Python aiohttp raise RuntimeError(‘Event loop is closed‘)

aiohttp 的 getting started 入门案例是这样写的

import aiohttp
import asyncio


async def main():
    async with aiohttp.ClientSession() as session:
        async with session.get('https://python.org') as response:
            print("Status:", response.status)
            print("Content-type:", response.headers['content-type'])

            html = await response.text()
            print("Body:", html[:15], "...")


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

运行结果为

Status: 200
Content-type: text/html; charset=utf-8
Body:  ...

看上去没问题，但是在 Python3.7 后对 asyncio 进行了改进，可以直接调用 asyncio.run() 执行协程程序，而不需管底层 API 如事件循环 loop 的操作，所以上述代码的

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

可以直接替换为

asyncio.run()

Linux 和 Mac 上这样运行是没问题的，但是在 Windows 上运行会报如下错误

Exception ignored in: 
Traceback (most recent call last):
  File "D:DevToolsPythonlibasyncioproactor_events.py", line 116, in __del__
    self.close()
  File "D:DevToolsPythonlibasyncioproactor_events.py", line 108, in close
    self._loop.call_soon(self._call_connection_lost, None)
  File "D:DevToolsPythonlibasynciobase_events.py", line 746, in call_soon
    self._check_closed()
  File "D:DevToolsPythonlibasynciobase_events.py", line 510, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

像 aiohttp 这类第三方协程库都是依赖于标准库 asyncio 的，而 asyncio 对 Windows 的支持本来就不好。Python3.8 后默认 Windows 系统上的事件循环采用 ProactorEventLoop （仅用于 Windows ）这篇文档描述了其在 Windos 下的缺陷：https://docs.python.org/zh-cn/3/library/asyncio-platforms.html#windows 

引发异常的函数是 _ProactorbasePipeTransport.__del__ ，所以 aiohttp 铁定使用了 _ProactorbasePipeTransport，并且在程序退出释放内存时自动调用了其__del__ 方法

就是上述一串连环反应最终抛出了 RuntimeError: Event loop is closed

一般的协程程序是不会使用 _ProactorbasePipeTransport 的，所以下面的代码还是可以正常的运行

import asyncio


async def main():
    print('Hello...')
    await asyncio.sleep(1)
    print('...World')


asyncio.run(main())

我特意写了个装饰器来验证这一点：

import asyncio
from asyncio.proactor_events import _ProactorbasePipeTransport
from functools import wraps


def explicit_call(func):
    @wraps(func)
    def wrappers(self, *args, **kwargs):
        print('call _ProactorbasePipeTransport.__del__')
        return func(self, *args, **kwargs)

    return wrappers


_ProactorbasePipeTransport.__del__ = explicit_call(_ProactorbasePipeTransport.__del__)


async def main():
    print('Hello...')
    await asyncio.sleep(1)
    print('...World')


asyncio.run(main())

正常执行，没有使用 _ProactorbasePipeTransport

Hello...
...World

import asyncio
from asyncio.proactor_events import _ProactorbasePipeTransport
from functools import wraps

import aiohttp


async def main():
    async with aiohttp.ClientSession() as session:
        async with session.get('https://www.baidu.com') as response:
            print("Status:", response.status)
            print("Content-type:", response.headers['content-type'])

            html = await response.text()
            print("Body:", html[:15], "...")


def explicit_call(func):
    @wraps(func)
    def wrappers(self, *args, **kwargs):
        print('call _ProactorbasePipeTransport.__del__')
        return func(self, *args, **kwargs)

    return wrappers


_ProactorbasePipeTransport.__del__ = explicit_call(_ProactorbasePipeTransport.__del__)

asyncio.run(main())

先打印 call _ProactorbasePipeTransport.__del__ 然后报错，说明使用了 _ProactorbasePipeTransport

Status: 200
Content-type: text/html
Body: 
 ...
call _ProactorbasePipeTransport.__del__
Exception ignored in: 
Traceback (most recent call last):
  File "D:CodesSpiderProjectBingImageSpiderdemo.py", line 16, in wrappers
    return func(self, *args, **kwargs)
  File "D:DevToolsPythonlibasyncioproactor_events.py", line 116, in __del__
    self.close()
  File "D:DevToolsPythonlibasyncioproactor_events.py", line 108, in close
    self._loop.call_soon(self._call_connection_lost, None)
  File "D:DevToolsPythonlibasynciobase_events.py", line 746, in call_soon
    self._check_closed()
  File "D:DevToolsPythonlibasynciobase_events.py", line 510, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

进程已结束，退出代码为 0

如果执意要在 Windows 下继续开发，有这几个方案可以选择

既然 _ProactorbasePipeTransport 会在程序结束后自动关闭事件循环，那就不要用 run 函数了，用官网的例子，乖乖使用 loop 吧

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

在调用 run 函数前，替换默认的 ProactorEventLoop 为 SelectorEventLoop

asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
asyncio.run(main())

但是 SelectorEventLoop 是有一些缺点的，比如不支持子进程等

这是 Github 上一个外国大佬的方法，在不改变源码的前提下，使用装饰器忽略掉异常

import asyncio
from asyncio.proactor_events import _ProactorbasePipeTransport
from functools import wraps

import aiohttp


async def main():
    async with aiohttp.ClientSession() as session:
        async with session.get('https://www.baidu.com') as response:
            print("Status:", response.status)
            print("Content-type:", response.headers['content-type'])

            html = await response.text()
            print("Body:", html[:15], "...")


def silence_event_loop_closed(func):
    @wraps(func)
    def wrapper(self, *args, **kwargs):
        try:
            return func(self, *args, **kwargs)
        except RuntimeError as e:
            if str(e) != 'Event loop is closed':
                raise

    return wrapper


_ProactorbasePipeTransport.__del__ = silence_event_loop_closed(_ProactorbasePipeTransport.__del__)

asyncio.run(main())

更详细的信息可以在这个 issue 上找到：https://github.com/aio-libs/aiohttp/issues/4324 

相关链接：

• https://github.com/aio-libs/aiohttp/issues/4324
• https://stackoverflow.com/questions/45600579/asyncio-event-loop-is-closed-when-getting-loop
• https://docs.python.org/zh-cn/3/library/asyncio-platforms.html#windows
• https://bugs.python.org/issue39232

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