python - 异步如何暂停协程直到调用发送

Question

假设我有一辆从某处接收消息的总线。每条消息都有target和msg，我想实现一个订阅机制，让其他协程可以订阅到特定的target

subscriptions = {}
async def subscribe(target):
    subscriptions[target]= wait_for_messages()

async def proc_msg(target,msg);
    subscriptions[target].send(msg)

async def wait_for_messages():
     while True:
        asyncio.sleep(1)

async def subscribe(target)
    async for msg in subscribe(target):
      print(msg)

我的问题是关于 wait_for_messages，即使 sleep 可以正常工作(代码仅供演示)，我如何才能等待发送命令而不经常 sleep 和醒来？

Answer 1

订阅机制来自回调世界和asyncio.Future - 是从基于回调的世界到基于协程的世界的某种桥梁:有了它你可以等待发生的事情。在 msg 到来时使用 set_result 并为 Future 使用 await 以在协程中接收 msg。

例如:

import asyncio
from random import randint
from collections import defaultdict
from contextlib import suppress


# MSG SUBSCRIBE/SEND API:
_futures = defaultdict(asyncio.Future)


async def msg_for_target(target):
    """Await for this coroutine to recieve msg."""
    return await _futures[target]


def send_msg(target, msg):
    _futures[target].set_result(msg)
    del _futures[target]  # We would need new Future for future subscribers


# TEST:
async def random_sender():
    """Send random messages."""
    i = 0
    while True:
        i += 1
        await asyncio.sleep(0.5)

        target = randint(0, 3)
        msg = f'msg {i}'        
        print(f'Sending msg "{msg}" for target "{target}" ...')
        send_msg(target, msg)


async def main():
    task = asyncio.ensure_future(random_sender())

    for target in (2, 0, 3, 1):
        print(f'> Subscribed for target "{target}"')
        msg = await msg_for_target(target)
        print(f'> Recieved "{msg}"')
        print()

    # Cleanup, see https://stackoverflow.com/a/43810272/1113207
    task.cancel()
    with suppress(asyncio.CancelledError):
        await task


if __name__ ==  '__main__':
    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(main())
    finally:
        loop.run_until_complete(loop.shutdown_asyncgens())
        loop.close()

输出:

> Subscribed for target "2"
Sending msg "msg 1" for target "2" ...
> Recieved "msg 1"

> Subscribed for target "0"
Sending msg "msg 2" for target "3" ...
Sending msg "msg 3" for target "0" ...
> Recieved "msg 3"

> Subscribed for target "3"
Sending msg "msg 4" for target "2" ...
Sending msg "msg 5" for target "2" ...
Sending msg "msg 6" for target "2" ...
Sending msg "msg 7" for target "1" ...
Sending msg "msg 8" for target "3" ...
> Recieved "msg 8"

> Subscribed for target "1"
Sending msg "msg 9" for target "0" ...
Sending msg "msg 10" for target "0" ...
Sending msg "msg 11" for target "2" ...
Sending msg "msg 12" for target "2" ...
Sending msg "msg 13" for target "1" ...
> Recieved "msg 13"