Dag tags use set instead of list

[zhongjiajie]

Dag tags store in db without duplicate name in each
dag, but list could contain duplicate elements, so
it's better to change tags attr in Dag from list to
set

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[codecov-io]

Codecov Report

Merging #8233 into master will decrease coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8233      +/-   ##
==========================================
- Coverage   88.35%   88.05%   -0.30%     
==========================================
  Files         936      936              
  Lines       45305    45316      +11     
==========================================
- Hits        40030    39905     -125     
- Misses       5275     5411     +136     

Impacted Files	Coverage Δ
airflow/example_dags/example_bash_operator.py	94.44% <ø> (ø)
airflow/example_dags/example_branch_operator.py	100.00% <ø> (ø)
...ample_dags/example_branch_python_dop_operator_3.py	75.00% <ø> (ø)
airflow/example_dags/example_complex.py	100.00% <ø> (ø)
...w/example_dags/example_external_task_marker_dag.py	100.00% <ø> (ø)
...irflow/example_dags/example_kubernetes_executor.py	85.00% <ø> (ø)
...example_dags/example_kubernetes_executor_config.py	75.00% <ø> (ø)
airflow/example_dags/example_latest_only.py	100.00% <ø> (ø)
...w/example_dags/example_latest_only_with_trigger.py	100.00% <ø> (ø)
...le_dags/example_passing_params_via_test_command.py	100.00% <ø> (ø)
... and 76 more

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[kaxil]

We don't need to do this: Check https://github.com/apache/airflow/pull/6489/files#r347050841

[BasPH]

@kaxil I think it might be worth changing this call to a set: https://github.com/apache/airflow/blob/master/airflow/models/dag.py#L1532. It seems currently the DagTags are made unique when fetching from the DB.

Other than that, we can close this pull request IMO.

[zhongjiajie]

Sorry for reply late due to have cold. @kaxil @BasPH I change dag tags from list to set due to two main reason:

1. In

   airflow/airflow/models/dag.py

   Lines 1654 to 1660 in 6ba672e

   	class DagTag(Base):
   	"""
   	A tag name per dag, to allow quick filtering in the DAG view.
   	"""
   	__tablename__ = "dag_tag"
   	name = Column(String(100), primary_key=True)
   	dag_id = Column(String(ID_LEN), ForeignKey('dag.dag_id'), primary_key=True)

   we have unique key on name and dag_id but in

   airflow/airflow/models/dag.py

   Line 234 in 6ba672e

   tags: Optional[List[str]] = None

   we define tags in list(which may contain duplicate element). Since each dag can not contain duplicate tag, should we define Dag.tags without duplicate element and using set would be a solution
2. Users use tag when they define DAG file with set, them may came to their mind DAG tags is a set and no duplicate element. I think will be more clear. I know we will drop the duplicate element by code or database constraint, but maybe avoid it by user define side would be better.

WDYT

[zhongjiajie]

BTW, I submit this when I review bulk sync to database code in #8231

[kaxil]

Sorry for reply late due to have cold. @kaxil @BasPH I change dag tags from list to set due to two main reason:

1. In

   airflow/airflow/models/dag.py

   Lines 1654 to 1660 in 6ba672e

   	class DagTag(Base):
   	"""
   	A tag name per dag, to allow quick filtering in the DAG view.
   	"""
   	__tablename__ = "dag_tag"
   	name = Column(String(100), primary_key=True)
   	dag_id = Column(String(ID_LEN), ForeignKey('dag.dag_id'), primary_key=True)

   
   we have unique key on name and dag_id but in

   airflow/airflow/models/dag.py

   Line 234 in 6ba672e

   tags: Optional[List[str]] = None

   
   we define tags in list(which may contain duplicate element). Since each dag can not contain duplicate tag, should we define Dag.tags without duplicate element and using set would be a solution
2. Users use tag when they define DAG file with set, them may came to their mind DAG tags is a set and no duplicate element. I think will be more clear. I know we will drop the duplicate element by code or database constraint, but maybe avoid it by user define side would be better.

WDYT

I think we can just change the line @BasPH mentioned (https://github.com/apache/airflow/blob/master/airflow/models/dag.py#L1532. ) to a set. We can allow user to submit any Iterable (List, Set etc)

[zhongjiajie]

@kaxil Ok, if we just need to change one line, I prefer to do it in #8231 and close this PR