This could be useful, for example, to expose the same API under different prefixes, e.g. `/api/v1` and `/api/latest`.

This is an advanced usage that you might not really need, but it's there in case you do.

## Include an `APIRouter` in another

The same way you can include an `APIRouter` in a `FastAPI` application, you can include an `APIRouter` in another `APIRouter` using:

    * Generating **automatic errors** returned to the client when the data is invalid.
* **Document** the API using OpenAPI:
    * which is then used by the automatic interactive documentation user interfaces.

This might all sound abstract. Don't worry. You'll see all this in action in the [Tutorial - User Guide](tutorial/index.md){.internal-link target=_blank}.

    * If you depended on that previous behavior, you might need to update your code. As always, make sure your tests pass before merging the upgrade.

## 0.90.1

### Upgrades

### Suggest solutions

* After being able to understand the question, you can give them a possible **answer**.

* In many cases, it's better to understand their **underlying problem or use case**, because there might be a better way to solve it than what they are trying to do.

---

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.

Anyway, in any of the cases above, FastAPI will still work asynchronously and be extremely fast.

But by following the steps above, it will be able to do some performance optimizations.

<div style="text-align: right; margin-right: 10%;">Kabir Khan - <strong>Microsoft</strong> <a href="https://github.com/tiangolo/fastapi/pull/26" target="_blank"><small>(ref)</small></a></div>

---

"_Chúng tôi tích hợp thư viện **FastAPI** để sinh ra một **REST** server, nó có thể được truy vấn để thu được những **dự đoán**._ [bởi Ludwid] "

<div style="text-align: right; margin-right: 10%;">Kabir Khan - <strong>Microsoft</strong> <a href="https://github.com/tiangolo/fastapi/pull/26" target="_blank"><small>(ref)</small></a></div>

---

"_**FastAPI** kitabxanasını **Proqnozlar** əldə etmək üçün sorğulana bilən **REST** serverini yaratmaqda istifadə etdik._"

<div style="text-align: right; margin-right: 10%;">Kabir Khan - <strong>Microsoft</strong> <a href="https://github.com/tiangolo/fastapi/pull/26" target="_blank"><small>(ref)</small></a></div>

---

"_A gba àwọn ohun èlò ìwé afọwọkọ **FastAPI** tí kò yí padà láti ṣẹ̀dá olùpín **REST** tí a lè béèrè lọ́wọ́ rẹ̀ láti gba **àsọtẹ́lẹ̀**. [fún Ludwig]_"

<div style="text-align: right; margin-right: 10%;">Kabir Khan - <strong>Microsoft</strong> <a href="https://github.com/tiangolo/fastapi/pull/26" target="_blank"><small>(ref)</small></a></div>

---

"*Estou extremamente entusiasmado com o **FastAPI**. É tão divertido!*"

<div style="text-align: right; margin-right: 10%;">Kabir Khan - <strong>Microsoft</strong> <a href="https://github.com/tiangolo/fastapi/pull/26" target="_blank"><small>(ref)</small></a></div>

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"_FastAPIライブラリを採用し、クエリで**予測値**を取得できる**REST**サーバを構築しました。 [for Ludwig]_"