`secrets.compare_digest()` needs to take `bytes` or a `str` that only contains ASCII characters (the ones in English), this means it wouldn't work with characters like `á`, as in `Sebastián`.

To handle that, we first convert the `username` and `password` to `bytes` encoding them with UTF-8.

And `threading.local` is not compatible with the new async features of modern Python.

!!! note "Technical Details"

    ```

=== "Python 3.9 and above"

    ```Python hl_lines="6"
    {!> ../../../docs_src/extra_models/tutorial005_py39.py!}
    ```

## Em resumo

Use vários modelos Pydantic e herde livremente para cada caso.

    Those details are implementation specific.

    For OAuth2 they are just strings.

## Code to get the `username` and `password`

Now let's use the utilities provided by **FastAPI** to handle this.

### `OAuth2PasswordRequestForm`

First, import `OAuth2PasswordRequestForm`, and use it as a dependency with `Depends` in the *path operation* for `/token`:

=== "Python 3.10+"

!!! warning
    This is a more or less advanced section. If you are just starting, you can skip it.

    You don't necessarily need OAuth2 scopes, and you can handle authentication and authorization however you want.

    But OAuth2 with scopes can be nicely integrated into your API (with OpenAPI) and your API docs.

    def exception_handler(
        self,
        exc_class_or_status_code: Annotated[
            Union[int, Type[Exception]],
            Doc(
                """
                The Exception class this would handle, or a status code.
                """
            ),
        ],
    ) -> Callable[[DecoratedCallable], DecoratedCallable]:
        """
        Add an exception handler to the app.

## A dependency with `yield` and `try`

If you use a `try` block in a dependency with `yield`, you'll receive any exception that was thrown when using the dependency.

For example, if some code at some point in the middle, in another dependency or in a *path operation*, made a database transaction "rollback" or create any other error, you will receive the exception in your dependency.

# OpenAPI `docs`

Utilities to handle OpenAPI automatic UI documentation, including Swagger UI (by default at `/docs`) and ReDoc (by default at `/redoc`).

::: fastapi.openapi.docs.get_swagger_ui_html

::: fastapi.openapi.docs.get_redoc_html

::: fastapi.openapi.docs.get_swagger_ui_oauth2_redirect_html

## Use Case

Let's start with an example **use case** and then see how to solve it with this.

Let's imagine that you have some **machine learning models** that you want to use to handle requests. 🤖

The same models are shared among requests, so, it's not one model per request, or one per user or something similar.

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