Python - 如何使用asyncio同时运行多个协同程序？

Question

我正在使用该websockets库在Python 3.4中创建一个websocket服务器.这是一个简单的echo服务器:

import asyncio
import websockets

@asyncio.coroutine
def connection_handler(websocket, path):
    while True:
        msg = yield from websocket.recv()
        if msg is None:  # connection lost
            break
        yield from websocket.send(msg)

start_server = websockets.serve(connection_handler, 'localhost', 8000)
asyncio.get_event_loop().run_until_complete(start_server)
asyncio.get_event_loop().run_forever()

假设我们 - 另外 - 想要在某些事件发生时向客户端发送消息.为简单起见,让我们每隔60秒定期发送一条消息.我们怎么做？我的意思是,因为connection_handler经常等待收到的消息,服务器只能在收到客户端的消息后才采取行动,对吗？我在这里错过了什么？

也许这种情况需要一个基于事件/回调的框架,而不是一个基于协同程序的框架？龙卷风？

Answer 1

TL; DR用于asyncio.ensure_future()同时运行多个协同程序.

也许这种情况需要一个基于事件/回调的框架,而不是一个基于协同程序的框架？龙卷风？

不,你不需要任何其他框架.异步应用程序vs同步的整个想法是它在等待结果时不会阻塞.使用协同程序或回调并不重要.

我的意思是,因为connection_handler一直在等待传入消息,所以服务器只能在收到来自客户端的消息后才采取行动,对吗？我在这里错过了什么？

在同步应用程序中,您将编写类似的内容msg = websocket.recv(),这将阻止整个应用程序,直到您收到消息(如您​​所述).但在异步应用程序中它完全不同.

当你这样做时,msg = yield from websocket.recv()你会说:暂停执行connection_handler()直到websocket.recv()会产生一些东西.使用yield from内部协同程序将控制权返回给事件循环,因此可以执行其他一些代码,而我们正在等待结果websocket.recv().请参阅文档以更好地了解协程如何工作.

假设我们 - 另外 - 想要在某些事件发生时向客户端发送消息.为简单起见,让我们每隔60秒定期发送一条消息.我们怎么做？

asyncio.async()在执行阻塞调用以启动事件循环之前,您可以使用它来运行任意数量的协同程序.

import asyncio

import websockets

# here we'll store all active connections to use for sending periodic messages
connections = []


@asyncio.coroutine
def connection_handler(connection, path):
    connections.append(connection)  # add connection to pool
    while True:
        msg = yield from connection.recv()
        if msg is None:  # connection lost
            connections.remove(connection)  # remove connection from pool, when client disconnects
            break
        else:
            print('< {}'.format(msg))
        yield from connection.send(msg)
        print('> {}'.format(msg))


@asyncio.coroutine
def send_periodically():
    while True:
        yield from asyncio.sleep(5)  # switch to other code and continue execution in 5 seconds
        for connection in connections:
            print('> Periodic event happened.')
            yield from connection.send('Periodic event happened.')  # send message to each connected client


start_server = websockets.serve(connection_handler, 'localhost', 8000)
asyncio.get_event_loop().run_until_complete(start_server)
asyncio.async(send_periodically())  # before blocking call we schedule our coroutine for sending periodic messages
asyncio.get_event_loop().run_forever()

这是一个示例客户端实现.它要求您输入名称,从echo服务器接收它,等待来自服务器的两个消息(这是我们的定期消息)并关闭连接.

import asyncio

import websockets


@asyncio.coroutine
def hello():
    connection = yield from websockets.connect('ws://localhost:8000/')
    name = input("What's your name? ")
    yield from connection.send(name)
    print("> {}".format(name))
    for _ in range(3):
        msg = yield from connection.recv()
        print("< {}".format(msg))

    yield from connection.close()


asyncio.get_event_loop().run_until_complete(hello())

重点:

1. 在Python 3.4.4 asyncio.async()中重命名为asyncio.ensure_future().
2. 有一些特殊的方法来安排延迟调用,但它们不适用于协同程序.

Answer 2

我很惊讶gather没有提到。

从Python 文档：

import asyncio

async def factorial(name, number):
    f = 1
    for i in range(2, number + 1):
        print(f"Task {name}: Compute factorial({i})...")
        await asyncio.sleep(1)
        f *= i
    print(f"Task {name}: factorial({number}) = {f}")

async def main():
    # Schedule three calls *concurrently*:
    await asyncio.gather(
        factorial("A", 2),
        factorial("B", 3),
        factorial("C", 4),
    )

asyncio.run(main())

# Expected output:
#
#     Task A: Compute factorial(2)...
#     Task B: Compute factorial(2)...
#     Task C: Compute factorial(2)...
#     Task A: factorial(2) = 2
#     Task B: Compute factorial(3)...
#     Task C: Compute factorial(3)...
#     Task B: factorial(3) = 6
#     Task C: Compute factorial(4)...
#     Task C: factorial(4) = 24

Answer 3

同样的问题,直到我在这里看到完美的样品才能得到解决方案:http://websockets.readthedocs.io/en/stable/intro.html#both

 done, pending = await asyncio.wait(
        [listener_task, producer_task],
        return_when=asyncio.FIRST_COMPLETED)  # Important

所以,我可以处理多个协程任务,如心跳和redis订阅.