t1 = t.override(task_id="t1")(va)
            t2 = t.override(task_id="t2")(t1)
            with pytest.raises(NotImplementedError) as ctx:
                t.override(task_id="t4").expand(value=va)
            assert str(ctx.value) == "operator expansion in an expanded task group is not yet supported"
            return t2
        
        t2 = tg.expand(va=[["a", "b"], [4], ["z"]])
        
        t.override(task_id="t3")(t2)
    dr: DagRun = dag_maker.create_dagrun()
    results = {}
    decision = dr.task_instance_scheduling_decisions(session=session)
    for ti in decision.schedulable_tis:
        ti.run()
    assert results == {("tg.t1", 0): ["a", "b"], ("tg.t1", 1): [4], ("tg.t1", 2): ["z"]}
    results = {}
    decision = dr.task_instance_scheduling_decisions(session=session)
    for ti in decision.schedulable_tis:
        ti.run()
    assert results == {("tg.t2", 0): ["a", "b"], ("tg.t2", 1): [4], ("tg.t2", 2): ["z"]}
    results = {}
    decision = dr.task_instance_scheduling_decisions(session=session)
    for ti in decision.schedulable_tis:
        ti.run()
    assert len(results) == 1
    assert list(results[("t3", -1)]) == [["a", "b"], [4], ["z"]]
def test_mapping_against_empty_list(dag_maker, session):