code: def prepare_model(self, sess, allow_initialize=True):
      
      if self._follower:
        self.wait_for_initialization()
      else:
        self._init_model(sess, allow_initialize)
  
      if sess is not self._sess:
        if self.threads:
          raise ValueError('You must call stop_queues() before '
                           'starting a new session with QueueRunners.')
        self._sess = sess
      self._start_threads(sess)
docs: Initialize the model and if necessary launch the queue runners.