsync.py:8> wait_for=<Future finished result=None> cb=[gather.<locals>._done_callback() at /usr/lib/python3.11/asyncio/tasks.py:759]>
1668844830.9250844
world!
loop end
loop start
<Task pending name='Task-3' coro=<heavy_job() running at /home/home/PycharmProjects/sandbox/notsync.py:27> cb=[gather.<locals>._done_callback() at /usr/lib/python3.11/asyncio/tasks.py:759]>
heavy halt ended
heavy done
<Handle gather.<locals>._done_callback(<Task finishe...> result=None>) at /usr/lib/python3.11/asyncio/tasks.py:759>
loop end
loop start
<Handle gather.<locals>._done_callback(<Task finishe...> result=None>) at /usr/lib/python3.11/asyncio/tasks.py:759>
loop end
loop start
<Task pending name='Task-1' coro=<test() running at /home/home/PycharmProjects/sandbox/notsync.py:35> wait_for=<_GatheringFuture finished result=[None, None]> cb=[_run_until_complete_cb() at /usr/lib/python3.11/asyncio/base_events.py:180]>
loop end
loop start
<Handle _run_until_complete_cb(<Task finishe...> result=None>) at /usr/lib/python3.11/asyncio/base_events.py:180>
loop end
loop start
<Task pending name='Task-4' coro=<BaseEventLoop.shutdown_asyncgens() running at /usr/lib/python3.11/asyncio/base_events.py:539> cb=[_run_until_complete_cb() at /usr/lib/python3.11/asyncio/base_events.py:180]>
loop end
loop start
<Handle _run_until_complete_cb(<Task finishe...> result=None>) at /usr/lib/python3.11/asyncio/base_events.py:180>
loop end
loop start
<Task pending name='Task-5' coro=<BaseEventLoop.shutdown_default_executor() running at /usr/lib/python3.11/asyncio/base_events.py:564> cb=[_run_until_complete_cb() at /usr/lib/python3.11/asyncio/base_events.py:180]>
loop end
loop start
<Handle _run_until_complete_cb(<Task finishe...> result=None>) at /usr/lib/python3.11/asyncio/base_events.py:180>
loop end

【讨论】：

虽然这是一项了不起的侦探工作，但作为一种解决方案，它是巫毒教……不要用对asyncio.sleep(0)的循环调用来乱丢你的代码库