* 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 }

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 }
        »»»

```

////

//// 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!}
```

////

#### Using `Union` or `Optional` { #using-union-or-optional }

    assertThat(okHttpCertificate.tbsCertificate.validity).isEqualTo(Validity(-1000L, 2000L))
    assertThat(okHttpCertificate.tbsCertificate.serialNumber).isEqualTo(BigInteger("17"))
  }

  @Test
  fun `missing subject alternative names`() {
    val certificate =
      HeldCertificate
        .Builder()
        .certificateAuthority(3)
        .commonName("Jurassic Park")
        .organizationalUnit("Gene Research")

* `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.

        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.

Par exemple, le générateur de projet <a href="https://github.com/tiangolo/full-stack-fastapi-postgresql" class="external-link" target="_blank">Full Stack FastAPI PostgreSQL</a>  peut être une meilleure alternative, étant activement maintenu et utilisé et comprenant toutes les nouvelles fonctionnalités et améliorations.

{* ../../docs_src/events/tutorial003_py39.py hl[22] *}

## Eventos Alternativos (obsoleto) { #alternative-events-deprecated }

/// warning | Advertencia

## 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).

 *
 * <ul>
 *   <li>The raw {@code long} values returned by {@code nanoTime} are meaningless and unsafe to use
 *       in any other way than how {@code Stopwatch} uses them.
 *   <li>An alternative source of nanosecond ticks can be substituted, for example for testing or
 *       performance reasons, without affecting most of your code.
 * </ul>
 *