Python 如何并发执行多个任务？

Question

Python 如何并发执行多个任务？

仓乃文 · Accepted Answer

Python 并发有三种方式：多线程（IO 密集型）、多进程（CPU 密集型）、asyncio（异步 IO）。根据任务类型选择合适方案。

多线程（适合 IO 密集型，如网络请求）：
```python
from concurrent.futures import ThreadPoolExecutor, as_completed

def fetch_url(url):
    return requests.get(url).status_code

urls = ["https://api1.com", "https://api2.com", "https://api3.com"]

with ThreadPoolExecutor(max_workers=5) as executor:
    futures = {executor.submit(fetch_url, url): url for url in urls}
    for future in as_completed(futures):
        url = futures[future]
        print(f"{url}: {future.result()}")
```

asyncio（适合高并发 IO，推荐 Python 3.7+）：
```python
import asyncio
import aiohttp

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

async def main():
    urls = ["https://api1.com", "https://api2.com", "https://api3.com"]
    async with aiohttp.ClientSession() as session:
        tasks = [fetch(session, url) for url in urls]
        results = await asyncio.gather(*tasks)  # 并发执行
        return results

asyncio.run(main())
```

补充说明：
- GIL 限制导致多线程不适合 CPU 密集型任务
- ThreadPoolExecutor 是线程池的标准用法
- asyncio 单线程协程，适合高并发 IO，性能优于多线程
- CPU 密集型用 ProcessPoolExecutor 或 multiprocessing