In particular, this may be a good alternative to logic in a middleware.

For example, if you want to read or manipulate the request body before it is processed by your application.

!!! danger
    This is an "advanced" feature.

    If you are just starting with **FastAPI** you might want to skip this section.

## Use cases

Some use cases include:

!!! warning "Deprecated"
    This tutorial is deprecated and will be removed in a future version.

You can also use <a href="https://github.com/encode/databases" class="external-link" target="_blank">`encode/databases`</a> with **FastAPI** to connect to databases using `async` and `await`.

It is compatible with:

* PostgreSQL
* MySQL
* SQLite

* There's also an `app/dependencies.py` file, just like `app/main.py`, it is a "module": `app.dependencies`.
* There's a subdirectory `app/routers/` with another file `__init__.py`, so it's a "Python subpackage": `app.routers`.
* The file `app/routers/items.py` is inside a package, `app/routers/`, so, it's a submodule: `app.routers.items`.

Copyright (c) 2018 Sebastián Ramírez

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

```

It is not encrypted, so, anyone could recover the information from the contents.

But it's signed. So, when you receive a token that you emitted, you can verify that you actually emitted it.

That way, you can create a token with an expiration of, let's say, 1 week. And then when the user comes back the next day with the token, you know that user is still logged in to your system.

## Singular values in body

The same way there is a `Query` and `Path` to define extra data for query and path parameters, **FastAPI** provides an equivalent `Body`.

For example, extending the previous model, you could decide that you want to have another key `importance` in the same body, besides the `item` and `user`.

If you declare it as is, because it is a singular value, **FastAPI** will assume that it is a query parameter.

<td><code>name</code></td><td><code>str</code></td><td><strong>REQUIRED</strong> (if a <code>license_info</code> is set). The license name used for the API.</td></tr><tr><td><code>identifier</code></td><td><code>str</code></td><td>An <a href="https://spdx.org/licenses/" class="external-link" target="_blank">SPDX</a> license expression for the API. The <code>identifier</code> field is mutually exclusive of the <code>url</code> field. <small>Available since OpenAPI 3.1.0, FastAPI 0.99.0.</small></...

filterwarnings = [
    "error",
    # TODO: needed by asyncio in Python 3.9.7 https://bugs.python.org/issue45097, try to remove on 3.9.8
    'ignore:The loop argument is deprecated since Python 3\.8, and scheduled for removal in Python 3\.10:DeprecationWarning:asyncio',
    'ignore:starlette.middleware.wsgi is deprecated and will be removed in a future release\..*:DeprecationWarning:starlette',
    # TODO: remove after upgrading HTTPX to a version newer than 0.23.0

---

"_If anyone is looking to build a production Python API, I would highly recommend **FastAPI**. It is **beautifully designed**, **simple to use** and **highly scalable**, it has become a **key component** in our API first development strategy and is driving many automations and services such as our Virtual TAC Engineer._"

```Python hl_lines="3  6-8"
{!../../../docs_src/sub_applications/tutorial001.py!}
```

### Sub-application

Then, create your sub-application, and its *path operations*.

This sub-application is just another standard FastAPI application, but this is the one that will be "mounted":

```Python hl_lines="11  14-16"
{!../../../docs_src/sub_applications/tutorial001.py!}
```

### Mount the sub-application