Should I always use `async def`?

No. Only use `async def` if you are actually going to `await` an async library (like `httpx`, `asyncpg`, or `motor`). If your code does not `await` anything, using `async def` is technically a slight performance penalty.

How do I sleep in an `async def` function?

Never use `time.sleep()`. You must use the asynchronous version: `await asyncio.sleep()`. This pauses the specific coroutine without blocking the main event loop.

Can I mix `def` and `async def` endpoints?

Absolutely. This is the recommended architecture. Use `async def` for your fast, I/O bound endpoints, and use standard `def` for heavy CPU processing endpoints.

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Async Programming

Master Asynchronous programming in FastAPI. Learn the profound differences between async def and def, how to use the await keyword, and how to avoid the catastrophic mistake of blocking the main event loop.

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