How Can I Customize Error Responses for Specific Routes in FastAPI?

Customizing Error Responses for Specific Routes in FastAPI

Overview

FastAPI ^{[1](#sources)} provides a versatile platform for customizing error responses for specific routes. This allows developers to tailor error handling according to specific application requirements. By overriding the default exception handler or employing other techniques like sub-applications, one can create custom error responses that enhance user experience and provide meaningful feedback.

Option 1: Override Default Exception Handler

One approach involves overriding the default exception handler for RequestValidationError. This exception is raised when a request contains invalid data. By implementing a custom handler, you can check for specific errors related to your required custom header and return a custom error response.

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

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:
        # check whether the error relates to the `some_custom_header` parameter
        for err in exc.errors():
            if err['loc'][0] == 'header' and err['loc'][1] == 'some-custom-header':
                return JSONResponse(content={'401': 'Unauthorized'}, status_code=401)
            
    return JSONResponse(
        status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
        content=jsonable_encoder({'detail': exc.errors(), 'body': exc.body}),
    )

Option 2: Sub-Applications

Another alternative involves creating sub-applications and mounting them to the main application. This allows for custom error handling specific to routes within the sub-application, without affecting other routes in the main application.

# main API
app = FastAPI()

# sub-application with custom error handling
subapi = FastAPI()
@subapi.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
    # Handle error specific to sub-application's routes
    return JSONResponse(content={'401': 'Unauthorized'}, status_code=401)

# mount sub-application
app.mount('/sub', subapi)

Option 3: Custom APIRoute Class

This approach involves creating a custom APIRoute class that handles request validation within a try-except block. If a RequestValidationError is raised, a custom error response can be returned.

from fastapi import FastAPI, APIRouter, APIRoute, Request, Header, HTTPException
from fastapi.responses import JSONResponse
from fastapi.exceptions import RequestValidationError

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

        async def custom_route_handler(request: Request) -> Response:
            try:
                return await original_route_handler(request)
            except RequestValidationError as e:
                raise HTTPException(status_code=401, detail='401 Unauthorized')

# create new router with custom error handling
router = APIRouter(route_class=ValidationErrorHandlingRoute)

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

# add router to app 
app.include_router(router)

Conclusion

By exploring these techniques, developers can customize error responses in FastAPI to suit specific requirements, providing a more tailored user experience and enhanced error handling capabilities.

Sources

1. [FastAPI Documentation: Handling HTTP Exceptions](https://fastapi.tiangolo.com/exception-handlers/)

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