how to tell browser screen size in python
I am not sure about what you are trying to do, but I have pretty good reasons to believe that this is something you want to handle in your css/js side of the application.
I am not familiar with app-engine, but the browser does not send it's viewport size with requests (unless you put it yourself in request data).
So if you are using ajax request - put the data about viewport in the data send to backend. If not - use css or js to handle your frontend.
I am using WebApp2 with python (and app-engine). Is there a way to tell the screen size of the browser in python? I am trying to figure if the browser size is less than 768.