[GitHub] [airflow] turbaszek edited a comment on pull request #8805: Resolve upstream tasks when template field is XComArg

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

turbaszek edited a comment on pull request #8805:
URL: https://github.com/apache/airflow/pull/8805#issuecomment-636327497


   @ashb  I forgot what was the problem with metaclass:
   ```python
           with DAG(dag_id='xcomargs_test', default_args={"start_date": datetime.today()}):
               op1 = DummyOperator(task_id="op1")
               op2 = CustomOp(task_id="op2")
               op2.field = op1.output  # value is set after init
   
           assert op1 in op2.upstream_list
   ```
   this needs to be solved (your comment https://github.com/apache/airflow/pull/8805#discussion_r423205936)
   
   But this hints another possible solution without metaclass:
   ```python
   class BaseOperator():
       def __setattr__(self, key, value):
           super().__setattr__(key, value)
           # Resolve upstreams set by assigning an XComArg after initializing
           # an operator, example:
           #   op = BashOperator()
           #   op.bash_command = "sleep 1"
           from airflow.models.xcom_arg import XComArg
           if key in self.template_fields:
               self.set_xcomargs_dependencies()
   ```
   This one is nice and simple, however I am not sure if this will degradate performance in any way .
   
   @evgenyshulman @casassg @ashb @mik-laj thoughts? 


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