* This will be done by sending a POST request (from *your API*) to some *external API* provided by that external developer (this is the "callback").

## The normal **FastAPI** app

Let's first see how the normal API app would look like before adding the callback.

It will have a *path operation* that will receive an `Invoice` body, and a query parameter `callback_url` that will contain the URL for the callback.

The files will be uploaded as "form data".

If you declare the type of your *path operation function* parameter as `bytes`, **FastAPI** will read the file for you and you will receive the contents as `bytes`.

Keep in mind that this means that the whole contents will be stored in memory. This will work well for small files.

## `fastapi dev`

When you run `fastapi dev`, it will run on development mode.

By default, it will have **auto-reload** enabled, so it will automatically reload the server when you make changes to your code. This is resource intensive and could be less stable than without it, you should only use it for development.

The dependencies will keep working as expected, and the **best part** is that the **type information will be preserved**, which means that your editor will be able to keep providing you with **autocompletion**, **inline errors**, etc. The same for other tools like `mypy`.

!!! tip
    All this might not seem as useful with these simple examples.

    But you will see how useful it is in the chapters about **security**.

    ```

=== "Python 3.8+"

    ```Python hl_lines="3  17"
    {!> ../../../docs_src/path_operation_configuration/tutorial001.py!}
    ```

That status code will be used in the response and will be added to the OpenAPI schema.

!!! note "Technical Details"
    You could also use `from starlette import status`.

You will see a simple page like:

<img src="/img/tutorial/websockets/image01.png">

You can type messages in the input box, and send them:

<img src="/img/tutorial/websockets/image02.png">

And your **FastAPI** application with WebSockets will respond back:

<img src="/img/tutorial/websockets/image03.png">

    ```

In this case, the function parameter `q` will be optional, and will be `None` by default.

!!! check
    Also notice that **FastAPI** is smart enough to notice that the path parameter `item_id` is a path parameter and `q` is not, so, it's a query parameter.

## Query parameter type conversion

You can also declare `bool` types, and they will be converted:

=== "Python 3.10+"

    ```Python hl_lines="7"

* If the PR can be simplified in a way, you can ask for that, but there's no need to be too picky, there might be a lot of subjective points of view (and I will have my own as well 🙈), so it's better if you can focus on the fundamental things.

### Tests

* Help me check that the PR has **tests**.

* Check that the tests **fail** before the PR. 🚨

This is normally called a **webhook**.

## Webhooks steps

The process normally is that **you define** in your code what is the message that you will send, the **body of the request**.

You also define in some way at which **moments** your app will send those requests or events.

And **your users** define in some way (for example in a web dashboard somewhere) the **URL** where your app should send those requests.