name: str
    description: Union[str, None] = None
    price: float
    tax: Union[float, None] = None


class User(BaseModel):
    username: str
    full_name: Union[str, None] = None


@app.put("/items/{item_id}")
async def update_item(
    *,
    item_id: int,
    item: Item,
    user: User,
    importance: Annotated[int, Body(gt=0)],
    q: Union[str, None] = None,
):

    name: str
    description: Union[str, None] = None
    price: float
    tax: Union[float, None] = None


class User(BaseModel):
    username: str
    full_name: Union[str, None] = None


@app.put("/items/{item_id}")
async def update_item(
    *,
    item_id: int,
    item: Item,
    user: User,
    importance: Annotated[int, Body(gt=0)],
    q: Union[str, None] = None,
):

    access_token: str
    token_type: str


class TokenData(BaseModel):
    username: Union[str, None] = None
    scopes: list[str] = []


class User(BaseModel):
    username: str
    email: Union[str, None] = None
    full_name: Union[str, None] = None
    disabled: Union[bool, None] = None


class UserInDB(User):
    hashed_password: str

@app.get("/")
async def get():
    return HTMLResponse(html)


async def get_cookie_or_token(
    websocket: WebSocket,
    session: Annotated[str | None, Cookie()] = None,
    token: Annotated[str | None, Query()] = None,
):
    if session is None and token is None:
        raise WebSocketException(code=status.WS_1008_POLICY_VIOLATION)
    return session or token


@app.websocket("/items/{item_id}/ws")

    websocket: WebSocket,
    session: str | None = Cookie(default=None),
    token: str | None = Query(default=None),
):
    if session is None and token is None:
        raise WebSocketException(code=status.WS_1008_POLICY_VIOLATION)
    return session or token


@app.websocket("/items/{item_id}/ws")
async def websocket_endpoint(
    websocket: WebSocket,
    item_id: str,
    q: int | None = None,

def create_model_field(
    name: str,
    type_: Any,
    class_validators: Optional[Dict[str, Validator]] = None,
    default: Optional[Any] = Undefined,
    required: Union[bool, UndefinedType] = Undefined,
    model_config: Type[BaseConfig] = BaseConfig,
    field_info: Optional[FieldInfo] = None,
    alias: Optional[str] = None,
    mode: Literal["validation", "serialization"] = "validation",
) -> ModelField:

    username: str
    password: str
    email: EmailStr
    full_name: str | None = None


class UserOut(BaseModel):
    username: str
    email: EmailStr
    full_name: str | None = None


class UserInDB(BaseModel):
    username: str
    hashed_password: str
    email: EmailStr
    full_name: str | None = None


def fake_password_hasher(raw_password: str):
    return "supersecret" + raw_password

@app.get("/")
async def get():
    return HTMLResponse(html)


async def get_cookie_or_token(
    websocket: WebSocket,
    session: Annotated[Union[str, None], Cookie()] = None,
    token: Annotated[Union[str, None], Query()] = None,
):
    if session is None and token is None:
        raise WebSocketException(code=status.WS_1008_POLICY_VIOLATION)
    return session or token


@app.websocket("/items/{item_id}/ws")

@app.get("/")
async def get():
    return HTMLResponse(html)


async def get_cookie_or_token(
    websocket: WebSocket,
    session: Annotated[Union[str, None], Cookie()] = None,
    token: Annotated[Union[str, None], Query()] = None,
):
    if session is None and token is None:
        raise WebSocketException(code=status.WS_1008_POLICY_VIOLATION)
    return session or token


@app.websocket("/items/{item_id}/ws")

Le paramètre de requête `q` a pour type `Union[str, None]` (ou `str | None` en Python 3.10), signifiant qu'il est de type `str` mais pourrait aussi être égal à `None`, et bien sûr, la valeur par défaut est `None`, donc **FastAPI** saura qu'il n'est pas requis.

/// note

**FastAPI** saura que la valeur de `q` n'est pas requise grâce à la valeur par défaut `= None`.