[AIRFLOW-6162] Add back serialization as a module

airflow/models/baseoperator.py

@@ -405,6 +405,11 @@ def __init__(
 
         self.dag = dag or DagContext.get_current_dag()
 
+        # subdag parameter is only set for SubDagOperator.
+        # Setting it to None by default as other Operators do not have that field
+        from airflow.models.dag import DAG
+        self.subdag: Optional[DAG] = None
+
         self._log = logging.getLogger("airflow.task.operators")
 
         # Lineage

airflow/models/dag.py

@@ -244,7 +244,7 @@ def __init__(
         self._description = description
         # set file location to caller source path
         self.fileloc = sys._getframe().f_back.f_code.co_filename
-        self.task_dict = dict()  # type: Dict[str, BaseOperator]
+        self.task_dict: Dict[str, BaseOperator] = dict()
 
         # set timezone from start_date
         if start_date and start_date.tzinfo:

airflow/models/dagbag.py

@@ -159,7 +159,7 @@ def get_dag(self, dag_id, from_file_only=False):
         # Needs to load from file for a store_serialized_dags dagbag.
         enforce_from_file = False
         if self.store_serialized_dags and dag is not None:
-            from airflow.serialization.serialized_dag import SerializedDAG
+            from airflow.serialization.serialized_objects import SerializedDAG
             enforce_from_file = isinstance(dag, SerializedDAG)
 
         # If the dag corresponding to root_dag_id is absent or expired

airflow/models/serialized_dag.py

@@ -29,7 +29,7 @@
 
 from airflow import DAG
 from airflow.models.base import ID_LEN, Base
-from airflow.serialization.serialized_dag import SerializedDAG
+from airflow.serialization.serialized_objects import SerializedDAG
 from airflow.settings import json
 from airflow.utils import db, timezone
 from airflow.utils.log.logging_mixin import LoggingMixin

airflow/serialization/__init__.py

@@ -0,0 +1,19 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+"""DAG serialization."""