Python异步IO之asyncio举例深度解析

作者：袖梨 2026-08-08

处理Python异步IO之asyncio举例深度解析这类问题时，先确认目标场景，再按步骤核对配置或玩法细节。

1. 异步编程概述

异步编程是一种编程范式，它允许程序在等待某个操作完成时继续执行其他任务，而不是阻塞等待。在Python中，asyncio库是实现异步编程的核心。

核心概念

同步编程：代码按顺序执行，一个操作完成后才开始下一个操作
异步编程：代码可以在等待某些操作（如I/O）时执行其他任务
协程：轻量级的并发执行单元
事件循环：管理和调度协程的执行
Future：表示异步操作的结果
Task：对协程的封装，用于并发执行

2. asyncio基础

2.1 协程定义

在Python中，协程可以通过async def关键字定义：

import asyncioasync def hello():    print('Hello')    await asyncio.sleep(1)    print('World')# 运行协程asyncio.run(hello())

2.2 事件循环

事件循环是asyncio的核心，它负责调度协程的执行：

import asyncioasync def task1():    print('Task 1 started')    await asyncio.sleep(2)    print('Task 1 completed')async def task2():    print('Task 2 started')    await asyncio.sleep(1)    print('Task 2 completed')async def main():    # 创建任务    t1 = asyncio.create_task(task1())    t2 = asyncio.create_task(task2())        # 等待任务完成    await t1    await t2# 运行主协程asyncio.run(main())

2.3 await关键字

await关键字用于等待一个异步操作完成：

async def fetch_data(url):    print(f'Fetching data from {url}')    # 模拟网络请求    await asyncio.sleep(2)    return f'Data from {url}'async def main():    # 串行执行    data1 = await fetch_data('https://api.example.com/data1')    data2 = await fetch_data('https://api.example.com/data2')    print(data1, data2)asyncio.run(main())

3. 并发执行

3.1 asyncio.gather

asyncio.gather用于并发执行多个协程：

async def fetch_data(url):    print(f'Fetching data from {url}')    await asyncio.sleep(2)    return f'Data from {url}'async def main():    # 并发执行    results = await asyncio.gather(        fetch_data('https://api.example.com/data1'),        fetch_data('https://api.example.com/data2'),        fetch_data('https://api.example.com/data3')    )    print(results)asyncio.run(main())

3.2 asyncio.create_task

asyncio.create_task用于创建后台任务：

async def background_task():    while True:        print('Background task running')        await asyncio.sleep(1)async def main():    # 创建后台任务    task = asyncio.create_task(background_task())        # 执行其他操作    print('Main task running')    await asyncio.sleep(3)        # 取消后台任务    task.cancel()    try:        await task    except asyncio.CancelledError:        print('Background task cancelled')asyncio.run(main())

3.3 asyncio.wait

asyncio.wait用于等待多个协程完成：

async def task1():    await asyncio.sleep(2)    return 'Task 1 result'async def task2():    await asyncio.sleep(1)    return 'Task 2 result'async def main():    tasks = [task1(), task2()]    done, pending = await asyncio.wait(tasks, timeout=1.5)        print('Done tasks:', len(done))    print('Pending tasks:', len(pending))        for task in done:        print('Result:', await task)asyncio.run(main())

4. 异步IO操作

4.1 文件IO

使用aiofiles库进行异步文件操作：

import asyncioimport aiofilesasync def read_file(filename):    async with aiofiles.open(filename, 'r') as f:        content = await f.read()    return contentasync def write_file(filename, content):    async with aiofiles.open(filename, 'w') as f:        await f.write(content)async def main():    # 读取文件    content = await read_file('example.txt')    print('File content:', content)        # 写入文件    await write_file('output.txt', 'Hello, asyncio!')    print('File written')asyncio.run(main())

4.2 网络IO

使用aiohttp库进行异步网络请求：

import asyncioimport aiohttpasync 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://example.com')        print('HTML length:', len(html))asyncio.run(main())

4.3 数据库操作

使用asyncpg库进行异步数据库操作：

import asyncioimport asyncpgasync def main():    # 连接数据库    conn = await asyncpg.connect(        host='localhost',        port=5432,        user='postgres',        password='password',        database='mydb'    )        # 执行查询    rows = await conn.fetch('SELECT * FROM users')    for row in rows:        print(row)        # 关闭连接    await conn.close()asyncio.run(main())

5. 高级特性

5.1 异步上下文管理器

使用async with语句创建异步上下文管理器：

import asyncioclass AsyncContextManager:    async def __aenter__(self):        print('Entering context')        await asyncio.sleep(1)        return self        async def __aexit__(self, exc_type, exc_val, exc_tb):        print('Exiting context')        await asyncio.sleep(1)async def main():    async with AsyncContextManager() as cm:        print('Inside context')        await asyncio.sleep(2)asyncio.run(main())

5.2 异步迭代器

使用async for语句创建异步迭代器：

import asyncioclass AsyncIterator:    def __init__(self, start, end):        self.start = start        self.end = end        def __aiter__(self):        self.current = self.start        return self        async def __anext__(self):        if self.current >= self.end:            raise StopAsyncIteration        value = self.current        self.current += 1        await asyncio.sleep(0.5)        return valueasync def main():    async for num in AsyncIterator(1, 5):        print(num)asyncio.run(main())

