results = some_library()
    return results
```

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If your application (somehow) doesn't have to communicate with anything else and wait for it to respond, use `async def`.

---

If you just don't know, use normal `def`.

---

**Note**: You can mix `def` and `async def` in your *path operation functions* as much as you need and define each one using the best option for you. FastAPI will do the right thing with them.