Pydantic, which is what powers all the data validation and serialization in FastAPI, has a special behavior when you use `Optional` or `Union[Something, None]` without a default value, you can read more about it in the Pydantic docs about <a href="https://docs.pydantic.dev/2.3/usage/models/#required-optional-fields" class="external-link" target="_blank">Required fields</a>.

///

/// tip

	// https://istio.io/docs/setup/platform-setup/
	MinK8SVersion               = 28
	UnSupportedK8SVersionLogMsg = "\nThe Kubernetes version %s is not supported by Istio %s. The minimum supported Kubernetes version is 1.%d.\n" +
		"Proceeding with the installation, but you might experience problems. " +
		"See https://istio.io/latest/docs/releases/supported-releases/ for a list of supported versions.\n"
)

///

## Check the automatic API docs

Now, run your app:

<div class="termy">

```console
$ fastapi dev app/main.py

<span style="color: green;">INFO</span>:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
```

</div>

And open the docs at <a href="http://127.0.0.1:8000/docs" class="external-link" target="_blank">http://127.0.0.1:8000/docs</a>.

So, the line:

```Python
    uvicorn.run(app, host="0.0.0.0", port=8000)
```

will not be executed.

/// info

For more information, check <a href="https://docs.python.org/3/library/__main__.html" class="external-link" target="_blank">the official Python docs</a>.

///

## Run your code with your debugger

`@lru_cache` is part of `functools` which is part of Python's standard library, you can read more about it in the <a href="https://docs.python.org/3/library/functools.html#functools.lru_cache" class="external-link" target="_blank">Python docs for `@lru_cache`</a>.

## Recap

You can use Pydantic Settings to handle the settings or configurations for your application, with all the power of Pydantic models.

							data-order="${s.index}"
						> <img src="${fe:url('/images/blank.png')}" alt="thumbnail"
							data-src="${fe:url('/thumbnail/')}?docId=${f:u(doc.doc_id)}&queryId=${f:u(queryId)}" class="thumbnail"
						>
						</a>
					</div>
					</c:if>
					<div class="description">${doc.content_description}</div>
				</div>
				<div class="site text-truncate">

```

////

Notice that the *path operations* define the models they use for request payload and response payload, using the models `Item` and `ResponseMessage`.

### API Docs

If you go to the API docs, you will see that it has the **schemas** for the data to be sent in requests and received in responses:

<img src="/img/tutorial/generate-clients/image01.png">

from fastapi import FastAPI
from fastapi.responses import RedirectResponse

app = FastAPI()


@app.get("/pydantic", response_class=RedirectResponse, status_code=302)
async def redirect_pydantic():

# 📣 📨 🖼 💽

👆 💪 📣 🖼 💽 👆 📱 💪 📨.

📥 📚 🌌 ⚫️.

## Pydantic `schema_extra`

👆 💪 📣 `example` Pydantic 🏷 ⚙️ `Config` &amp; `schema_extra`, 🔬 <a href="https://docs.pydantic.dev/latest/concepts/json_schema/#customizing-json-schema" class="external-link" target="_blank">Pydantic 🩺: 🔗 🛃</a>:

//// tab | 🐍 3️⃣.6️⃣ &amp; 🔛

```Python hl_lines="15-23"
{!> ../../docs_src/schema_extra_example/tutorial001.py!}
```

////

You will see the JSON response as:

```JSON
{"message": "Hello World"}
```

### Interactive API docs

Now go to <a href="http://127.0.0.1:8000/docs" class="external-link" target="_blank">http://127.0.0.1:8000/docs</a>.