## Use Case { #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.

## Check the username { #check-the-username }

Here's a more complete example.

Use a dependency to check if the username and password are correct.

For this, use the Python standard module <a href="https://docs.python.org/3/library/secrets.html" class="external-link" target="_blank">`secrets`</a> to check the username and password.

Os atributos da classe são declarados com anotações de tipo, e possíveis valores padrão, da mesma maneira que os modelos do Pydantic.

Você pode utilizar todas as ferramentas e funcionalidades de validação que são utilizadas nos modelos do Pydantic, como tipos de dados diferentes e validações adicionei com `Field()`.

//// tab | Pydantic v2

///

## Tests separieren

In einer echten Anwendung würden Sie Ihre Tests wahrscheinlich in einer anderen Datei haben.

Und Ihre **FastAPI**-Anwendung könnte auch aus mehreren Dateien/Modulen, usw. bestehen.

### **FastAPI** Anwendungsdatei

Nehmen wir an, Sie haben eine Dateistruktur wie in [Größere Anwendungen](bigger-applications.md){.internal-link target=_blank} beschrieben:

```
.
├── app

         *
         * So this is probably just the "Windows Java 8" case. In that case, if we wanted *another*
         * layer of fallback before consulting the system property, we could try
         * com.sun.security.auth.module.NTSystem.
         *
         * But for now, we use the value from the system property as our best guess.
         */
        return fromSystemProperty;
      } catch (InvocationTargetException e) {

Remember that when you import `Query`, `Path`, and others from `fastapi`, those are actually functions that return special classes.

///

/// tip

Notice how each model's attribute with a type, default value and `Field` has the same structure as a *path operation function's* parameter, with `Field` instead of `Path`, `Query` and `Body`.

///

## Add extra information { #add-extra-information }

	logger.LogIf(ctx, "tier", err, errKind...)
}

func kmsLogIf(ctx context.Context, err error, errKind ...any) {
	logger.LogIf(ctx, "kms", err, errKind...)
}

// KMSLogger permits access to kms module specific logging
type KMSLogger struct{}

// LogOnceIf is the implementation of LogOnceIf, accessible using the Logger interface
func (l KMSLogger) LogOnceIf(ctx context.Context, err error, id string, errKind ...any) {

            if (typeof moment !== 'function' && moment.hasOwnProperty('default')) moment = moment['default']
            return factory(moment, jquery);
        });
    } else if (typeof module === 'object' && module.exports) {
        // Node / Browserify
        //isomorphic issue
        var jQuery = (typeof window != 'undefined') ? window.jQuery : undefined;
        if (!jQuery) {
            jQuery = require('jquery');

# a service account that has the right permissions to be able to do so.
set +x
if [[ -n "${GOOGLE_APPLICATION_CREDENTIALS:-}" ]]; then
  # Python 3.12 removed the module `imp` which is needed by gcloud CLI so we set
  # `CLOUDSDK_PYTHON` to Python 3.11 which is the system Python on TFCI Mac
  # VMs.
  pyenv install -s "3.11"
  if [[ $? -eq 0 ]]; then
    pyenv local "3.11"

Si revisas la nueva (falsa) base de datos `fake_users_db`, verás cómo se ve ahora la contraseña con hash: `"$2b$12$EixZaYVK1fsbw1ZfbX3OXePaWxn96p36WQoeG6Lruj3vjPGga31lW"`.

///

## Manejo de tokens JWT

Importa los módulos instalados.

Crea una clave secreta aleatoria que se usará para firmar los tokens JWT.

Para generar una clave secreta segura al azar usa el comando:

<div class="termy">

```console