Python 异步编程与协程实战指南

创建与取消任务

import asyncio

async def task_func():
    try:
        print('Task running...')
        await asyncio.sleep(5)
    except asyncio.CancelledError:
        print('Task cancelled!')

async def main():
    task = asyncio.create_task(task_func())
    await asyncio.sleep(0.5)
    task.cancel()
    try:
        await task
    except asyncio.CancelledError:
        print('Main: Task cancelled!')

asyncio.run(main())

超时控制

防止任务无限期挂起，可以使用 wait_for 设置超时：

import asyncio

async def slow_task():
    await asyncio.sleep(2)
    return 'Done'

async def main():
    try:
        result = await asyncio.wait_for(slow_task(), timeout=1)
        print(result)
    except asyncio.TimeoutError:
        print('Task timed out!')

asyncio.run(main())

事件循环：异步的心脏

事件循环负责调度所有协程的执行。虽然现代 Python 推荐使用 asyncio.run() 自动管理循环，但理解其原理有助于调试复杂场景。

import asyncio

loop = asyncio.get_event_loop()

async def hello():
    print('Hello from loop')

loop.run_until_complete(hello())

核心库：asyncio 与 aiohttp

asyncio 是标准库，提供基础支持；aiohttp 则是基于 asyncio 的高性能 HTTP 客户端/服务端库。

安装依赖

pip install aiohttp

发送 HTTP 请求

使用 ClientSession 管理连接池，避免重复握手开销：

import aiohttp
import asyncio

async def fetch(session, url):
    async with session.get(url) as response:
        return await response.text()

async def main():
    async with aiohttp.ClientSession() as session:
        html = await fetch(session, 'https://www.example.com')
        print(html)

asyncio.run(main())

支持 POST 和 JSON 数据传递：

import aiohttp
import asyncio
import json

async def post_json(session, url, data):
    async with session.post(url, json=data) as response:
        return await response.text()

async def main():
    async with aiohttp.ClientSession() as session:
        payload = {'name': '张三', 'age': 25}
        response = await post_json(session, 'https://httpbin.org/post', payload)
        print(response)

asyncio.run(main())

实战案例

1. 异步 HTTP 客户端

批量抓取多个网站信息，对比耗时差异：

import aiohttp
import asyncio
import time

async def fetch(session, url):
    start_time = time.time()
    async with session.get(url) as response:
        text = await response.text()
        elapsed_time = time.time() - start_time
        return url, len(text), elapsed_time

async def main():
    urls = [
        'https://www.example.com',
        'https://www.google.com',
        'https://www.github.com',
        'https://www.python.org'
    ]
    
    async with aiohttp.ClientSession() as session:
        tasks = [asyncio.create_task(fetch(session, url)) for url in urls]
        results = await asyncio.gather(*tasks)
        
        for url, length, elapsed_time in results:
            print(f'URL: {url}, Length: {length}, Time: {elapsed_time:.2f}s')

if __name__ == '__main__':
    start_time = time.time()
    asyncio.run(main())
    print(f'Total Time: {time.time() - start_time:.2f}s')

2. 异步 Web 服务器

利用 aiohttp.web 快速构建高性能 API 服务：

from aiohttp import web
import asyncio

async def handle_root(request):
    return web.Response(text='Hello, World!')

async def handle_api(request):
    data = {'name': '张三', 'age': 25}
    return web.json_response(data)

async def create_app():
    app = web.Application()
    app.add_routes([
        web.get('/', handle_root),
        web.get('/api', handle_api),
    ])
    return app

if __name__ == '__main__':
    # 启动服务监听 localhost:8080
    asyncio.run(web.run_app(create_app(), host='localhost', port=8080))

结语

掌握异步编程的关键在于理解事件循环的调度机制以及 await 的语义。在实际开发中，合理使用 gather、create_task 等工具，配合 aiohttp 等成熟库，能够轻松应对高并发网络需求。建议多动手编写示例，体会非阻塞带来的性能提升。