request.url_for in a BaseHTTPMiddleware raises an exception

[Jdsleppy]

Request.url_for looks into self.scope["router"] .

In a BaseHTTPMiddleware, the "router" key is missing from the request scope so url_for() always raises an exception.

[Kludex]

MRE?

[Jdsleppy]

If this gets promoted to an issue and if the fix doesn't end up requiring a PhD in Starlette, I'd try to fix it myself. Let me know if that would be appropriate.

[Kludex]

I'm not sure if this is an issue, or just a limitation.

If you move the print to after the call_next on your example, does it work?

[Jdsleppy]

When I move the request.url_for() call to after call_next it does work correctly and no exception is thrown:

class ReproduceMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next) -> Response:
        response = await call_next(request)
        print(f'Saw a request for {request.url_for("index")}')
        return response

I see why this happens though: it's the Router itself that puts scope["router"] = self, but the Router gets called after all of the middleware.

But let's see what gets run when we call request.url_for()

    def url_for(self, name: str, /, **path_params: typing.Any) -> URL:
        router: Router = self.scope["router"]
        url_path = router.url_path_for(name, **path_params)
        return url_path.make_absolute_url(base_url=self.base_url)

So maybe if we can find something else that provides url_path_for() we can avoid the "router" lookup that fails in middleware. I see the application itself (Starlette class) has def url_path_for(), and the application does put itself in the scope as the first step in handling a request

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        scope["app"] = self
        if self.middleware_stack is None:
            self.middleware_stack = self.build_middleware_stack()
        await self.middleware_stack(scope, receive, send)

@Kludex I think this limitation could be fixed by having request.url_for() use the app instead of the router, like

    def url_for(self, name: str, /, **path_params: typing.Any) -> URL:
        app: Starlette = self.scope["app"]
        url_path = app.url_path_for(name, **path_params)
        return url_path.make_absolute_url(base_url=self.base_url)

If you think it's worth pursuing, I'll give it a try.

[Kludex]

I think you'll have the same issue. Isn't it the case?

[Jdsleppy]

I think it will work with the app instead of the router because the app is placed in the scope before the middleware stack runs. I'll try it and update here.