如何使 asyncio cancel() 真正取消任务？

Question

这段代码陷入死循环，self.task.cancel()似乎没有效果：

import asyncio

from unittest.mock import AsyncMock, patch


async def loop():
    while True:
        await asyncio.sleep(1)


class AsyncSleepMock(AsyncMock):
    def __init__(self):
        super(AsyncMock, self).__init__()
        self.task = None

    async def __call__(self, delay, *args, **kwargs):
        self.task.cancel()
        return await super(AsyncMock, self).__call__(delay, *args, **kwargs)


def create_async_sleep_mock():
    return AsyncSleepMock()


@patch("asyncio.sleep", new_callable=create_async_sleep_mock)
def main(async_sleep_mock):
    loop_task = asyncio.get_event_loop().create_task(loop())
    async_sleep_mock.task = loop_task
    asyncio.get_event_loop().run_until_complete(loop_task)


if __name__ == "__main__":
    main()

目标是制作一个模拟 asyncio.sleep()，可以从被测应用程序拥有的无限 loop() 中跳出来。怎么做？

Answer 1

self.task.cancel() 将任务标记为已取消，但此时这是 CPU 上的活动任务。必须发生任务切换以允许调度程序处理取消。

来自 cancel() 文档：

Request the Task to be cancelled.

This arranges for a CancelledError exception to be thrown into the wrapped coroutine on the next cycle of the event loop.

我插入了一个未模拟的 await asyncio.sleep(0) 以确保所需的任务切换，现在它不再循环：

realsleep = asyncio.sleep

class AsyncSleepMock(AsyncMock):
    def __init__(self):
        super(AsyncMock, self).__init__()
        self.task = None

    async def __call__(self, delay, *args, **kwargs):
        self.task.cancel()
        await realsleep(0)
        return await super(AsyncMock, self).__call__(delay, *args, **kwargs)

为了完整起见，我添加了 asyncio.sleep() 描述中的引述：

sleep() always suspends the current task, allowing other tasks to run.

Setting the delay to 0 provides an optimized path to allow other tasks to run. This can be used by long-running functions to avoid blocking the event loop for the full duration of the function call.

如何使 asyncio cancel() 真正取消任务？

How to make asyncio cancel() to actually cancel the task?

python

python-asyncio

coroutine

python-mock