---

Imagine you are the computer / program 🤖 in that story.

While you are at the line, you are just idle 😴, waiting for your turn, not doing anything very "productive". But the line is fast because the cashier is only taking the orders (not preparing them), so that's fine.

👆 💪 ✍ & ⚙️ 🌐 🔢 🐚, 🍵 💆‍♂ 🐍:

=== "💾, 🇸🇻, 🚪 🎉"

    <div class="termy">

    ```console
    // You could create an env var MY_NAME with
    $ export MY_NAME="Wade Wilson"

    // Then you could use it with other programs, like
    $ echo "Hello $MY_NAME"

So, instead of adding all that to each *path operation*, we can add it to the `APIRouter`.

```Python hl_lines="5-10  16  21" title="app/routers/items.py"
{!../../../docs_src/bigger_applications/app/routers/items.py!}
```

As the path of each *path operation* has to start with `/`, like in:

```Python hl_lines="1"
@router.get("/{item_id}")
async def read_item(item_id: str):
    ...
```

=== "Python 3.10+"

    ```Python hl_lines="1"
    {!> ../../../docs_src/python_types/tutorial009_py310.py!}
    ```

=== "Python 3.8+"

    ```Python hl_lines="1  4"
    {!> ../../../docs_src/python_types/tutorial009.py!}
    ```

=== "Python 3.8+ alternative"

    ```Python hl_lines="1  4"
    {!> ../../../docs_src/python_types/tutorial009b.py!}
    ```

```

</div>

That command line program called `fastapi` is **FastAPI CLI**.

FastAPI CLI takes the path to your Python program and automatically detects the variable with the FastAPI (commonly named `app`) and how to import it, and then serves it.

For production you would use `fastapi run` instead. 🚀

Sebastián Ramírez <******@****.***> 1714435394 -0700

```Python hl_lines="6"
{!../../../docs_src/events/tutorial002.py!}
```

Here, the `shutdown` event handler function will write a text line `"Application shutdown"` to a file `log.txt`.

!!! info
    In the `open()` function, the `mode="a"` means "append", so, the line will be added after whatever is on that file, without overwriting the previous contents.

আপনি যদি <abbr title="Command Line Interface">CLI</abbr> অ্যাপ বানাতে চান, যা কিনা ওয়েব API এর পরিবর্তে টার্মিনালে ব্যবহার হবে, তাহলে দেখুন<a href="https://typer.tiangolo.com/" class="external-link" target="_blank">**Typer**</a>.

**টাইপার** হল FastAPI এর ছোট ভাইয়ের মত। এবং এটির উদ্দেশ্য ছিল **CLIs এর FastAPI** হওয়া। ⌨️ 🚀

    When implementing the callback yourself, you could use something like <a href="https://www.python-httpx.org" class="external-link" target="_blank">HTTPX</a> or <a href="https://requests.readthedocs.io/" class="external-link" target="_blank">Requests</a>.

## Write the callback documentation code

This code won't be executed in your app, we only need it to *document* how that *external API* should look like.

* <a href="https://hub.docker.com/_/mysql" class="external-link" target="_blank">MySQL</a>
* <a href="https://hub.docker.com/_/mongo" class="external-link" target="_blank">MongoDB</a>
* <a href="https://hub.docker.com/_/redis" class="external-link" target="_blank">Redis</a>, etc.