It will have a *path operation* that will receive an `Invoice` body, and a query parameter `callback_url` that will contain the URL for the callback.

This part is pretty normal, most of the code is probably already familiar to you:

```Python hl_lines="9-13  36-53"
{!../../../docs_src/openapi_callbacks/tutorial001.py!}
```

!!! tip

In most cases you would (and should) have a "termination proxy" handling HTTPS for you on top, this will depend on how you deploy your application, your provider might do this for you, or you might need to set it up yourself.

!!! tip

!!! note
    When defining a <a href="https://docs.pydantic.dev/latest/concepts/types/#unions" class="external-link" target="_blank">`Union`</a>, include the most specific type first, followed by the less specific type. In the example below, the more specific `PlaneItem` comes before `CarItem` in `Union[PlaneItem, CarItem]`.

=== "Python 3.10+"

...makes the parameter optional, with a default value of `None`, the same as:

```Python
q: str | None = None
```

But it declares it explicitly as being a query parameter.

!!! info
    Keep in mind that the most important part to make a parameter optional is the part:

    ```Python
    = None
    ```

    or the:

    ```Python
    = Query(default=None)
    ```

Actually, all (or most) of the web frameworks work in this same way.

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

!!! note "Technical Details"
    You could also use `from starlette.responses import HTMLResponse`.

    **FastAPI** provides the same `starlette.responses` as `fastapi.responses` just as a convenience for you, the developer. But most of the available responses come directly from Starlette.

### Multiple File Uploads with Additional Metadata

And the same way as before, you can use `File()` to set additional parameters, even for `UploadFile`:

Uma salva de palmas para eles. 👏 🙇

## Usuários mais ativos do ultimo mês

Estes são os usuários que estão [helping others the most with issues (questions) in GitHub](help-fastapi.md#responda-perguntas-no-github){.internal-link target=_blank} durante o ultimo mês. ☕

{% if people %}
<div class="user-list user-list-center">

---

If you are using a third party library that communicates with something (a database, an API, the file system, etc.) and doesn't have support for using `await`, (this is currently the case for most database libraries), then declare your *path operation functions* as normally, with just `def`, like:

```Python hl_lines="2"
@app.get('/')
def results():
    results = some_library()
    return results
```

!!! tip
    The next sections are **not necessarily "advanced"**.

    And it's possible that for your use case, the solution is in one of them.

## Read the Tutorial first

You could still use most of the features in **FastAPI** with the knowledge from the main [Tutorial - User Guide](../tutorial/index.md){.internal-link target=_blank}.

And the next sections assume you already read it, and assume that you know those main ideas.

    **FastAPI** provides the same `starlette.responses` as `fastapi.responses` just as a convenience for you, the developer. But most of the available responses come directly from Starlette.

    And as the `Response` can be used frequently to set headers and cookies, **FastAPI** also provides it at `fastapi.Response`.

## Custom Headers