code: def get_callback_task(self, *args, **kwargs):
      
      if not self.async:
        raise UnexpectedPipelineError(
            'May only call get_callback_task() method for asynchronous pipelines.')
  
      params = kwargs.get('params', {})
      kwargs['params'] = params
      params['pipeline_id'] = self._pipeline_key.name()
      kwargs['url'] = self.base_path + '/callback'
      kwargs['method'] = 'POST'
      return taskqueue.Task(*args, **kwargs)
docs: Returns a task for calling back this Pipeline.
  
      Args:
        params: Keyword argument containing a dictionary of key/value pairs
          that will be passed to the callback when it is executed.
        args, kwargs: Passed to the taskqueue.Task constructor. Use these
          arguments to set the task name (for idempotence), etc.
  
      Returns:
        A taskqueue.Task instance that must be enqueued by the caller.