What is asyncio.Semaphore and when to use it?

Question

Accepted Answer

**asyncio.Semaphore** limits the **number of concurrent tasks**.

```python
import asyncio

async def fetch(url, sem):
    async with sem:  # Acquire semaphore
        print(f"Fetching {url}")
        await asyncio.sleep(1)  # Simulate network request
        return f"Data from {url}"

async def main():
    sem = asyncio.Semaphore(3)  # Max 3 concurrent
    urls = [f"url{i}" for i in range(10)]

tasks = [fetch(url, sem) for url in urls]
    results = await asyncio.gather(*tasks)
    print(results)

asyncio.run(main())
# Only 3 requests run simultaneously
```

**Methods:**
- `acquire()` — wait for a slot.
- `release()` — free a slot.
- `locked()` — check if full.

**Use cases:**
- Rate limiting API calls.
- Connection pool limits.
- Preventing resource exhaustion.

Python: What is asyncio.Semaphore and when to use it?