def get_callback_task(self, *args, **kwargs):
    """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.
    """
    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)
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.