Jian Daoyun can keep the common part of the page such as navigation unchanged, but only change the main part of the page when you click on the navigation bar (or navigation pane) to switch pages. At the same time, if you look at the address bar, you can see that the address (routing) of the page has changed with the navigation switch, rather than maintaining the same address.
the details can be compared with the red boxes of the two pictures below, which shows that my own changes in the left navigation bar have not been reset because of the navigation switch, while the address of the address bar has changed.
what should I do if I use Flask,? Don"t consider AJAX, for the time being because as far as I know, using AJAX ensures that the address of the address bar is always unique, but the address changes when you switch navigation in the cloud.