Python Asyncio阻止了协同程序

我正在尝试编写基于Asyncio的简单程序和使用ZeroMQ实现的发布/订阅设计模式.出版商有2个协同程序;一个用于侦听传入的订阅,另一个用于将值(通过HTTP请求获取)发布到订阅者.订户订阅特定参数(在这种情况下为城市名称),并等待该值(该城市的温度).

这是我的代码：

publisher.py

#!/usr/bin/env python

import json
import aiohttp
import aiozmq
import asyncio
import zmq


class Publisher:
    BIND_ADDRESS = 'tcp://*:10000'

    def __init__(self):
        self.stream = None
        self.parameter = ""

    @asyncio.coroutine
    def main(self):
        self.stream = yield from aiozmq.create_zmq_stream(zmq.XPUB, bind=Publisher.BIND_ADDRESS)
        tasks = [
            asyncio.async(self.subscriptions()),
            asyncio.async(self.publish())]
        print("before wait")
        yield from asyncio.wait(tasks)
        print("after wait")

    @asyncio.coroutine
    def subscriptions(self):
        print("Entered subscriptions coroutine")
        while True:
            print("New iteration of subscriptions loop")
            received = yield from self.stream.read()
            first_byte = received[0][0]
            self.parameter = received[0][-len(received[0])+1:].decode("utf-8")
            # Subscribe request
            if first_byte == 1:
                print("subscription request received for parameter "+self.parameter)
            # Unsubscribe request
            elif first_byte == 0:
                print("Unsubscription request received for parameter "+self.parameter)


    @asyncio.coroutine
    def publish(self):
        print("Entered publish coroutine")
        while True:
            if self.parameter:
                print("New iteration of publish loop")

                # Make HTTP request
                url = "http://api.openweathermap.org/data/2.5/weather?q="+self.parameter
                response = yield from aiohttp.request('GET', url)
                assert response.status == 200
                content = yield from response.read()

                # Decode JSON string
                decoded_json = json.loads(content.decode())

                # Get parameter value
                value = decoded_json["main"]["temp"]

                # Publish fetched values to subscribers
                message = bytearray(self.parameter+":"+str(value),"utf-8")
                print(message)
                pack = [message]

                print("before write")
                yield from self.stream.write(pack)
                print("after write")

            yield from asyncio.sleep(10)

test = Publisher()
loop = asyncio.get_event_loop()
loop.run_until_complete(test.main())

subscriber.py

#!/usr/bin/env python

import zmq

class Subscriber:
    XSUB_CONNECT = 'tcp://localhost:10000'

    def __init__(self):
        self.context = zmq.Context()
        self.socket = self.context.socket(zmq.XSUB)
        self.socket.connect(Subscriber.XSUB_CONNECT)

    def loop(self):
        print(self.socket.recv())
        self.socket.close()

    def subscribe(self, parameter):
        self.socket.send_string('\x01'+parameter)
        print("Subscribed to parameter "+parameter)

    def unsubscribe(self, parameter):
        self.socket.send_string('\x00'+parameter)
        print("Unsubscribed to parameter "+parameter)

test = Subscriber()
test.subscribe("London")
while True:
    print(test.socket.recv())

这是输出：

订阅方：

$python3 subscriber.py 
    Subscribed to parameter London
    b'London:288.15'

出版方：

$python3 publisher.py 
    before wait
    Entered subscriptions coroutine
    New iteration of subscriptions loop
    Entered publish coroutine
    subscription request received for parameter London
    New iteration of subscriptions loop
    New iteration of publish loop
    bytearray(b'London:288.15')
    before write

该计划被困在那里.

如您所见,输出中出现“before write”并发送消息,但不会出现“after write”.所以,我认为可能会引发异常,并在self.stream.write(pack)调用堆栈中的某处捕获.

如果我向发布者发送KeyboardInterrupt,这是我得到的：

Traceback (most recent call last):
  File "publisher.py", line 73, in <module>
    loop.run_until_complete(test.main())
  File "/usr/lib/python3.4/asyncio/base_events.py", line 304, in run_until_complete
    self.run_forever()
  File "/usr/lib/python3.4/asyncio/base_events.py", line 276, in run_forever
    self._run_once()
  File "/usr/lib/python3.4/asyncio/base_events.py", line 1136, in _run_once
    event_list = self._selector.select(timeout)
  File "/usr/lib/python3.4/selectors.py", line 432, in select
    fd_event_list = self._epoll.poll(timeout, max_ev)
KeyboardInterrupt
Task exception was never retrieved
future: <Task finished coro=<publish() done, defined at publisher.py:43> exception=TypeError("'NoneType' object is not iterable",)>
Traceback (most recent call last):
  File "/usr/lib/python3.4/asyncio/tasks.py", line 236, in _step
    result = coro.send(value)
  File "publisher.py", line 66, in publish
    yield from self.stream.write(pack)
TypeError: 'NoneType' object is not iterable
Task was destroyed but it is pending!
task: <Task pending coro=<subscriptions() running at publisher.py:32> wait_for=<Future pending cb=[Task._wakeup()]> cb=[_wait.<locals>._on_completion() at /usr/lib/python3.4/asyncio/tasks.py:399]>

所以我猜我的问题实际上是这个错误：TypeError：’NoneType’对象不可迭代,但我不知道是什么导致它.

这里出了什么问题？

解决方法:

问题是你试图从调用self.stream.write(),但stream.write isn’t actually a coroutine.当你从一个项目调用yield时,Python内部调用iter(item).在这种情况下,对write()的调用返回None,因此Python正在尝试执行iter(None) – 因此您看到的异常.

要修复它,你应该像普通函数一样调用write().如果您想实际等到刷新写入并发送到阅读器,请在调用write()后使用yield from stream.drain()：

print("before write")
self.stream.write(pack)
yield from self.stream.drain()
print("after write")

此外,要确保在不需要Ctrl C的情况下引发发布中的异常,请使用asyncio.gather而不是asyncio.wait：

    yield from asyncio.gather(*tasks)

使用asyncio.gather,任务内部任务抛出的任何异常都将重新引发.

标签：python,asynchronous,zeromq,publish-subscribe,python-asyncio
来源： https://codeday.me/bug/20190528/1170012.html