[Python] Starlette/FastAPI Default Exception Handler Overriding Error

I'm trying to override the default exception handler in a FastAPI application so I can log any exceptions that happen. The problem that I run into is that my handler seems to run as it logs what I want but I'm getting a 500 back to the client because of an error in the built-it ServerErrorMiddleware:

Traceback (most recent call last):
File "c:\users\jtaft\appdata\local\programs\python\python38\lib\site-packages\hypercorn\asyncio\task_group.py", line 27, in _handle
await app(scope, receive, send, sync_spawn, call_soon)
File "c:\users\jtaft\appdata\local\programs\python\python38\lib\site-packages\hypercorn\app_wrappers.py", line 34, in __call__
await self.app(scope, receive, send)
File "c:\users\jtaft\appdata\local\programs\python\python38\lib\site-packages\fastapi\applications.py", line 1054, in __call__
await super().__call__(scope, receive, send)
File "c:\users\jtaft\appdata\local\programs\python\python38\lib\site-packages\starlette\applications.py", line 112, in __call__
await self.middleware_stack(scope, receive, send)
File "c:\users\jtaft\appdata\local\programs\python\python38\lib\site-packages\starlette\middleware\errors.py", line 189, in __call__
await response(scope, receive, send)
TypeError: 'NoneType' object is not callable


This is my handler:

async def default_exception_handler(request: Request,
exc: Exception,
logger:logging.Logger) -> JSONResponse:
tempPath: str = request['path']
tempScriptPathList: List[str] = sys.argv[0].split("\\")

# The [0] is neded to get the value, slicing the list, still returns a list.
if tempScriptPathList[-1:][0] in ['hypercorn']:
tempScriptPath = sys.argv[1]
else:
tempScriptPath = sys.argv[0]

for pp in request.path_params:
if request.path_params[pp] in tempPath:
tempPath = tempPath.replace(request.path_params[pp], f"{{{pp}}}")
tempLogDict = {"message": exc.args[0],
"api_request_id": str(uuid.uuid4()),
"method": request.method,
"client_ip": request.client.host,
"log_type": "alert",
"path": tempPath,
"script": tempScriptPath,
"query_params": request.query_params._dict,
"path_params": request.path_params,
"error":{
"error_code": 500,
"error_message": exc.args[0],
"stack_trace": traceback.format_exc().splitlines()
}
}
logger.error(f'{json.dumps(tempLogDict)}')
return JSONResponse(status_code=HTTP_500_INTERNAL_SERVER_ERROR,
content=jsonable_encoder({"detail":tempLogDict['message']}),
headers={'x-api-request-id':tempLogDict['api_request_id']})


The logger.error line works because it will write to the logger, but I'm not getting the JSONresponse back. I've change the status code to see if I was getting the code but not the content or the headers but that didn't change that I'm getting the generic 500 from ServerErrorMiddleware.

Best I can tell, my handler runs but doesn't return what ServerErrorMiddleware expects and that causes the general 500, but I cant figure out why.

This is how I'm adding my handler:

@app.exception_handler(Exception)
async def exception_callback(request: Request, exc: Exception):
await default_exception_handler(request, exc, ql.logger)


Starlette: 0.44.0

FastAPI: 0.115.12

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