def dependency_list() -> List[str]:
    return []


DepList = Annotated[List[str], Depends(dependency_list)]


def create_dependency(name: str):
    def fun(deps: DepList):
        deps.append(name)

    return Depends(fun)


router = APIRouter(dependencies=[create_dependency("router")])
prefix_router = APIRouter(dependencies=[create_dependency("prefix_router")])

    caught = []

    @websocket_middleware
    async def catcher(websocket, call_next):
        try:
            return await call_next()
        except Exception as e:  # pragma: no cover
            caught.append(e)
            raise

    myapp = make_app(middleware=[Middleware(catcher)])

    client = TestClient(myapp)
    with pytest.raises(WebSocketDisconnect) as e:

Sie sehen eine einfache Seite wie:

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

Sie können Nachrichten in das Eingabefeld tippen und absenden:

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

Und Ihre **FastAPI**-Anwendung mit WebSockets antwortet:

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

{!../../../docs_src/events/tutorial002.py!}
```

Hier schreibt die `shutdown`-Eventhandler-Funktion eine Textzeile `"Application shutdown"` in eine Datei `log.txt`.

!!! info
    In der Funktion `open()` bedeutet `mode="a"` „append“ („anhängen“), sodass die Zeile nach dem, was sich in dieser Datei befindet, hinzugefügt wird, ohne den vorherigen Inhalt zu überschreiben.

!!! tip "Tipp"

    path.write_bytes(b"<file content>")
    app = FastAPI()

    testing_file_store: List[UploadFile] = []

    @app.post("/uploadfile/")
    def create_upload_file(file: UploadFile):
        testing_file_store.append(file)
        return {"filename": file.filename}

    client = TestClient(app)
    with path.open("rb") as file:
        response = client.post("/uploadfile/", files={"file": file})

Including external libraries also based on Pydantic, as <abbr title="Object-Relational Mapper">ORM</abbr>s, <abbr title="Object-Document Mapper">ODM</abbr>s for databases.

This also means that in many cases you can pass the same object you get from a request **directly to the database**, as everything is validated automatically.

✅ 🔢 🗃 ⚓️ 🔛 Pydantic, <abbr title="Object-Relational Mapper">🐜</abbr>Ⓜ, <abbr title="Object-Document Mapper">🏭</abbr>Ⓜ 💽.

👉 ⛓ 👈 📚 💼 👆 💪 🚶‍♀️ 🎏 🎚 👆 🤚 ⚪️➡️ 📨 **🔗 💽**, 🌐 ✔ 🔁.

🎏 ✔ 🎏 🌌 🤭, 📚 💼 👆 💪 🚶‍♀️ 🎚 👆 🤚 ⚪️➡️ 💽 **🔗 👩‍💻**.

# OpenAPI Callbacks

You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).

    id: description
    attributes:
      label: Description
      description: |
        What is the problem, question, or error?

        Write a short description telling me what you are doing, what you expect to happen, and what is currently happening.
      placeholder: |
        * Open the browser and call the endpoint `/`.
        * It returns a JSON with `{"Hello": "World"}`.
        * But I expected it to return `{"Hello": "Sara"}`.

!!! tip
    The `shutdown` would happen when you are **stopping** the application.

    Maybe you need to start a new version, or you just got tired of running it. 🤷

### Lifespan function