Python异步: second coroutine wrapped in a Task is not scheduled (but first is)

Question

我编写了一个基于asyncio.Protocol的Echo客户端。我有兴趣在 data_received 收到一些数据时触发回调。

我试图通过在 data_received 中的协程中创建 Task 来实现此目的，然后通过 add_done_callback 进行回调。

这是客户端的代码:

import asyncio

class EchoClientProtocol(asyncio.Protocol):

    def __init__(self, loop):
        self.loop = loop
        self.tasks = None

        # To be initialised upon connection
        self.transport = None

    def send(self, msg):
        print('>', msg)
        self.transport.write(msg)

    def connection_made(self, transport):
        self.transport = transport
        self.logon()

    def data_received(self, msg):
        print('<', msg)
        if msg == b'logon':
            task = self.loop.create_task(next(self.tasks)())
            task.add_done_callback(next(self.tasks))
        else:
            try:
                task = self.loop.create_task(next(self.tasks)())
                task.add_done_callback(next(self.tasks))
            except StopIteration:
                print('no more messages')
                return

    def eof_received(self):
        """Triggered when the other end signals it won't send any more data"""
        print('EOF received')

    def connection_lost(self, exc):
        """Triggered when connection is lost"""
        print('Exception: %s', exc)
        self.loop.stop()

    def logon(self):
        self.send(b'logon')

主要内容如下:

def main():
    loop = asyncio.get_event_loop()
    EchoClient = EchoClientProtocol(loop)

    # access recv and send here
    first_msg = b'message 1'
    second_msg = b'message 2'

    @asyncio.coroutine
    def first():
        yield EchoClient.send(first_msg)

    @asyncio.coroutine
    def second(*a):
        yield EchoClient.send(second_msg)

    def tasks():
        yield first
        yield second

    EchoClient.tasks = tasks()

    loop = asyncio.get_event_loop()
    coro = loop.create_connection(lambda: EchoClient, '127.0.0.1', 2000)
    loop.run_until_complete(coro)
    loop.run_forever()
    loop.close()

if __name__ == '__main__':
    main()

这是我运行客户端时的输出(它假设 Echo 服务器正在端口 2000 上运行 - 如果您没有，这将解决问题:socat TCP4-LISTEN:2000,fork EXEC :猫):

jir@klo01 $ python3 echo_client_asyncio3.py 
> b'logon'
< b'logon'
> b'message 1'
< b'message 1'
no more messages
^CTraceback (most recent call last):
  File "echo_client_asyncio3.py", line 76, in <module>
    main()
  File "echo_client_asyncio3.py", line 72, in main
    loop.run_forever()
  File "/home/jir/apps/lib/python3.6/asyncio/base_events.py", line 421, in run_forever
    self._run_once()
  File "/home/jir/apps/lib/python3.6/asyncio/base_events.py", line 1389, in _run_once
    event_list = self._selector.select(timeout)
  File "/home/jir/apps/lib/python3.6/selectors.py", line 445, in select
    fd_event_list = self._epoll.poll(timeout, max_ev)
KeyboardInterrupt

如您所见，第一个协程已正确调度:logon 被发送和接收，message1 被发送和接收。但是，message2 似乎从未得到安排。

知道我在这里做错了什么吗？

Answer 1

task.add_done_callback(f) 接受常规函数而不是协程。

假设进行以下更改:

def tasks():
    yield first()
    yield second()

这会起作用:

    task = self.loop.create_task(next(self.tasks))
    next_task = next(self.tasks)
    task.add_done_callback(
        lambda fut: self.loop.create_task(next_task))

请记住，您可以使用包装器任务而不是使用 add_done_callback:

async def first_and_second():
    await first()
    await second()