evgenyshulman commented on a change in pull request #8805:
URL: https://github.com/apache/airflow/pull/8805#discussion_r422693309



##########
File path: airflow/models/baseoperator.py
##########
@@ -60,9 +60,25 @@
 ScheduleInterval = Union[str, timedelta, relativedelta]
 
 
+class BaseOperatorMeta(type):
+    """
+    Base metaclass of BaseOperator.
+    """
+    def __call__(cls, *args, **kwargs):
+        """
+        Called when you call BaseOperator(). In this way we are able to 
perform an action
+        after initializing an operator no matter where  the 
``super().__init__`` is called
+        (before or after assign of new attributes in a custom operator).
+        """
+        obj = type.__call__(cls, *args, **kwargs)
+        # Set upstream task defined by XComArgs passed to template fields of 
an operator
+        obj._set_xcomargs_dependencies()  # pylint: disable=protected-access

Review comment:
       for this specific case (fixing apply_defaults probably should be done 
via metaclass) I would vote for dagbag "post" action. people can assign values 
not only after super().__init__ in subclass, but also in the DAG definition 
file as well. 
   something like 
   a = Op()
   a.b = other.c




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