],
        scope: Annotated[
            str,
            Form(),
            Doc(
                """
                A single string with actually several scopes separated by spaces. Each
                scope is also a string.

                For example, a single string with:

                ```python
                "items:read items:write users:read profile openid"
                ````

            'Whitelist' validation step. The parameter field will be the single one
            allowed by the alias or set of aliases defined.
            """
        ),
    ] = None,
    serialization_alias: Annotated[
        Union[str, None],
        Doc(
            """
            'Blacklist' validation step. The vanilla parameter field will be the
            single one of the alias' or set of aliases' fields and all the other

But in most cases, you will want to perform these steps only **once**.

So, you will want to have a **single process** to perform those **previous steps**, before starting the application.

## Use the `fastapi run` Command

In short, use `fastapi run` to serve your FastAPI application:

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> run <u style="text-decoration-style:single">main.py</u>
<font color="#3465A4">INFO    </font> Using path <font color="#3465A4">main.py</font>

# Bigger Applications - Multiple Files

If you are building an application or a web API, it's rarely the case that you can put everything on a single file.

**FastAPI** provides a convenience tool to structure your application while keeping all the flexibility.

!!! info
    If you come from Flask, this would be the equivalent of Flask's Blueprints.

## An example file structure

Let's say you have a file structure like this:

```
.
├── app

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

Copy that to a file `main.py`.

Run the live server:

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> dev <u style="text-decoration-style:single">main.py</u>
<font color="#3465A4">INFO    </font> Using path <font color="#3465A4">main.py</font>

<a href="https://github.com/containous/traefik/releases" class="external-link" target="_blank">Download Traefik</a>, it's a single binary, you can extract the compressed file and run it directly from the terminal.

Then create a file `traefik.toml` with:

```TOML hl_lines="3"
[entryPoints]
  [entryPoints.http]
    address = ":9999"

It is compatible with:

* PostgreSQL
* MySQL
* SQLite

In this example, we'll use **SQLite**, because it uses a single file and Python has integrated support. So, you can copy this example and run it as is.

Later, for your production application, you might want to use a database server like **PostgreSQL**.

!!! tip

=== "Python 3.8+"

    ```Python hl_lines="10"
    {!> ../../../docs_src/response_model/tutorial003_04.py!}
    ```

...this fails because the type annotation is not a Pydantic type and is not just a single `Response` class or subclass, it's a union (any of the two) between a `Response` and a `dict`.

### Disable Response Model

But, sadly, you get nothing useful:

<img src="/img/python-types/image01.png">

### Add types

Let's modify a single line from the previous version.

We will change exactly this fragment, the parameters of the function, from:

```Python
    first_name, last_name
```

to:

```Python
    first_name: str, last_name: str