[Python] How to handle exceptions for all the sub apps in FastAPI

I have a FastAPI project containing multiple sub apps (The sample includes just one sub app).

main_app = FastAPI()

class CustomException(Exception):
def __init__(self, message: str, status_code: int, name: str = "Exception"):
Exception.__init__(self)
self.name = name
self.status_code = status_code
self.message = message

@main_app.exception_handler(CustomException)
async def custom_exception_handler(exception: CustomException) -> JSONResponse:
return JSONResponse(
status_code=exception.status_code, content={"error": exception.message}
)
main_app.mount("/subapp", subapp1)


I've handled the exceptions in main app, but not in subapp1. Now if I use the CustomException in subapp1:

raise CustomException(
status_code=status.HTTP_404_NOT_FOUND,
message=f"{self.model.__name__} not found",
)


I get this error:

RuntimeError: Caught handled exception, but response already started.
​

It seems like when raising CustomException in a sub app, it won't be handled by the main app exception handler. So how can I handle the exceptions from all the sub app using the main app exception handler?

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