[BUG] Overwriting HTTPException and RequestValidationError does not work via FastAPI exceptions handlers init

When initilizing FastAPI, an exception_handlers dictionary can be passed, mapping Exceptions/Error Codes to callables. When using this method to set the exception handlers, the HTTPException and RequestValidationError can not be overwritten.

Code to Reproducte

import logging

import uvicorn
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from fastapi.exceptions import RequestValidationError
from fastapi.exception_handlers import request_validation_exception_handler


logger = logging.getLogger(__name__)

# app = FastAPI()
# @app.exception_handler(RequestValidationError)
async def my_request_validation_exception_handler(
    req: Request, exc: RequestValidationError
):
    logger.info("my_request_validation_exception_handler")
    return await request_validation_exception_handler(req, exc)

app = FastAPI(exception_handlers={RequestValidationError: my_request_validation_exception_handler})

@app.get("/test")
def test(param: int):

    return JSONResponse(
        status_code=200,
        content={"message": "successful test with param {}".format(param)},
    )


if __name__ == "__main__":
    uvicorn.run("api:app", host="localhost", port=2000, reload=True)

Now a curl "http://localhost:2000/test" will not show the logger.info("my_request_validation_exception_handler") log statement. However, when using the commented out code it works fine. I want to use the other way to keep the handlers in a separate file.

Environment

• OS: Windows/WSL
• FastAPI Version: 0.54.2
• Python version: 3.6.9

Origin

As far as i can see this is coming from the setup method

self.add_exception_handler(HTTPException, http_exception_handler)
self.add_exception_handler(
    RequestValidationError, request_validation_exception_handler
)

Starlette will overwrite the custom handlers in the add_exception_handler method

def add_exception_handler(
        self,
        exc_class_or_status_code: typing.Union[int, typing.Type[Exception]],
        handler: typing.Callable,
    ) -> None:
        self.exception_handlers[exc_class_or_status_code] = handler
        self.middleware_stack = self.build_middleware_stack()

Would be great if setup() could first check whether a handler for this exception exists already.

[tiangolo]

Thanks for all the discussion here everyone! ☕

@uriyyo fixed this in https://github.com/tiangolo/fastapi/pull/1924, available in FastAPI version 0.61.2. 🎉

[Kludex]

Well, in the past when @tiangolo saw one first, he basically merged that one (if it was good) and the next was discharged. So I recommend you to mention #1887 on your solution and explain that you’ve implemented it without knowing that it was already implemented. Then you just wait. 😎 👍