GAE Python Server initiating Angular Routes from Responses
Make both GAE and Angular understand your routes. You will need to define them for one, why not both?
You just have to organise your markup and structure so it can support complete page loading and ajax loading. For example, initial load is done on any route by GAE, then Angular can take over, loading each page "content" as it goes. 
This has the additional advantage of public pages being crawler friendly while real users get ajax loading (which should reduce bandwidth once you scale).
You may need to load user state in via the server, and or force a page reload on log in or out to do so.
I have done the above on a few apps, and it works well.
I'm currently building a web application using AngularJS, Webapp2, and the Python Google App Engine environment. This app is supposed to have all the features of modern social networks (users, posts, likes, comments). I want the page hierarchy to look like this, the main pages are from the server and the sub pages are supposed to be angular routes:
Index

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The problem is that when a user wants to signup I want them to be able to go to /signup and get the index page with the signup route loaded. How can I get the server to preload an angular route from the response