```

!!! tip
    All this might seem contrived. And it might not be very clear how is it useful yet.

    These examples are intentionally simple, but show how it all works.

    In the chapters about security, there are utility functions that are implemented in this same way.

Later, when reading about how to structure bigger applications ([Bigger Applications - Multiple Files](../../tutorial/bigger-applications.md){.internal-link target=_blank}), possibly with multiple files, you will learn how to declare a single `dependencies` parameter for a group of *path operations*.

## Global Dependencies

So, what we will do next is add the code to document how that *external API* should look like to receive the callback from *your API*.

That documentation will show up in the Swagger UI at `/docs` in your API, and it will let external developers know how to build the *external API*.

FastAPI will now:

* **Validate** the data making sure that the max length is 50 characters
* Show a **clear error** for the client when the data is not valid
* **Document** the parameter in the OpenAPI schema *path operation* (so it will show up in the **automatic docs UI**)

## Alternative (old) `Query` as the default value

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

* <a href="https://twitter.com/tiangolo" class="external-link" target="_blank">Follow me on **Twitter**</a> or <a href="https://fosstodon.org/@tiangolo" class="external-link" target="_blank">Mastodon</a>.
    * Tell me how you use FastAPI (I love to hear that).
    * Hear when I make announcements or release new tools.

You can learn more about [FastAPI versions and how to pin and upgrade them](https://fastapi.tiangolo.com/deployment/versions/) for your project in the docs.

## Reporting a Vulnerability

### Just Modern Python

It's all based on standard **Python type** declarations (thanks to Pydantic). No new syntax to learn. Just standard modern Python.

If you need a 2 minute refresher of how to use Python types (even if you don't use FastAPI), check the short tutorial: [Python Types](python-types.md){.internal-link target=_blank}.

You write standard Python with types:

```Python
from datetime import date

- 📫 Email based password recovery.
- ✅ Tests with [Pytest](https://pytest.org).
- 📞 [Traefik](https://traefik.io) as a reverse proxy / load balancer.
- 🚢 Deployment instructions using Docker Compose, including how to set up a frontend Traefik proxy to handle automatic HTTPS certificates.

    {!> ../../../docs_src/dependencies/tutorial005.py!}
    ```

It declares an optional query parameter `q` as a `str`, and then it just returns it.

This is quite simple (not very useful), but will help us focus on how the sub-dependencies work.

## Second dependency, "dependable" and "dependant"

Then you can create another dependency function (a "dependable") that at the same time declares a dependency of its own (so it is a "dependant" too):