异步CancelledError与KeyboardInterrupt

Question

我正在尝试两种方法来阻止无限循环的运行：

• supervisor_1：任务按程序取消
• supervisor_2：任务用Ctrl+C停止

虽然supervisor_2不会在中断时抛出任何错误，但我不能从获取Task was destroyed but it is pending!中获得supervisor_1。知道为什么吗？

以下是代码：

import asyncio
import aioredis
from functools import partial



class Listener:
    def __init__(self, redis_conn):
        self.redis_conn = redis_conn

    async def forever(self, loop_name):
        counter = 0
        try:
            while True:
                print('{}: {}'.format(loop_name, counter))
                counter += 1
                await asyncio.sleep(1)
        except asyncio.CancelledError:
            print('Task Cancelled')
            self.redis_conn.close()
            await self.redis_conn.wait_closed()


async def supervisor_1(redis_conn):
    redis_conn = await redis_conn

    l = Listener(redis_conn)

    task = asyncio.ensure_future(
        asyncio.gather(l.forever('loop_1'), 
                       l.forever('loop_2')))
    await asyncio.sleep(2)
    task.cancel()


async def supervisor_2(redis_conn):
    redis_conn = await redis_conn

    l = Listener(redis_conn)
    await asyncio.gather(l.forever('loop_1'), 
                         l.forever('loop_2'))


if __name__ == '__main__':
    redis_conn = aioredis.create_pool(('localhost', 5003), db=1)

    loop = asyncio.get_event_loop()
    run = partial(supervisor_2, redis_conn=redis_conn)
    task = asyncio.ensure_future(run())
    try:
        loop.run_until_complete(task)
    except KeyboardInterrupt:
        print('Interruped !')
        task.cancel()
        loop.run_forever()
    finally:
        loop.close()

@update

多亏@Gerasimov，这里有一个解决问题的版本，但在KeyboardInterrupt上仍然不时地引发错误：

async def supervisor(redis_conn):
    redis_conn = await redis_conn

    l = Listener(redis_conn)

    task = asyncio.ensure_future(
        asyncio.gather(l.forever('loop_1'), 
                       l.forever('loop_2'))
    )
    await asyncio.sleep(10)
    task.cancel()
    with suppress(asyncio.CancelledError):
        await task

async def kill_tasks():
    pending = asyncio.Task.all_tasks()
    for task in pending:
        task.cancel()
        with suppress(asyncio.CancelledError):
            await task 

和

if __name__ == '__main__':
    redis_conn = aioredis.create_pool(('localhost', 5003), db=1)

    loop = asyncio.get_event_loop()
    run = partial(supervisor, redis_conn=redis_conn)
    task = asyncio.ensure_future(run())
    try:
        loop.run_until_complete(task)
    except KeyboardInterrupt:
        print('Interruped !')
        loop.run_until_complete(kill_tasks())
    finally:
        loop.close()

Answer 1

task.cancel()本身并没有完成任务:它只是对任务说，CancelledError应该在任务中被启动并立即返回。当任务实际被取消时，您应该调用它并等待它(虽然它会引发CancelledError)。

您也不应该在任务中禁止CancelledError。

阅读this answer，在这里我试图展示处理任务的不同方法。例如，要取消某些任务并等待其取消，您可以：

from contextlib import suppress


task = ...  # remember, task doesn't suppress CancelledError itself

task.cancel()  # returns immediately, we should await task raised CancelledError.

with suppress(asyncio.CancelledError):
    await task  # or loop.run_until_complete(task) if it happens after event loop stopped

# Now when we awaited for CancelledError and handled it, 
# task is finally over and we can close event loop without warning.