old_contents: import webapp2
  
  class MainPage(webapp2.RequestHandler):
      def get(self):
          self.response.headers['Content-Type'] = 'text/plain'
          self.response.write('Hello, World!')
  
  application = webapp2.WSGIApplication([
      ('/', MainPage),
  ], debug=True)
  
new_contents: from google.appengine.api import users 
  import webapp2
  
  class MainPage(webapp2.RequestHandler):
   def get(self):
          # [START get_current_user]
          # Checks for active Google account session
          user = users.get_current_user()
          # [END get_current_user]
  
          # [START if_user]
          if user:
              self.response.headers['Content-Type'] = 'text/plain'
              self.response.write('Hello, ' + user.nickname())
          # [END if_user]
          # [START if_not_user]
          else:
              self.redirect(users.create_login_url(self.request.uri))
          # [END if_not_user]
  
  
  application = webapp2.WSGIApplication([
      ('/', MainPage),
  ], debug=True)
  
subject: add user model