child_dag = DAG(
            "parent_dag.child_dag",
            schedule="@daily",
            start_date=DEFAULT_DATE,
        )
        with child_dag:
            EmptyOperator(task_id="a_child")
            EmptyOperator(task_id="b_child")
        parent_dag = DAG(
            "parent_dag",
            schedule="@daily",
            start_date=DEFAULT_DATE,
        )
        
        with parent_dag:
            op1 = EmptyOperator(task_id="a_parent")
            op2 = SubDagOperator(task_id="child_dag", subdag=child_dag)
            op3 = EmptyOperator(task_id="b_parent")
            op1 >> op2 >> op3
        topological_list = parent_dag.topological_sort(include_subdag_tasks=True)
        assert self._occur_before("a_parent", "child_dag", topological_list)
        assert self._occur_before("child_dag", "a_child", topological_list)
        assert self._occur_before("child_dag", "b_child", topological_list)
        assert self._occur_before("a_child", "b_parent", topological_list)
        assert self._occur_before("b_child", "b_parent", topological_list)
    def test_dag_topological_sort_dag_without_tasks(self):
        dag = DAG("dag", start_date=DEFAULT_DATE, default_args={"owner": "owner1"})
        assert () == dag.topological_sort()
    def test_dag_naive_start_date_string(self):