Without a data validation system, you would have to do all the checks by hand, in code.

These features are what Marshmallow was built to provide. It is a great library, and I have used it a lot before.

But it was created before there existed Python type hints. So, to define every <abbr title="the definition of how data should be formed">schema</abbr> you need to use specific utils and classes provided by Marshmallow.

And then you can also have a path `/users/{user_id}` to get data about a specific user by some user ID.

Because *path operations* are evaluated in order, you need to make sure that the path for `/users/me` is declared before the one for `/users/{user_id}`:

```Python hl_lines="6  11"
{!../../../docs_src/path_params/tutorial003.py!}
```

    **FastAPI** provides it as a convenience for you, the developer. But it comes directly from Starlette.

### Before and after the `response`

You can add code to be run with the `request`,  before any *path operation* receives it.

And also after the `response` is generated, before returning it.

```

And the proxy would be **"stripping"** the **path prefix** on the fly before transmitting the request to the app server (probably Uvicorn via FastAPI CLI), keeping your application convinced that it is being served at `/app`, so that you don't have to update all your code to include the prefix `/api/v1`.

Up to here, everything would work as normally.

# Middleware

Sie können Middleware zu **FastAPI**-Anwendungen hinzufügen.

Eine „Middleware“ ist eine Funktion, die mit jedem **Request** arbeitet, bevor er von einer bestimmten *Pfadoperation* verarbeitet wird. Und auch mit jeder **Response**, bevor sie zurückgegeben wird.

* Sie nimmt jeden **Request** entgegen, der an Ihre Anwendung gesendet wird.
* Sie kann dann etwas mit diesem **Request** tun oder beliebigen Code ausführen.

* Security - HTTPS
* Running on startup
* Restarts
* Replication (the number of processes running)
* Memory
* Previous steps before starting

{!../../../docs_src/app_testing/test_main.py!}
```

...and have the code for the tests just like before.

## Testing: extended example

Now let's extend this example and add more details to see how to test different parts.

### Extended **FastAPI** app file

Let's continue with the same file structure as before:

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

Let's create a dependency `get_current_user`.

Remember that dependencies can have sub-dependencies?

`get_current_user` will have a dependency with the same `oauth2_scheme` we created before.

The same as we were doing before in the *path operation* directly, our new dependency `get_current_user` will receive a `token` as a `str` from the sub-dependency `oauth2_scheme`:

=== "Python 3.10+"

    ```Python hl_lines="25"

    これらのエンコーディングやフォームフィールドの詳細については、<a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST" class="external-link" target="_blank"><abbr title="Mozilla Developer Network">MDN</abbr>の<code>POST</code></a>のウェブドキュメントを参照してください。

!!! warning "注意"

As **FastAPI** is based on Starlette and implements the <abbr title="Asynchronous Server Gateway Interface">ASGI</abbr> specification, you can use any ASGI middleware.

A middleware doesn't have to be made for FastAPI or Starlette to work, as long as it follows the ASGI spec.

In general, ASGI middlewares are classes that expect to receive an ASGI app as the first argument.