///

/// info

In this example we use invented custom headers `X-Key` and `X-Token`.

But in real cases, when implementing security, you would get more benefits from using the integrated [Security utilities (the next chapter)](../security/index.md){.internal-link target=_blank}.

///

## Dependencies errors and return values

You can use the same dependency *functions* you use normally.

### Dependency requirements

   *
   * <p><i>Notes:</i> This is an implementation of the algorithm for Lexicographical Permutations
   * Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
   * Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
   * first permutation will be in ascending order, and the last will be in descending order.
   *

 * character encoding. UTF-8 is defined in section D92 of <a
 * href="http://www.unicode.org/versions/Unicode6.2.0/ch03.pdf">The Unicode Standard Core
 * Specification, Chapter 3</a>.
 *
 * <p>The variant of UTF-8 implemented by this class is the restricted definition of UTF-8
 * introduced in Unicode 3.1. One implication of this is that it rejects <a

And that function takes parameters in the same way that *path operation functions* do.

/// tip

You'll see what other "things", apart from functions, can be used as dependencies in the next chapter.

///

Whenever a new request arrives, **FastAPI** will take care of:

* Calling your dependency ("dependable") function with the correct parameters.
* Get the result from your function.

That defines the metadata about the main response of a *path operation*.

You can also declare additional responses with their models, status codes, etc.

There's a whole chapter here in the documentation about it, you can read it at [Additional Responses in OpenAPI](additional-responses.md){.internal-link target=_blank}.

## OpenAPI Extra

# Get Current User

In the previous chapter the security system (which is based on the dependency injection system) was giving the *path operation function* a `token` as a `str`:

//// tab | Python 3.9+

```Python hl_lines="12"
{!> ../../docs_src/security/tutorial001_an_py39.py!}
```

////

//// tab | Python 3.8+

```Python hl_lines="11"
{!> ../../docs_src/security/tutorial001_an.py!}
```

////

To see all the options you have, checkout <a href="https://docs.pydantic.dev/latest/concepts/types/" class="external-link" target="_blank">Pydantic's Type Overview</a>. You will see some examples in the next chapter.

For example, as in the `Image` model we have a `url` field, we can declare it to be an instance of Pydantic's `HttpUrl` instead of a `str`:

//// tab | Python 3.10+

```Python hl_lines="2  8"

This code is something you can actually use in your application, save the password hashes in your database, etc.

We are going to start from where we left in the previous chapter and increment it.

## About JWT

JWT means "JSON Web Tokens".

It's a standard to codify a JSON object in a long dense string without spaces. It looks like this:

```

  `Take off your hat,' the King said to the Hatter.

  `It isn't mine,' said the Hatter.

  `Stolen!' the King exclaimed, turning to the jury, who
instantly made a memorandum of the fact.

  `I keep them to sell,' the Hatter added as an explanation;
`I've none of my own.  I'm a hatter.'

  Here the Queen put on her spectacles, and began staring at the
Hatter, who turned pale and fidgeted.

But even if you never use **FastAPI**, you would benefit from learning a bit about them.

/// note

If you are a Python expert, and you already know everything about type hints, skip to the next chapter.

///

## Motivation

Let's start with a simple example:

{* ../../docs_src/python_types/tutorial001.py *}

Calling this program outputs:

```
John Doe
```