Cannot add a custom handler for HTTPExceptions with status_code=500

[clickonben]

I found this through FastAPI but the code causing the problem appears to be in Starlette.

from fastapi import FastAPI, HTTPException

app = FastAPI()

@app.exception_handler(500)
def custom_exception_handler(request, exc):
    print(f"Oops! An HTTP error occurred: {exc}")
    return JSONResponse({"detail":"Internal Server Error"}, status_code=500)


@app.get("/")
def read_root():
    raise HTTPException(status_code=500, detail="Internal Server Error")

This exception handler never gets called. When I dug into it I found the problem was this:

def build_middleware_stack(self) -> ASGIApp:
        debug = self.debug
        error_handler = None
        exception_handlers: dict[
            typing.Any, typing.Callable[[Request, Exception], Response]
        ] = {}

        for key, value in self.exception_handlers.items():
            if key in (500, Exception):
                error_handler = value
            else:
                exception_handlers[key] = value

        middleware = (
            [Middleware(ServerErrorMiddleware, handler=error_handler, debug=debug)]
            + self.user_middleware
            + [
                Middleware(
                    ExceptionMiddleware, handlers=exception_handlers, debug=debug
                )
            ]
        )

        app = self.router
        for cls, args, kwargs in reversed(middleware):
            app = cls(app=app, *args, **kwargs)
        return app

in this file https://github.com/encode/starlette/blob/master/starlette/applications.py

I don't understand why it is doing this:

for key, value in self.exception_handlers.items():
    if key in (500, Exception):
        error_handler = value
    else:
        exception_handlers[key] = value

because if I change it to this:

for key, value in self.exception_handlers.items():
    exception_handlers[key] = value

Everything works as expected and my error handler handles 500 errors the same as any other status code.