Response.background should be BackgroundTasks instead of BackgroundTask | None

[Stefano-Nicoli]

I found myself in the situation where I receive a Response.background object (I don't know in advance what it is excatly) and I simply need to add a task to it.
The current implementation (BackgroundTask | None) makes me do something like:

task: BackgroundTask
response_background: BackgroundTask | None
if response_background:
    if not isinstance(response_background, BackgroundTasks):
        response_background = BackgroundTasks([response_background])
    response_background.add_task(task)
else:
    response_background = BackgroundTasks([task])

while I would like to have something like:

task: BackgroundTask
response_background: BackgroundTasks

response_background.add_task(task)

In order to achieve so, I would implement some simple changes like:

class BackgroundTasks:
    def __init__(self, tasks: typing.Sequence[BackgroundTask] | None = None):
        self.tasks = list(tasks) if tasks else []

    def add_task(self, func: typing.Callable[P, typing.Any], *args: P.args, **kwargs: P.kwargs) -> None:
        task = BackgroundTask(func, *args, **kwargs)
        self.tasks.append(task)
    
    def __bool__(self) -> bool:
        return bool(self.tasks)

    async def __call__(self) -> None:
        for task in self.tasks:
            await task()

and

class Response:
    media_type = None
    charset = "utf-8"

    def __init__(
        self,
        content: typing.Any = None,
        status_code: int = 200,
        headers: typing.Mapping[str, str] | None = None,
        media_type: str | None = None,
        background: BackgroundTasks = BackgroundTasks(),
    ) -> None:

I checked locally and with a few changes the code would adapt nicely to this.

I'm not an expert in async web services, so I might be missing some details, but I hope this can help :)

edit: added a small commit in a fork: master...Stefano-Nicoli:starlette:sn/response_background