### Étape 2 : créer une "instance" `FastAPI`

```Python hl_lines="3"
{!../../../docs_src/first_steps/tutorial001.py!}
```

Ici la variable `app` sera une "instance" de la classe `FastAPI`.

Ce sera le point principal d'interaction pour créer toute votre API.

# Custom Response Classes - File, HTML, Redirect, Streaming, etc.

There are several custom response classes you can use to create an instance and return them directly from your *path operations*.

Read more about it in the [FastAPI docs for Custom Response - HTML, Stream, File, others](https://fastapi.tiangolo.com/advanced/custom-response/).

You can import them directly from `fastapi.responses`:

```python
from fastapi.responses import (
    FileResponse,

!!! tip
    Peewee also uses the term "**model**" to refer to these classes and instances that interact with the database.

    But Pydantic also uses the term "**model**" to refer to something different, the data validation, conversion, and documentation classes and instances.

Import `db` from `database` (the file `database.py` from above) and use it here.

```Python hl_lines="3  6-12  15-21"

There are some cases where you might need to modify the generated OpenAPI schema.

In this section you will see how.

## The normal process

The normal (default) process, is as follows.

A `FastAPI` application (instance) has an `.openapi()` method that is expected to return the OpenAPI schema.

As part of the application object creation, a *path operation* for `/openapi.json` (or for whatever you set your `openapi_url`) is registered.

    * Porque las estructuras de datos de Pydantic son solo <abbr title='En español: ejemplares. Aunque a veces los llaman incorrectamente "instancias"'>instances</abbr> de clases que tu defines, el auto-completado, el linting, mypy y tu intuición deberían funcionar bien con tus datos validados.
* Valida **estructuras complejas**:

{
  "message": "Hello World"
}
```

Of course, you will probably find much better ways to take advantage of this than formatting JSON. 😉

## Default response class

When creating a **FastAPI** class instance or an `APIRouter` you can specify which response class to use by default.

The parameter that defines this is `default_response_class`.

    But as it's a common use case, it is provided by **FastAPI** directly, just to make it easier.

### Use the form data

!!! tip
    The instance of the dependency class `OAuth2PasswordRequestForm` won't have an attribute `scope` with the long string separated by spaces, instead, it will have a `scopes` attribute with the actual list of strings for each scope sent.

                This is simply inherited from Starlette.

                Read more about it in the
                [Starlette docs for Applications](https://www.starlette.io/applications/#storing-state-on-the-app-instance).
                """
            ),
        ] = State()
        self.dependency_overrides: Annotated[
            Dict[Callable[..., Any], Callable[..., Any]],
            Doc(
                """

### Paso 2: crea un "instance" de `FastAPI`

```Python hl_lines="3"
{!../../../docs_src/first_steps/tutorial001.py!}
```

Aquí la variable `app` será un instance de la clase `FastAPI`.

Este será el punto de interacción principal para crear todo tu API.

from starlette.requests import Request
from starlette.status import HTTP_403_FORBIDDEN
from typing_extensions import Annotated, Doc


class OpenIdConnect(SecurityBase):
    """
    OpenID Connect authentication class. An instance of it would be used as a
    dependency.
    """

    def __init__(
        self,
        *,
        openIdConnectUrl: Annotated[
            str,
            Doc(
                """