并发编程之 Asyncio

import asyncio
import aiohttp
import time

async def download_one(url):
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as resp:
            print('Read {} from {}'.format(resp.content_length, url))

async def download_all(sites):
    tasks = [asyncio.create_task(download_one(site)) for site in sites]
    await asyncio.gather(*tasks)

def main():
    sites = [
        'https://en.wikipedia.org/wiki/Portal:Arts',
        'https://en.wikipedia.org/wiki/Portal:History',
        'https://en.wikipedia.org/wiki/Portal:Society',
        'https://en.wikipedia.org/wiki/Portal:Biography',
        'https://en.wikipedia.org/wiki/Portal:Mathematics',
        'https://en.wikipedia.org/wiki/Portal:Technology',
        'https://en.wikipedia.org/wiki/Portal:Geography',
        'https://en.wikipedia.org/wiki/Portal:Science',
        'https://en.wikipedia.org/wiki/Computer_science',
        'https://en.wikipedia.org/wiki/Python_(programming_language)',
        'https://en.wikipedia.org/wiki/Java_(programming_language)',
        'https://en.wikipedia.org/wiki/PHP',
        'https://en.wikipedia.org/wiki/Node.js',
        'https://en.wikipedia.org/wiki/The_C_Programming_Language',
        'https://en.wikipedia.org/wiki/Go_(programming_language)'
    ]
    start_time = time.perf_counter()
    asyncio.run(download_all(sites))
    end_time = time.perf_counter()
    print('Download {} sites in {} seconds'.format(len(sites), end_time - start_time))
    
if __name__ == '__main__':
    main()

## 输出
Read 63153 from https://en.wikipedia.org/wiki/Java_(programming_language)
Read 31461 from https://en.wikipedia.org/wiki/Portal:Society
Read 23965 from https://en.wikipedia.org/wiki/Portal:Biography
Read 36312 from https://en.wikipedia.org/wiki/Portal:History
Read 25203 from https://en.wikipedia.org/wiki/Portal:Arts
Read 15160 from https://en.wikipedia.org/wiki/The_C_Programming_Language
Read 28749 from https://en.wikipedia.org/wiki/Portal:Mathematics
Read 29587 from https://en.wikipedia.org/wiki/Portal:Technology
Read 79318 from https://en.wikipedia.org/wiki/PHP
Read 30298 from https://en.wikipedia.org/wiki/Portal:Geography
Read 73914 from https://en.wikipedia.org/wiki/Python_(programming_language)
Read 62218 from https://en.wikipedia.org/wiki/Go_(programming_language)
Read 22318 from https://en.wikipedia.org/wiki/Portal:Science
Read 36800 from https://en.wikipedia.org/wiki/Node.js
Read 67028 from https://en.wikipedia.org/wiki/Computer_science
Download 15 sites in 0.062144195078872144 seconds

Asyncio 是单线程的，但其内部 event loop 的机制，可以让它并发地运行多个不同的任务，并且比多线程享有更大的自主控制权。

Asyncio 中的任务， 在运行过程中不会被打断，因此不会出现 race condition 的情况。尤其是在 I/O 操作 heavy 的场景下， Asyncio 比多线程的运行效率更高，因此 Asyncio 内部任务切换的损耗，远比线程切换的损耗要小，并且 Asyncio 可以开启的任务数量，也比多线程中的线程数列多得多。

但需要注意的是，很多情况下，使用 Asyncio 需要特定第三方库的支持，而如果 I/O 操作很快，并不 heavy ，建议使用多线程来解决问题。

GIL（全局解释器锁）

CPython 引进 GIL 主要是由于：