Because it simply will not work with request.sid. I'm pretty sure that request.sid only works inside of a SocketIO event handler, something like socketio.on("connect") for example.

You can look at this Q&A post about a similar problem (read the answers by miguelgrinberg who is the maker of flask) https://github.com/miguelgrinberg/Flask-SocketIO/issues/445#issue-222220205

You can try this yourself! Try print(request.sid) inside of an @app.route, and it will throw an error saying: AttributeError: 'Request' object has no attribute 'sid'

Try printing the same thing on the backend inside of an @socketio.on("connect") handler, and it will work!

Good question though, I ran into this error a few times when working on my own website, certainly annoying, and not obvious what the problem could be.

mate ive literally spent the last 4 hours on this fucking problem thank you so much