PROPAGATE_EXCEPTIONS

 

PROPAGATE_EXCEPTIONS는 flask config option으로 default 는 None이고 DEBUG가 True면 True로 setting된다.

 

flask_restful/__init__.py를 보면 이렇게 구현되어 있다.

        if not isinstance(e, HTTPException) and current_app.propagate_exceptions:
            exc_type, exc_value, tb = sys.exc_info()
            if exc_value is e:
                raise
            else:
                raise e

        headers = Headers()
        if isinstance(e, HTTPException):
            code = e.code
            default_data = {
                'message': getattr(e, 'description', http_status_message(code))
            }
            headers = e.get_response().headers
        else:
            code = 500
            default_data = {
                'message': http_status_message(code),
            }

PROPAGATE_EXCEPTIONS가 set일 때, flask-restful의 handler가 있다면 처리하고 그렇지 않다면 flask로 던져서 flask의 handler가 처리한다.

 

PROPAGATE_EXCEPTIONS가 unset일 땐, flask-restful의 handler가 없더라도 flask로 던져서 처리하지 않고 바로 error message를 만들어 반환한다.

 

 

(잘 이해는 안되지만 언젠간 되겠지..)

The workaround in #280 also works, but does so by completely bypassing flask_restful error handling which I think is a bit of a blunt solution.

Setting PROPAGATE_EXCEPTIONS = True also works, but is a bad solution. When flask_restful is initialised with a flask app, it hijacks Flask's native error handler functions - app.handle_user_exception and app.handle_exception.

When PROPAGATE_EXCEPTIONS = True, flask_restful will forego it's own error handler, and fall back to these original flask app.handle_user_exception and app.handle_exception implementations. This means you can either use flask errorhandlers OR flask_restful's error handler (you might want both).

What these original Flask error handler functions do is check to see if there is a defined custom handler for that given error (a function decorated by @app.errorhandler) and let that function handle the error - which is what we want. Hence why setting PROPAGATE_EXCEPTIONS = True makes @app.errorhandler functions work.

The problem with this is that Flask also does something with PROPAGATE_EXCEPTIONS. In Flask when this is True and there is no defined handler for the error, it will reraise the exception instead of returning a stock werkzeug.exceptions.InternalServerError when the flag is False. This is bad because this single flag does 2 distinct but deceptively similar things, and there is no way to do one, and not the other.

I propose:
- The flask_restful error handler should check if there is a flask error handler for the given error and use that.
- Additionally if we actually want to forego the flask_restful error handler, flask_restful should do so using a different config flag value. Something like USE_FLASK_ERROR_HANDLER perhaps.

https://github.com/flask-restful/flask-restful/issues/578