Generate a random username for each session [duplicate]

I have created a discussion type web app with Flask. Everything works perfectly fine, but I want that when you enter the website, you press a button that generates you a random username. From then, each post until you close the page and come back will be post with this username. I figured how to generate random username with the button, but I can't manage to assign it to the username.

EDIT: I can now assign it to the username, but each time a user generate a username, it changes it for all the curent posts.