About behavior HTTPException of sub-applications

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Example Code

from fastapi import FastAPI, Depends, HTTPException
from fastapi.responses import JSONResponse
from starlette.middleware.cors import CORSMiddleware


app = FastAPI()
app.add_middleware(
    CORSMiddleware,
    allow_origins=['*'],
    allow_credentials=True,
    allow_methods=['*'],
    allow_headers=['*'],
    expose_headers=['*'],
)

sub = FastAPI()


@sub.get('/', status_code=200)
async def get():
    raise HTTPException(400, 'error handling sub app')


async def http_exception_handler(_, exc):
    return JSONResponse(exc.detail, status_code=exc.status_code)

# not working
app.add_exception_handler(HTTPException, http_exception_handler)

# working
# sub.add_exception_handler(HTTPException, http_exception_handler)

app.mount('/sub', sub)


@app.get('/healthcheck', status_code=200)
async def healthcheck():
    return 'health check'

Description

I expect response of GET /sub endpoint is “error handling sub app”.

But I got {detail: “error handling sub app”}.

The cause of this, http exception handler attached to root app will not handling sub-app exceptions.

Actually, http exception handler attached to sub app worked fine.

Is this intended behavior?

Operating System

macOS

Operating System Details

MacOS Monterey v 12.3.1

FastAPI Version

0.80.0

Python Version

Python 3.10.4

Additional Context

No response

[iudeen]

I think error handling behavior should be same range as middleware, what do you think?

I’m not sure if this is the right justification. I think middleware sits between your apps and the server, while your exception handlers are within your app’s scope.

[yoshiya0503]

Sure, I’m not particularly troubled at this behavior, If that is intended, It’s ok.