Just use any kind of model, any kind of class, any kind of database that you need for your application. **FastAPI** has you covered with the dependency injection system.

## Code size

This example might seem verbose. Keep in mind that we are mixing security, data models, utility functions and *path operations* in the same file.

But here's the key point.

The security and dependency injection stuff is written once.

It can't handle nested models very well. So, if the JSON body in the request is a JSON object that has inner fields that in turn are nested JSON objects, it cannot be properly documented and validated.

añadir headers propietarios personalizados <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers" class="external-link" target="_blank">usando el prefijo 'X-'</a>.

Si tienes headers personalizados y deseas que un cliente pueda verlos en el navegador, es necesario que los añadas a tus configuraciones de CORS (puedes leer más en [CORS (Cross-Origin Resource Sharing)](../tutorial/cors.md){.internal-link target=_blank}), usando el parámetro `expose_headers` documentado en <a href...

    assert response.status_code == 200, response.text
    assert response.json() == [
        {
            "name": "Portal Gun",
            "description": "Device to travel through the multi-rick-verse",
        },
        {"name": "Plumbus", "description": None},
    ]


@needs_py39
@needs_pydanticv2
def test_openapi_schema(client: TestClient) -> None:
    response = client.get("/openapi.json")

    return item


@app.get("/items/")
def read_items() -> list[Item]:
    return [
        Item(
            name="Portal Gun",
            description="Device to travel through the multi-rick-verse",
        ),
        Item(name="Plumbus"),

    return item


@app.get("/items/")
def read_items() -> list[Item]:
    return [
        Item(
            name="Portal Gun",
            description="Device to travel through the multi-rick-verse",
        ),
        Item(name="Plumbus"),

    return item


@app.get("/items/")
def read_items() -> List[Item]:
    return [
        Item(
            name="Portal Gun",
            description="Device to travel through the multi-rick-verse",
        ),
        Item(name="Plumbus"),

    assert response.status_code == 200, response.text
    assert response.json() == [
        {
            "name": "Portal Gun",
            "description": "Device to travel through the multi-rick-verse",
        },
        {"name": "Plumbus", "description": None},
    ]


@needs_pydanticv2
def test_openapi_schema(client: TestClient) -> None:
    response = client.get("/openapi.json")

    assert response.status_code == 200, response.text
    assert response.json() == [
        {
            "name": "Portal Gun",
            "description": "Device to travel through the multi-rick-verse",
        },
        {"name": "Plumbus", "description": None},
    ]


@needs_py39
@needs_pydanticv2
def test_openapi_schema(client: TestClient) -> None:
    response = client.get("/openapi.json")

# Primeros pasos

Un archivo muy simple de FastAPI podría verse así:

```Python
{!../../../docs_src/first_steps/tutorial001.py!}
```

Copia eso a un archivo `main.py`.

Corre el servidor en vivo:

<div class="termy">

```console
$ uvicorn main:app --reload

<span style="color: green;">INFO</span>:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)