how to use customer exception handler by APIRouter?

[WhyTimePass]

I want to write different handler with different APIRouter, but when i use APIRouter().exception_handler，the ide notice me no that "AttributeError : 'APIRouter' object has no attribute 'exception_handler'...
how can i use customer exception handler with different APIRouter?

[MacMacky]

@WhyTimePass
Hi, You can't add an exception handler in an APIRouter instance only on the FastAPI instance.
Maybe this will change in the future if the maintainer adds this functionality.

[Kludex]

@WhyTimePass I suggest you to create different errors and handle them in different ways. Example:

from fastapi import FastAPI, HTTPException, status
from fastapi.respones import JSONResponse

class PerfectException(HTTPException):
    pass

class TooPerfectException(HTTPException):
    pass

def perfect_exception_handler(request: Request, exc: HTTPException):
    return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": "hi"})

def too_perfect_exception_handler(request: Request, exc: HTTPException):
    return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": "hi, my friend"})

app = FastAPI()

app.add_exception_handler(PerfectException, perfect_exception_handler)
app.add_exception_handler(TooPerfectExceptionHandler, too_perfect_exception_handler)

[WhyTimePass]

thanks,

[Ma233]

According to the documentation: https://fastapi.tiangolo.com/advanced/custom-request-and-route/
You can also implement it by changing the behaviour of route_handler.

[WhyTimePass]

According to the documentation: https://fastapi.tiangolo.com/advanced/custom-request-and-route/
You can also implement it by changing the behaviour of route_handler.

Thank you ! this ways can help me, but it is still hard.
Has there been exist use decorator to handle custom exception

[michaelkariv]

I would like to suggest a use case where such functionality is important. I am porting the existing implementation from Google App Engine/Python 2.7. I have errors that are custom. So for some routes 422 error will have a response JSON formatted specifically for that route. So having error handlers be attached to @router, thar then @app would be a huge reprieve

[james77777778]

I figured out the APIRouter exception handler just like Flask
Hope this helps

(ex: add prometheus_client and logger when HTTPException occurs)

from fastapi import APIRouter, HTTPException, Request, Response
from fastapi.routing import APIRoute
from prometheus_client import Counter

system_fail_count = Counter('system_fail_count', 'Counts the number of system fail')

class PrometheusRoute(APIRoute):
    def get_route_handler(self) -> Callable:
        original_route_handler = super().get_route_handler()

        async def custom_route_handler(request: Request) -> Response:
            app = request.app
            try:
                return await original_route_handler(request)
            except HTTPException as exc:
                system_fail_count.inc()
                app.logger.warning('{}'.format(exc.detail))
                raise exc

        return custom_route_handler


router = APIRouter(prefix='/services', route_class=PrometheusRoute)

[wangxin688]

still want to know if there is a

@WhyTimePass Hi, You can't add an exception handler in an APIRouter instance only on the FastAPI instance. Maybe this will change in the future if the maintainer adds this functionality.

Also hope this feature will comming soon. Now, it's difficult to add customize Exception for APIRouter .

[VikashKothary]

The reason I wanted to do this is to split the exceptions logic and the main app, so I customised @Kludex's example to support multiple files so it works closer to the include_router logic when working with bigger applications.

exceptions.py:

from fastapi import FastAPI, HTTPException, status
from fastapi.respones import JSONResponse

class PerfectException(HTTPException):
    pass

class TooPerfectException(HTTPException):
    pass

def perfect_exception_handler(request: Request, exc: HTTPException):
    return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": "hi"})

def too_perfect_exception_handler(request: Request, exc: HTTPException):
    return JSONResponse(status_code=status.HTTP_404_NOT_FOUND, content={"message": "hi, my friend"})

def include_app(app):
	app.add_exception_handler(PerfectException, perfect_exception_handler)
	app.add_exception_handler(TooPerfectExceptionHandler, too_perfect_exception_handler)

app.py

import exceptions

app = FastAPI()
exceptions.include_app(app)

[tiangolo]

Thanks for the discussion everyone! ☕

About the original question, I think it's probably a misunderstanding. If you check the docs about custom APIRoute class you can see about custom handlers: https://fastapi.tiangolo.com/advanced/custom-request-and-route/#accessing-the-request-body-in-an-exception-handler

But there's not mention of a exception_handler attribute in the APIRouter class.

Note: have in mind that APIRoute and APIRouter are two different things.

If anyone else is still having related problems, please create a new issue, with a self-contained example, etc. To be able to discuss each particular case independently. ☕

Sorry for the long delay! 🙈 I wanted to personally address each issue and they piled up through time, but now I'm checking each one in order.

[github-actions]

Assuming the original need was handled, this will be automatically closed now. But feel free to add more comments or create new issues or PRs.