[GitHub] [airflow] ashb commented on pull request #8805: Resolve upstream tasks when template field is XComArg

[GitBox <gi...@apache.org>]

ashb commented on pull request #8805:
URL: https://github.com/apache/airflow/pull/8805#issuecomment-635400916


   > ```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
   > ```
   
   :interrobang:
   
   Fixing that usecase is like 90% of the case I would want to fix with this PR.
   
   Could we use the `__exit__` from the dag to fix this one case (but there are others that wouldn't be caught by that. 
   
   Given how rare metaprogramming is in Python, I think the metaclass approach gives a more complete solution, it catches all possible ways of assigning deps between a DAG.
   
   Yes, this is technically a breaking change, but I think it will affect almost no one. And I think the only "break" would be that using XComArg wouldn't work with a custom operator, unless your metaclass inherits this new metaclass. Is that correct? (i.e. the dag would still run, the user's metaclass would be the one that is used.)
   
   So I'm +1 for metaclass, if I've understood it all correctly.


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