[BUG] FastAPI exception handler only catches exact exceptions

[ashic]

Investigative information

Please provide the following:

• Timestamp: Nov 22, 2023
• Function App name: running locally (reproduction of a production issue)
• Function name(s) (as appropriate): see above
• Core Tools version: ❯ func --version: 4.0.5455

Repro steps

Provide the steps required to reproduce the problem:

I'm using the AsgiMiddleware to wrap a FastApi app. If I add an exception_handler for Exception, then specific errors like ZeroDivisionError are not caught when running with func start, however, it works if I use uvicorn to run the app instead of func start. Please see https://github.com/ashic/azfn-fastapi-demo/blob/main/src/azfn_fastapi_demo/main.py

The main function is here:

from fastapi import FastAPI, Body, Response
from fastapi.responses import JSONResponse
import azure.functions as func

app = FastAPI()

@app.exception_handler(Exception)
async def catch_all(req, e:Exception):
    return JSONResponse("Catch all handler")

# @app.exception_handler(ZeroDivisionError)
# async def zero_handler(req, e:ZeroDivisionError):
#     return JSONResponse("Zero handler")

@app.post("/test")
async def test():

    a = 20 / 0

    return {
        'content': "Hello"
    }

main = func.AsgiMiddleware(app).main

If I remove the comments on the ZeroDivisionError handler, then running with func start cathes the divide by zero exception. However, with the code as is, when running with uvicorn, the Exception handler catches is, but running with func start, doesn't.

I use scripts/run-local.sh to run via uvicorn and scripts/run-local-az.sh to run with the azure function host.

Expected behavior

Provide a description of the expected behavior.

When running with func start, an Exception handler should catch all exceptions. The behaviour should be the same as when running with uvicorn.

Actual behavior

Provide a description of the actual behavior observed.

The behaviour differs from running with uvicorn. Only specific exceptions are caught.

Known workarounds

Provide a description of any known workarounds.

Other than writing a handler for every possible error, there doesn't seem to be one.

Contents of the requirements.txt file:

Provide the requirements.txt file to help us find out module related issues.

azure-functions==1.17.0
fastapi==0.104.1
uvicorn==0.24.0.post1