Patch Pool.DEFAULT_POOL_NAME in BaseOperator (#8587)

Co-authored-by: Vishesh Jain <visheshj@twitter.com>

Author: vshshjn7

UPDATING.md

@@ -62,6 +62,16 @@ https://developers.google.com/style/inclusive-documentation
 
 -->
 
+### Ability to patch Pool.DEFAULT_POOL_NAME in BaseOperator
+It was not possible to patch pool in BaseOperator as the signature sets the default value of pool
+as Pool.DEFAULT_POOL_NAME.
+While using subdagoperator in unittest(without initializing the sqlite db), it was throwing the
+following error:
+```
+sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such table: slot_pool.
+```
+Fix for this, https://github.com/apache/airflow/pull/8587
+
 ### Change signature of BigQueryGetDatasetTablesOperator
 Was:
 ```python

airflow/models/baseoperator.py

@@ -316,7 +316,7 @@ def __init__(
         priority_weight: int = 1,
         weight_rule: str = WeightRule.DOWNSTREAM,
         queue: str = conf.get('celery', 'default_queue'),
-        pool: str = Pool.DEFAULT_POOL_NAME,
+        pool: Optional[str] = None,
         pool_slots: int = 1,
         sla: Optional[timedelta] = None,
         execution_timeout: Optional[timedelta] = None,
@@ -385,7 +385,7 @@ def __init__(
 
         self.retries = retries
         self.queue = queue
-        self.pool = pool
+        self.pool = Pool.DEFAULT_POOL_NAME if pool is None else pool
         self.pool_slots = pool_slots
         if self.pool_slots < 1:
             raise AirflowException("pool slots for %s in dag %s cannot be less than 1"

tests/serialization/test_dag_serialization.py

@@ -70,6 +70,7 @@
                 "bash_command": "echo {{ task.task_id }}",
                 "_task_type": "BashOperator",
                 "_task_module": "airflow.operators.bash",
+                "pool": "default_pool",
             },
             {
                 "task_id": "custom_task",
@@ -84,6 +85,7 @@
                 "template_fields": ['bash_command'],
                 "_task_type": "CustomOperator",
                 "_task_module": "tests.test_utils.mock_operators",
+                "pool": "default_pool",
             },
         ],
         "timezone": "UTC",