i want to integrate "google sign in" in my google app engine project

I want the google sign in method in my webapp2 application. Currently i can't find any method (in google searches) to do that. You can easily do that in javascript (but that's on the client side), i want the data on my server side so i can add user to the database (if not present already). Some example snippets would be great help. and is there official documentation to do that?

i want to do it in a controller like so

from google.appengine.api import users
import webapp2
class Google_sign_up(webapp2.RequestHandler):
    def get(self):
        # code for the google sign in goes here