```

The webhooks that you define will end up in the **OpenAPI** schema and the automatic **docs UI**.

!!! info
    The `app.webhooks` object is actually just an `APIRouter`, the same type you would use when structuring your app with multiple files.

```mermaid
graph LR
    subgraph Client Node
        Client
        CZ["Ztunnel"]
    end
    subgraph Server Node
        Server
        SZ["Ztunnel"]
    end
    Client--Plain-->CZ
    CZ-."HBONE (target)".->Server
    CZ--"HBONE (actual)"-->SZ
    SZ--Plain-->Server
```

The only new thing is the `callbacks=invoices_callback_router.routes` as an argument to the *path operation decorator*. We'll see what that is next.

## Documenting the callback

The actual callback code will depend heavily on your own API app.

And it will probably vary a lot from one app to the next.

It could be just one or two lines of code, like:

```Python

### Use the form data

!!! tip
    The instance of the dependency class `OAuth2PasswordRequestForm` won't have an attribute `scope` with the long string separated by spaces, instead, it will have a `scopes` attribute with the actual list of strings for each scope sent.

    We are not using `scopes` in this example, but the functionality is there if you need it.

Now, get the user data from the (fake) database, using the `username` from the form field.

	actualIPSetContents, err := hostsideProbeSet.ListEntriesByIP()
	if err != nil {
		log.Warnf("unable to list IPSet: %v", err)
		return err
	}
	actual := sets.New[netip.Addr](actualIPSetContents...)
	stales := actual.DifferenceInPlace(expected)

	for staleIP := range stales {
		if err := hostsideProbeSet.ClearEntriesWithIP(staleIP); err != nil {
			return err
		}

    assertEquals(0, actual.indexOf(list(1, 3)));
    assertEquals(1, actual.indexOf(list(1, 4)));
    assertEquals(2, actual.indexOf(list(2, 3)));
    assertEquals(3, actual.indexOf(list(2, 4)));
    assertEquals(-1, actual.indexOf(list(3, 1)));

    assertEquals(-1, actual.indexOf(list(1)));
    assertEquals(-1, actual.indexOf(list(1, 1, 1)));
  }

    assertEquals(0, actual.indexOf(list(1, 3)));
    assertEquals(1, actual.indexOf(list(1, 4)));
    assertEquals(2, actual.indexOf(list(2, 3)));
    assertEquals(3, actual.indexOf(list(2, 4)));
    assertEquals(-1, actual.indexOf(list(3, 1)));

    assertEquals(-1, actual.indexOf(list(1)));
    assertEquals(-1, actual.indexOf(list(1, 1, 1)));
  }

```

Using `Optional[str]` instead of just `str` will let the editor help you detecting errors where you could be assuming that a value is always a `str`, when it could actually be `None` too.

`Optional[Something]` is actually a shortcut for `Union[Something, None]`, they are equivalent.

This also means that in Python 3.10, you can use `Something | None`:

=== "Python 3.10+"

    ```Python hl_lines="1"

# 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
```

   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
   * each appears first in the source collection.
   *
   * <p><b>Performance note:</b> This method will sometimes recognize that the actual copy operation
   * is unnecessary; for example, {@code copyOf(copyOf(anArrayList))} will copy the data only once.
   * This reduces the expense of habitually making defensive copies at API boundaries. However, the