python | 多线程中的 event loop

这篇文章算是

的延伸。

import asyncio
import traceback
from concurrent.futures import ThreadPoolExecutor, as_completed

import aiohttp


class T:

    async def get(self, index):
        async with aiohttp.ClientSession() as session:
            for j in range(10):
                async with session.get("http://www.baidu.com") as r:
                    t = await r.text()
                    print(index)


test = T()


def main(index):
    asyncio.run(test.get(index))


if __name__ == '__main__':
    task_list = []
    pools = ThreadPoolExecutor(5)
    for i in range(5):
        task_list.append(pools.submit(main, i))

    # 阻塞主进程
    for result in as_completed(task_list):
        data = result.result()

    print("end")

上面的代码运行是没有问题的。

如果改成

import asyncio
import traceback
from concurrent.futures import ThreadPoolExecutor, as_completed

import aiohttp


class T:

    async def get(self, index):
        async with aiohttp.ClientSession() as session:
            for j in range(10):
                async with session.get("http://www.baidu.com") as r:
                    t = await r.text()
                    print(index)


test = T()


def main(index):
    loop = asyncio.get_event_loop()
    asyncio.set_event_loop(loop)
    loop.run_until_complete(test.get(index))


if __name__ == '__main__':
    task_list = []
    pools = ThreadPoolExecutor(5)
    for i in range(5):
        task_list.append(pools.submit(main, i))

    # 阻塞主进程
    for result in as_completed(task_list):
        data = result.result()

    print("end")

则会报错

RuntimeError: There is no current event loop in thread 'ThreadPoolExecutor-0_0'.

这是因为，主线程可以通过 get_event_loop 获得一个 event loop，但是，多线程是不行的，需要明面申请。

python | 事件循环

所以，可以改成

import asyncio
import traceback
from concurrent.futures import ThreadPoolExecutor, as_completed

import aiohttp


class T:

    async def get(self, index):
        async with aiohttp.ClientSession() as session:
            for j in range(10):
                async with session.get("http://www.baidu.com") as r:
                    t = await r.text()
                    print(index)


test = T()


def main(index):
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    loop.run_until_complete(test.get(index))


if __name__ == '__main__':
    task_list = []
    pools = ThreadPoolExecutor(5)
    for i in range(5):
        task_list.append(pools.submit(main, i))

    # 阻塞主进程
    for result in as_completed(task_list):
        data = result.result()

    print("end")

上面 3 个例子清晰的表明了如何在多线程中使用协程。

有的人认为这不是脱裤子放屁，多此一举吗？直接一个协程不就行了吗，而且，多线程还有切换的开销，这样做不是降低速度吗？

在多线程中用协程，是和使用场景有关系的，不过在这里不再展开。