In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.

## An app with callbacks

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

!!! info
    Unfortunately, I can't simply trust PRs that just have several approvals.

    Several times it has happened that there are PRs with 3, 5 or more approvals, probably because the description is appealing, but when I check the PRs, they are actually broken, have a bug, or don't solve the problem they claim to solve. 😅

    * And validators allow complex data schemas to be clearly and easily defined, checked and documented as JSON Schema.
    * You can have deeply **nested JSON** objects and have them all validated and annotated.
* **Extensible**:
    * Pydantic allows custom data types to be defined or you can extend validation with methods on a model decorated with the validator decorator.

### In production

In your production system, you probably have a frontend created with a modern framework like React, Vue.js or Angular.

And to communicate using WebSockets with your backend you would probably use your frontend's utilities.

Or you might have a native mobile application that communicates with your WebSocket backend directly, in native code.

Or you might have any other way to communicate with the WebSocket endpoint.

---

```Python hl_lines="2"
@app.get('/')
def results():
    results = some_library()
    return results
```

---

And then, that system (in this case **FastAPI**) will take care of doing whatever is needed to provide your code with those needed dependencies ("inject" the dependencies).

This is very useful when you need to:

* Have shared logic (the same code logic again and again).
* Share database connections.
* Enforce security, authentication, role requirements, etc.
* And many other things...

All these, while minimizing code repetition.

    But `OAuth2PasswordRequestForm` is just a class dependency that you could have written yourself, or you could have declared `Form` parameters directly.

    But as it's a common use case, it is provided by **FastAPI** directly, just to make it easier.

### Use the form data

!!! tip

## 🛠️

↩️ 👥 ⚙️ 🇸🇲 🔗 &amp; 👥 🚫 🚚 🙆 😇 🔌-⚫️ 👷 ⏮️ **FastAPI**, 👥 💪 🛠️ 💽 <abbr title="Automatically updating the database to have any new column we define in our models.">🛠️</abbr> ⏮️ <a href="https://alembic.sqlalchemy.org" class="external-link" target="_blank">⚗</a> 🔗.

It will have **auto-reload disabled** by default.

It will listen on the IP address `0.0.0.0`, which means all the available IP addresses, this way it will be publicly accessible to anyone that can communicate with the machine. This is how you would normally run it in production, for example, in a container.

                    "operationId": "read_items_items__get",
                    "parameters": [
                        {
                            "description": "Query string for the items to search in the database that have a good match",
                            "required": False,
                            "deprecated": True,
                            "schema": IsDict(
                                {