<img src="/img/tutorial/security/image12.png">

## Check the username

Here's a more complete example.

Use a dependency to check if the username and password are correct.

This will probably happen for more than one of those tabs.

If you had multiple clients talking to your app exactly at the same time, this is what could happen.

And as your app starts to handle more and more clients at the same time, the waiting time in a single request needs to be shorter and shorter to trigger the error.

But before that, handling asynchronous code was quite more complex and difficult.

In previous versions of Python, you could have used threads or <a href="https://www.gevent.org/" class="external-link" target="_blank">Gevent</a>. But the code is way more complex to understand, debug, and think about.

# Test Client - `TestClient`

You can use the `TestClient` class to test FastAPI applications without creating an actual HTTP and socket connection, just communicating directly with the FastAPI code.

Read more about it in the [FastAPI docs for Testing](https://fastapi.tiangolo.com/tutorial/testing/).

You can import it directly from `fastapi.testclient`:

```python
from fastapi.testclient import TestClient
```

    API docs (normally served at `/docs`).

    You would only call this function yourself if you needed to override some parts,
    for example the URLs to use to load Swagger UI's JavaScript and CSS.

    Read more about it in the
    [FastAPI docs for Configure Swagger UI](https://fastapi.tiangolo.com/how-to/configure-swagger-ui/)

# Extra Data Types

Up to now, you have been using common data types, like:

* `int`
* `float`
* `str`
* `bool`

But you can also use more complex data types.

And you will still have the same features as seen up to now:

* Great editor support.
* Data conversion from incoming requests.
* Data conversion for response data.
* Data validation.
* Automatic annotation and documentation.

## Other data types

# Custom Response Classes - File, HTML, Redirect, Streaming, etc.

There are several custom response classes you can use to create an instance and return them directly from your *path operations*.

Read more about it in the [FastAPI docs for Custom Response - HTML, Stream, File, others](https://fastapi.tiangolo.com/advanced/custom-response/).

You can import them directly from `fastapi.responses`:

```python
from fastapi.responses import (
    FileResponse,

org/en-US/docs/Web/HTTP/Headers" class="external-link" target="_blank">using the 'X-' prefix</a>.

But if you have custom headers that you want a client in a browser to be able to see, you need to add them to your CORS configurations (read more in [CORS (Cross-Origin Resource Sharing)](../tutorial/cors.md){.internal-link target=_blank}), using the parameter `expose_headers` documented in <a href="https://www.starlette.io/middleware/#corsmiddleware" class="external-link" target="_blank">Starlette's...

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      </a>
    </div>
    <div class="item">
      <a title="Auth, user management and more for your B2B product" style="display: block; position: relative;" href="https://www.propelauth.com/?utm_source=fastapi&utm_campaign=1223&utm_medium=topbanner" target="_blank">
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    email = user_dict["email"],
    full_name = user_dict["full_name"],
    disabled = user_dict["disabled"],
    hashed_password = user_dict["hashed_password"],
)
```

!!! info
    For a more complete explanation of `**user_dict` check back in [the documentation for **Extra Models**](../extra-models.md#about-user_indict){.internal-link target=_blank}.

## Return the token