使用 asyncio 使 websocket 回调异步

Question

我正在尝试使用 asyncio 和 websockets 和 Python 3.5.2 实现一个基本的 websocket 客户端。

基本上，我希望connect_to_dealer 成为阻塞调用，但在不同线程上等待 websocket 消息。

在阅读了一些文档（我对 Python 的经验很少）之后，我得出结论，asyncio.ensure_future() 传递协程 (listen_for_message) 是可行的方法。

现在，我可以在不同的线程上运行 listen_for_message，但在协程中我似乎无法使用 await 或任何其他机制来进行调用同步。如果我这样做，即使是简单的sleep，执行也会永远等待（挂起）。

我想知道我做错了什么。

async def listen_for_message(self, future, websocket):
    while (True):
        try:
            await asyncio.sleep(1) # It hangs here
            print('Listening for a message...')
            message = await websocket.recv() # If I remove the sleep, hangs here
            print("< {}".format(message))
            future.set_result(message)
            future.done()
        except websockets.ConnectionClosed as cc:
            print('Connection closed')
        except Exception as e:
            print('Something happened')

def handle_connect_message(self, future):
    # We must first remove the websocket-specific payload because we're only interested in the connect protocol msg
    print(future.result)

async def connect_to_dealer(self):
    print('connect to dealer')
    websocket = await websockets.connect('wss://mywebsocket'))
    hello_message = await websocket.recv()
    print("< {}".format(hello_message))
    # We need to parse the connection ID out of the message
    connection_id = hello_message['connectionId']
    print('Got connection id {}'.format(connection_id))
    sub_response = requests.put('https://subscribetotraffic{user_id}?connection={connection_id}'.format(user_id='username', connection_id=connection_id), headers=headers)
    if sub_response.status_code == 200:
        print('Now we\'re observing traffic')
    else:
        print('Oops request failed with {code}'.format(code=sub_response.status_code))
    # Now we need to handle messages but continue with the regular execution
    try:
        future = asyncio.get_event_loop().create_future()
        future.add_done_callback(self.handle_connect_message)
        asyncio.ensure_future(self.listen_for_message(future, websocket))
    except Exception as e:
        print(e)

Answer 1

您是否有特定原因需要使用显式期货？

使用asyncio，您可以结合使用coroutines 和Tasks 来实现大多数目的。任务本质上是封装的协程，它们在后台自行启动，独立于其他异步代码，因此您不必显式管理它们的流程或将它们与其他代码位混合。

我不完全确定您的最终目标，但也许下面阐述的方法可以为您提供一些工作：

import asyncio

async def listen_for_message(websocket):

    while True:

        await asyncio.sleep(0)

        try:
            print('Listening for a message...')
            message = await websocket.recv()

            print("< {}".format(message))

        except websockets.ConnectionClosed as cc:
            print('Connection closed')

        except Exception as e:
            print('Something happened')


async def connect_to_dealer():

    print('connect to dealer')
    websocket = await websockets.connect('wss://mywebsocket')

    hello_message = await websocket.recv()
    print("< {}".format(hello_message))

    # We need to parse the connection ID out of the message
    connection_id = hello_message['connectionId']
    print('Got connection id {}'.format(connection_id))

    sub_response = requests.put('https://subscribetotraffic{user_id}?connection={connection_id}'.format(
        user_id='username', connection_id=connection_id), headers=headers)

    if sub_response.status_code == 200:
        print('Now we\'re observing traffic')
    else:
        print('Oops request failed with {code}'.format(code=sub_response.status_code))


async def my_app():

    # this will block until connect_to_dealer() returns
    websocket = await connect_to_dealer()

    # start listen_for_message() in its own task wrapper, so doing it continues in the background
    asyncio.ensure_future(listen_for_message(websocket))

    # you can continue with other code here that can now coexist with listen_for_message()


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