Once TaskGroups (https://airflow.readthedocs.io/en/latest/concepts.html#taskgroup) that would be released in Airflow 2.0 reach feature parity with SubDags and we have wide adoption and feedback from users about Taskgroups we should deprecate Subdags and remove them eventually in Airflow 3.0.

Discussion Thread: https://lists.apache.org/thread.html/ra52746f9c8274469d343b5f0251199de776e75ab75ded6830886fb6a%40%3Cdev.airflow.apache.org%3E
tests/operators/test_subdag_operator.py
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import unittest
from unittest import mock
from unittest.mock import Mock
import pytest
from parameterized import parameterized
import airflow
from airflow.exceptions import AirflowException
from airflow.models import DAG, DagRun, TaskInstance
from airflow.operators.dummy import DummyOperator
from airflow.operators.subdag import SkippedStatePropagationOptions, SubDagOperator
from airflow.utils.session import create_session
from airflow.utils.state import State
from airflow.utils.timezone import datetime
from airflow.utils.types import DagRunType
from tests.test_utils.db import clear_db_runs
DEFAULT_DATE = datetime(2016, 1, 1)
default_args = dict(
    owner='airflow',
    start_date=DEFAULT_DATE,
)
class TestSubDagOperator(unittest.TestCase):