This method is used in DagFileProcessor.process_file - it may be easier to migrate all it's internal calls instead of the whole method.
subdag_a = DAG("parent.op_subdag_0.opSubdag_A", default_args=default_args)
                    EmptyOperator(task_id="subdag_a.task", dag=subdag_a)
                    return subdag_a
                def subdag_b():
                    subdag_b = DAG("parent.op_subdag_0.opSubdag_B", default_args=default_args)
                    EmptyOperator(task_id="subdag_b.task", dag=subdag_b)
                    return subdag_b
                def subdag_c():
                    subdag_c = DAG("parent.op_subdag_1.opSubdag_C", default_args=default_args)
                    EmptyOperator(task_id="subdag_c.task", dag=subdag_c)
                    return subdag_c
                def subdag_d():