And your **FastAPI** application might also be composed of several files/modules, etc.

### **FastAPI** app file

Let's say you have a file structure as described in [Bigger Applications](bigger-applications.md){.internal-link target=_blank}:

```
.
├── app
│   ├── __init__.py
│   └── main.py
```

In the file `main.py` you have your **FastAPI** app:

## Learn More

You can learn more about **GraphQL** in the <a href="https://graphql.org/" class="external-link" target="_blank">official GraphQL documentation</a>.

```

## Notas

Cuando devuelves una `Response` directamente, los datos no son validados, convertidos (serializados), ni documentados automáticamente.

Pero todavía es posible documentarlo como es descrito en [Respuestas adicionales en OpenAPI](additional-responses.md){.internal-link target=_blank}.

### Install requirements using pip

After activating the environment as described above:

<div class="termy">

```console
$ pip install -r requirements.txt

---> 100%
```

</div>

It will install all the dependencies and your local FastAPI in your local environment.

```Python hl_lines="9"
{!../../../docs_src/behind_a_proxy/tutorial004.py!}
```

and then it won't include it in the OpenAPI schema.

## Mounting a sub-application

If you need to mount a sub-application (as described in [Sub Applications - Mounts](sub-applications.md){.internal-link target=_blank}) while also using a proxy with `root_path`, you can do it normally, as you would expect.

!!! info
    You can also use:

    * `response_model_exclude_defaults=True`
    * `response_model_exclude_none=True`

    as described in <a href="https://docs.pydantic.dev/latest/concepts/serialization/#modeldict" class="external-link" target="_blank">the Pydantic docs</a> for `exclude_defaults` and `exclude_none`.

#### Data with values for fields with defaults

As the parameters are described with TypeScript types (similar to Python type hints), editor support is quite good.

We will talk a lot about the running "**process**", so it's useful to have clarity about what it means, and what's the difference with the word "**program**".

### What is a Program

The word **program** is commonly used to describe many things:

* The **code** that you write, the **Python files**.
* The **file** that can be **executed** by the operating system, for example: `python`, `python.exe` or `uvicorn`.

That means that, clients using your API don't have to check if the value exists or not, they can **assume the field will always be there**, but just that in some cases it will have the default value of `None`.

The way to describe this in OpenAPI, is to mark that field as **required**, because it will always be there.

Because of that, the JSON Schema for a model can be different depending on if it's used for **input or output**:

## Retornar una `Response` directamente

Adicionalmente, puedes añadir headers cuando se retorne una `Response` directamente.

Crea un response tal como se describe en [Retornar una respuesta directamente](response-directly.md){.internal-link target=_blank} y pasa los headers como un parámetro adicional:

```Python hl_lines="10-12"
{!../../../docs_src/response_headers/tutorial001.py!}
```