How to Customize FastAPI\'s Error Response for Specific Routes?

Customize Error Response for Specific Route in FastAPI

In FastAPI, when validating a request, if a parameter is missing or invalid, FastAPI raises a RequestValidationError exception. By default, this exception returns a 422 Unprocessable Entity response. However, you may want to customize this response for specific routes. Here are several approaches to achieve this:

Option 1: Override Default Exception Handler

Override the default exception handler for RequestValidationError and provide a custom response for the desired routes.

from fastapi import FastAPI, Request, status, HTTPException
from fastapi.responses import JSONResponse
from fastapi.encoders import jsonable_encoder
from fastapi.exceptions import RequestValidationError

app = FastAPI()
routes_with_custom_exception = ['/']

@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    if request.url.path in routes_with_custom_exception:
        return JSONResponse(content={'401': 'Unauthorized'}, status_code=401)

Option 2: Use Sub-Application(s)

Create a sub-application and mount it to the main app for the specific routes. Override the exception handler for RequestValidationError in the sub-application.

from fastapi import FastAPI, RequestValidationError, HTTPException
from fastapi.responses import JSONResponse

app = FastAPI()
subapi = FastAPI()

@subapi.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    return 
JSONResponse(content={'401': 'Unauthorized'}, status_code=401)

app.mount('/sub', subapi)

Option 3: Use Custom APIRouter and APIRoute

Create a custom APIRoute subclass and override the get_route_handler method. This allows you to intercept requests and handle RequestValidationError exceptions.

from fastapi import FastAPI
from fastapi.routing import APIRoute
from fastapi.responses import JSONResponse
from fastapi.exceptions import RequestValidationError
from fastapi.requests import Request

class ValidationErrorHandlingRoute(APIRoute):
    def get_route_handler(self):
        original_route_handler = super().get_route_handler()

        async def custom_route_handler(request: Request):
            try:
                return await original_route_handler(request)
            except RequestValidationError as e:
                return JSONResponse(content={'401': 'Unauthorized'}, status_code=401)

        return custom_route_handler

app = FastAPI()
router = APIRouter(route_class=ValidationErrorHandlingRoute)

@app.get('/')
async def main():
    return {'message': 'Hello from main API'}

@router.get('/custom')
async def custom_route(some_custom_header: str = Header(...)):
    return {'some-custom-header': some_custom_header}

app.include_router(router)

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