Python 异步编程与协程实战：asyncio 与 aiohttp 详解

取消与超时控制

任务支持取消操作，需捕获 CancelledError。同时可使用 wait_for 设置超时限制。

import asyncio

async def hello():
    try:
        print('Hello, World!')
        await asyncio.sleep(1)
        print('Hello again!')
    except asyncio.CancelledError:
        print('Task cancelled!')

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

asyncio.run(main())

import asyncio

async def hello():
    print('Hello, World!')
    await asyncio.sleep(2)
    print('Hello again!')

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

asyncio.run(main())

事件循环

事件循环是异步编程的核心，负责从队列取出任务并执行。虽然现代 Python 推荐使用 asyncio.run()，但理解底层机制依然重要。

import asyncio

loop = asyncio.get_event_loop()

async def hello():
    print('Hello, World!')
    await asyncio.sleep(1)
    print('Hello again!')

loop.run_until_complete(hello())

asyncio 库核心用法

asyncio 提供了丰富的工具函数：

• run(): 运行顶层协程。
• create_task(): 创建并发任务。
• gather(): 收集多个协程结果。
• wait_for(): 带超时的等待。
• sleep(): 暂停指定时间。

高级用法中，Future 对象可用于回调通知。

import asyncio

async def hello():
    print('Hello, World!')
    await asyncio.sleep(1)
    print('Hello again!')

async def main():
    future = asyncio.Future()
    task = asyncio.create_task(hello())
    task.add_done_callback(lambda t: future.set_result(t.result()))
    await future

asyncio.run(main())

aiohttp 库实战

安装依赖

pip install aiohttp

HTTP 请求示例

发送 GET 请求：

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())

发送 POST 请求：

import aiohttp
import asyncio
import json

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

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

asyncio.run(main())

发送 JSON 数据：

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:
        data = {'name': '张三', 'age': 25}
        response = await post_json(session, 'https://httpbin.org/post', data)
        print(response)

asyncio.run(main())

实战案例：异步 HTTP 客户端

构建一个支持高并发请求的客户端，统计响应长度与耗时。

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',
        'https://www.djangoproject.com'
    ]
    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}, 响应长度：{length}, 耗时：{elapsed_time:.2f}秒')

if __name__ == '__main__':
    start_time = time.time()
    asyncio.run(main())
    elapsed_time = time.time() - start_time
    print(f'总耗时：{elapsed_time:.2f}秒')

实战案例：异步 Web 服务器

基于 aiohttp.web 快速搭建支持路由和静态文件的服务器。

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 handle_static(request):
    return web.FileResponse('static/index.html')

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

if __name__ == '__main__':
    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与协程的生命周期。通过 asyncio 和 aiohttp，开发者能轻松构建高性能的网络应用。建议在实际项目中多尝试并发模式，根据业务场景选择合适的调度策略。