code: def start_dev_session(self):
          
          try:
              from .dev import DevServerSession
              session = DevServerSession.initialize(host=self.dev)
              session.start()
  
              #: Save a reference
              self._dev_session = session
          except:
              self.show_error(traceback.format_exc())
docs: Start a client that attempts to connect to the dev server
          running on the host `app.dev`