Ability to set response headers / cookies after yield

It feels inconsistent that we can change headers / cookies in dependencies before yield, but can’t do this afterwords. E.g. in the following example only before=yield cookie will be set:

def set_cookie(response: Response):
    response.set_cookie("before", "yield")
    yield
    response.set_cookie("after", "yield")

@app.get("/")
def index(_=Depends(set_cookie)):
    return {}

The possibility of setting them “after request” would eliminate a need for middlewares in cases when their order is not important.

Alternatively we can at least throw an exception to make the behavior explicit.

[sm-Fifteen]

How would that work? Context manager dependencies are only closed once the response has been fully rendered, I believe, at which point it’s already far too late to add headers. Is there any reason why you wouldn’t be able to set both headers before yielding?

[michitaro]

I found it when I tried to automatically save cookie-based session.

I’m guessing @imbolc were talking about codes something like this. It looks quite useful.

app = FastAPI()

def get_session_store(
    response: Response,
    FASTAPI_SESSION: Optional[str] = Cookie(None),
):
    store = {}
    if FASTAPI_SESSION is not None:
        store = json.loads(FASTAPI_SESSION)
    yield store
    response.set_cookie(key='FASTAPI_SESSION', value=json.dumps(store))


@app.get
def index(store: dict = Depends(get_session_store)):
    store['n'] = 42 # store will be automatically saved in cookie

@app.get
def clear(store: dict = Depends(get_session_store)):
    store.clear() # will work
    store = {'n': 42} # will not work

Is it possible to do something like this?

Anyway, Thank you very much for the nice framework!