turbaszek commented on pull request #8805:
URL: https://github.com/apache/airflow/pull/8805#issuecomment-635363551
@ashb currently proposed approach resolves the relation when creating a
DagBag:
```python
for task in dag.tasks:
settings.policy(task)
task.set_xcomargs_dependencies() # here
```
The problem is that in such case
```python
with DAG(dag_id='xcomargs_test_3', default_args={"start_date":
datetime.today()}) as dag3:
op1 = DummyOperator(task_id="op1")
op2 = CustomOp(task_id="op2", field=op1.output)
assert op1 in op2.upstream_list. # False
```
op1 is not in upstream of op2 because the relation is not resolved (no
dagbag created).
Same happens with polices but those are not as important as relations.
The metaclass approach would be more generic and would work always. A
possible problem is breaking backward compatibility. Users may use custom meta
in their operators and will have to upgrade.
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