asyncio 警告 DeprecationWarning: There is no current event loop

以下为实现异步抓取网页的代码段：

import asyncio
import aiohttp


async def fetch_page(url):
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as response:
            return response.status

tasks = [fetch_page('http://books.toscrape.com') for i in range(50)]

loop = asyncio.get_event_loop()

loop.run_until_complete(asyncio.gather(*tasks))

当前 Python 版本为 3.10.5，运行以上代码，会出现如下警告：

DeprecationWarning: There is no current event loop
  loop = asyncio.get_event_loop()
DeprecationWarning: There is no current event loop
  loop.run_until_complete(asyncio.gather(*tasks))

总之 asyncio.get_event_loop()， asyncio.gather(*tasks) 两个函数都不能用。

代码改为使用 asyncio.run() ，测试ok：

import asyncio
import aiohttp


async def fetch_page(url):
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as response:
            print(response.status)


async def main():
    tasks = [fetch_page('https://quotes.toscrape.com/') for i in range(50)]
    await asyncio.gather(*tasks)  # argument unpacking: (tasks[0], tasks[1], ... tasks[49])

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

以下语句必须要加，

# important
asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

否则 windows 系统对于 https 网站，会出现错误

RuntimeError: Event loop is closed