* `app`: the object created inside of `main.py` with the line `app = FastAPI()`.

It is equivalent to:

```Python
from main import app
```

///

Each alternative ASGI server program would have a similar command, you can read more in their respective documentation.

/// warning

Uvicorn and other servers support a `--reload` option that is useful during development.

![Swagger UI](https://fastapi.tiangolo.com/img/index/index-01-swagger-ui-simple.png)

### Documentación de API Alternativa { #alternative-api-docs }

Y ahora, ve a <a href="http://127.0.0.1:8000/redoc" class="external-link" target="_blank">http://127.0.0.1:8000/redoc</a>.

![Swagger UI](https://fastapi.tiangolo.com/img/index/index-01-swagger-ui-simple.png)

### Documentação Alternativa da API { #alternative-api-docs }

E agora, vá para <a href="http://127.0.0.1:8000/redoc" class="external-link" target="_blank">http://127.0.0.1:8000/redoc</a>.

```

////

//// tab | Python 3.9+

```Python hl_lines="1  4"
{!> ../../docs_src/python_types/tutorial009_py39.py!}
```

////

//// tab | Python 3.9+ Alternative

```Python hl_lines="1  4"
{!> ../../docs_src/python_types/tutorial009b_py39.py!}
```

////

#### `Union` oder `Optional` verwenden? { #using-union-or-optional }

* Show a **clear error** for the client when the data is not valid
* **Document** the parameter in the OpenAPI schema *path operation* (so it will show up in the **automatic docs UI**)

## Alternative (old): `Query` as the default value { #alternative-old-query-as-the-default-value }

        Doc(
            """
            Load and use Google Fonts.
            """
        ),
    ] = True,
) -> HTMLResponse:
    """
    Generate and return the HTML response that loads ReDoc for the alternative
    API docs (normally served at `/redoc`).

    You would only call this function yourself if you needed to override some parts,
    for example the URLs to use to load ReDoc's JavaScript and CSS.

## Alternative API docs { #alternative-api-docs }

And you can also go to <a href="http://192.168.99.100/redoc" class="external-link" target="_blank">http://192.168.99.100/redoc</a> or <a href="http://127.0.0.1/redoc" class="external-link" target="_blank">http://127.0.0.1/redoc</a> (or equivalent, using your Docker host).

Examples of how to translate a heading:

    Source (English):

        «««
        ## Alternative API docs { #alternative-api-docs }
        »»»

    Result (Spanish):

        «««
        ## Documentación de la API alternativa { #alternative-api-docs }
        »»»

    Source (English):

        «««
        ### Example { #example }
        »»»

![Swagger UI](https://fastapi.tiangolo.com/img/index/index-01-swagger-ui-simple.png)

### Альтернативная документация API { #alternative-api-docs }

Теперь откройте <a href="http://127.0.0.1:8000/redoc" class="external-link" target="_blank">http://127.0.0.1:8000/redoc</a>.

Обратите внимание, что параметр пути объявлен целочисленным.

///

## Преимущества стандартизации, альтернативная документация { #standards-based-benefits-alternative-documentation }