5.3 任务组

Python 3.11+引入了任务组，用于更安全地管理并发任务：

import asyncioasync def task(id, duration):    print(f'Task {id} started')    await asyncio.sleep(duration)    print(f'Task {id} completed')    return f'Task {id} result'async def main():    async with asyncio.TaskGroup() as tg:        # 创建任务        task1 = tg.create_task(task(1, 2))        task2 = tg.create_task(task(2, 1))        task3 = tg.create_task(task(3, 3))        # 任务组退出时，所有任务已完成    print('All tasks completed')    print('Task 1 result:', task1.result())    print('Task 2 result:', task2.result())    print('Task 3 result:', task3.result())asyncio.run(main())

6. 性能优化

6.1 避免阻塞操作

在异步代码中避免使用阻塞操作，如同步IO：

# 错误示例async def bad_example():    # 阻塞操作    time.sleep(1)  # 这会阻塞整个事件循环    print('Done')# 正确示例async def good_example():    # 异步操作    await asyncio.sleep(1)  # 这会释放事件循环    print('Done')

6.2 批量操作

对于多个IO操作，使用并发执行：

async def fetch_all(urls):    async with aiohttp.ClientSession() as session:        tasks = [fetch(session, url) for url in urls]        results = await asyncio.gather(*tasks)    return results

6.3 超时处理

为异步操作设置超时：

async def fetch_with_timeout(url, timeout=5):    try:        async with aiohttp.ClientSession() as session:            async with asyncio.timeout(timeout):                async with session.get(url) as response:                    return await response.text()    except asyncio.TimeoutError:        return 'Request timed out'

7. 常见陷阱

7.1 忘记await

忘记使用await会导致协程不会执行：

async def foo():    print('Foo')    await asyncio.sleep(1)    print('Bar')async def main():    foo()  # 错误：忘记await，协程不会执行    await foo()  # 正确：使用awaitasyncio.run(main())

7.2 阻塞事件循环

在协程中使用阻塞操作会阻塞整个事件循环：

async def blocking_operation():    # 错误：使用阻塞操作    time.sleep(1)  # 这会阻塞事件循环    return 'Done'async def main():    # 正确：使用线程池执行阻塞操作    loop = asyncio.get_event_loop()    result = await loop.run_in_executor(None, lambda: time.sleep(1))    return result

7.3 任务泄漏

创建的任务如果不等待或取消，会导致任务泄漏：

async def background_task():    while True:        await asyncio.sleep(1)        print('Background task')async def main():    # 错误：创建任务但不管理    asyncio.create_task(background_task())    await asyncio.sleep(5)    # 任务会继续运行，导致泄漏async def main_fixed():    # 正确：管理任务生命周期    task = asyncio.create_task(background_task())    await asyncio.sleep(5)    task.cancel()    try:        await task    except asyncio.CancelledError:        pass

8. 代码示例

8.1 异步Web服务器

使用aiohttp创建异步Web服务器：

from aiohttp import webasync def handle(request):    name = request.match_info.get('name', 'World')    # 模拟异步操作    await asyncio.sleep(0.5)    return web.Response(text=f'Hello, {name}!')async def main():    app = web.Application()    app.add_routes([        web.get('/', handle),        web.get('/{name}', handle)    ])    runner = web.AppRunner(app)    await runner.setup()    site = web.TCPSite(runner, 'localhost', 8080)    await site.start()    print('Server started on http://localhost:8080')    # 保持运行    await asyncio.Future()  # 无限等待if __name__ == '__main__':    asyncio.run(main())

8.2 异步爬虫

使用aiohttp创建异步爬虫：

import asyncioimport aiohttpfrom bs4 import BeautifulSoupasync def fetch_url(session, url):    try:        async with session.get(url) as response:            return await response.text()    except Exception as e:        print(f'Error fetching {url}: {e}')        return ''async def parse_page(html):    soup = BeautifulSoup(html, 'html.parser')    links = []    for a in soup.find_all('a', href=True):        links.append(a['href'])    return linksasync def crawl(start_url, max_depth=2):    visited = set()    queue = [(start_url, 0)]        async with aiohttp.ClientSession() as session:        while queue:            url, depth = queue.pop(0)            if url in visited or depth >= max_depth:                continue                        visited.add(url)            print(f'Crawling {url} (depth: {depth})')                        html = await fetch_url(session, url)            if not html:                continue                        links = await parse_page(html)            for link in links:                if link.startswith('http'):                    queue.append((link, depth + 1))async def main():    await crawl('https://example.com')asyncio.run(main())

8.3 异步数据库操作

使用asyncpg进行异步数据库操作：

import asyncioimport asyncpgasync def setup_database():    # 连接数据库    conn = await asyncpg.connect(        host='localhost',        port=5432,        user='postgres',        password='password',        database='mydb'    )        # 创建表    await conn.execute('''        CREATE TABLE IF NOT EXISTS users (            id SERIAL PRIMARY KEY,            name VARCHAR(100),            email VARCHAR(100) UNIQUE        )    ''')        # 插入数据    await conn.execute(        'INSERT INTO users (name, email) VALUES ($1, $2) ON CONFLICT DO NOTHING',        'Alice', '[email protected]'    )    await conn.execute(        'INSERT INTO users (name, email) VALUES ($1, $2) ON CONFLICT DO NOTHING',        'Bob', '[email protected]'    )        # 查询数据    rows = await conn.fetch('SELECT * FROM users')    print('Users:')    for row in rows:        print(f'ID: {row["id"]}, Name: {row["name"]}, Email: {row["email"]}')        # 关闭连接    await conn.close()asyncio.run(setup_database())