If you have a group of **query parameters** that are related, you can create a **Pydantic model** to declare them.

This would allow you to **re-use the model** in **multiple places** and also to declare validations and metadata for all the parameters at once. 😎

/// note

This is supported since FastAPI version `0.115.0`. 🤓

///

OAuth2 wurde so konzipiert, dass das Backend oder die API unabhängig vom Server sein kann, der den Benutzer authentifiziert.

In diesem Fall handhabt jedoch dieselbe **FastAPI**-Anwendung sowohl die API als auch die Authentifizierung.

Betrachten wir es also aus dieser vereinfachten Sicht:

* Der Benutzer gibt den `username` und das `password` im Frontend ein und drückt `Enter`.

Nehmen wir als Beispiel die folgende Anwendung:

{* ../../docs_src/query_params_str_validations/tutorial001_py310.py hl[7] *}

Der Query-Parameter `q` hat den Typ `Union[str, None]` (oder `str | None` in Python 3.10), was bedeutet, er ist entweder ein `str` oder `None`. Der Defaultwert ist `None`, also weiß FastAPI, der Parameter ist nicht erforderlich.

/// note | Hinweis

## License identifier { #license-identifier }

Since OpenAPI 3.1.0 and FastAPI 0.99.0, you can also set the `license_info` with an `identifier` instead of a `url`.

For example:

{* ../../docs_src/metadata/tutorial001_1.py hl[31] *}

## Metadata for tags { #metadata-for-tags }

You can also add additional metadata for the different tags used to group your path operations with the parameter `openapi_tags`.

///

/// note | Technical Details

You could also use `from starlette.requests import Request`.

**FastAPI** provides it as a convenience for you, the developer. But it comes directly from Starlette.

///

### Before and after the `response` { #before-and-after-the-response }

You can add code to be run with the `request`,  before any *path operation* receives it.

And also after the `response` is generated, before returning it.

    /**
     * Indicates whether Maven should also build the specified projects' dependencies.
     *
     * @return an {@link Optional} containing true if dependencies should also be built, false if not, or empty if not specified
     */
    @Nonnull
    Optional<Boolean> alsoMake();

    /**
     * Indicates whether Maven should also build the specified projects' dependents.
     *

**FastAPI** will use that *temporal* response to extract the status code (also cookies and headers), and will put them in the final response that contains the value you returned, filtered by any `response_model`.

Die gleichen Modelle werden von den Requests gemeinsam genutzt, es handelt sich also nicht um ein Modell pro Request, pro Benutzer, oder ähnliches.

Stellen wir uns vor, dass das Laden des Modells **eine ganze Weile dauern** kann, da viele **Daten von der Festplatte** gelesen werden müssen. Sie möchten das also nicht für jeden Request tun.

You never call those functions directly. They are called by your framework (in this case, **FastAPI**).

With the Dependency Injection system, you can also tell **FastAPI** that your *path operation function* also "depends" on something else that should be executed before your *path operation function*, and **FastAPI** will take care of executing it and "injecting" the results.

And then you can also have a path `/users/{user_id}` to get data about a specific user by some user ID.

Because *path operations* are evaluated in order, you need to make sure that the path for `/users/me` is declared before the one for `/users/{user_id}`:

{* ../../docs_src/path_params/tutorial003.py hl[6,11] *}