code: def process_callback(self, block=True):
          
          try:
              (callback, args) = self._queue.get(block=block)
              try:
                  callback(*args)
              finally:
                  self._queue.task_done()
          except queue.Empty:
              return False
          return True
docs: Dispatch a single callback in the current thread.
  
          :param boolean block: If True, blocks waiting for a callback to come.
          :return: True if a callback was processed; otherwise False.