This example doesn't implement the callback itself (that could be just a line of code), only the documentation part.

/// tip

The actual callback is just an HTTP request.

For that, use the standard Python `typing.List` (or just `list` in Python 3.9 and above):

{* ../../docs_src/extra_models/tutorial004_py39.py hl[18] *}

## Response with arbitrary `dict` { #response-with-arbitrary-dict }

You can also declare a response using a plain arbitrary `dict`, declaring just the type of the keys and values, without using a Pydantic model.

///

/// note | Technical Details

You could also use `from starlette.testclient import TestClient`.

**FastAPI** provides the same `starlette.testclient` as `fastapi.testclient` just as a convenience for you, the developer. But it comes directly from Starlette.

///

/// tip

That doesn't add any extra security to your API, the *path operations* will still be available where they are.

If there's a security flaw in your code, it will still exist.

They are just a convenience, they hold the same number, but that way you can use the editor's autocomplete to find them:

<img src="/img/tutorial/response-status-code/image02.png">

/// note | Technical Details

You could also use `from starlette import status`.

        assertTrue(CharUtil.isUrlChar('9')); // last digit

        // Test characters just outside ranges that are not valid
        assertFalse(CharUtil.isUrlChar('`')); // backtick (just before 'a')
        assertFalse(CharUtil.isUrlChar('{')); // left brace (just after 'z')
        assertFalse(CharUtil.isUrlChar('^')); // caret (before 'a' range)

      } else {
        // if our write lost the race, it must have lost to a nonzero value, so we can stop
        return countMap.putIfAbsent(element, new AtomicInteger(newCount)) == null;
      }
    }
    int oldValue = existingCounter.get();
    if (oldValue == expectedOldCount) {
      if (oldValue == 0) {
        if (newCount == 0) {
          // Just observed a 0; try to remove the entry to clean up the map

**FastAPI** will create the object of type `BackgroundTasks` for you and pass it as that parameter.

## Create a task function { #create-a-task-function }

Create a function to be run as the background task.

It is just a standard function that can receive parameters.

It can be an `async def` or normal `def` function, **FastAPI** will know how to handle it correctly.

Here we do it... just to show that we can 🤷:

{* ../../docs_src/bigger_applications/app_an_py39/main.py hl[21:23] title["app/main.py"] *}

and it will work correctly, together with all the other *path operations* added with `app.include_router()`.

/// info | Very Technical Details

**Note**: this is a very technical detail that you probably can **just skip**.

---

/// info

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

///