OperatorExample.py

    runonce = RunOnceBranchOperator(
        dag=dag,
        task_id='runonce_example',
        run_once_task_id='downstream_task_id_to_run_once',
        skip_task_id='dummy_task_id_used_to_skip_other_task',
    )
  
    runonce.set_downstream(downstream_to_run_once)
    runonce.set_downstream(dummy_task_used_to_skip_other_task)
       
    join = DummyOperator(
        task_id='join',
        # important
        trigger_rule='one_success',
        dag=dag
    )    
    join.set_upstream(downstream_to_run_once)
    join.set_upstream(dummy_task_used_to_skip_other_task)