turbaszek commented on a change in pull request #8805:
URL: https://github.com/apache/airflow/pull/8805#discussion_r422630352
##########
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
+ return obj
+
+
# pylint: disable=too-many-instance-attributes,too-many-public-methods
@functools.total_ordering
-class BaseOperator(Operator, LoggingMixin):
+class BaseOperator(Operator, LoggingMixin, metaclass=BaseOperatorMeta):
Review comment:
> Why metaclass instead of __init__?
The `_set_xcomargs_dependencies` method has to work for any operator, no
matter where someone calls `super().__init__(*args, **kwargs)`. In other words,
`_set_xcomargs_dependencies ` has to be called after initialization of an
operator.
Calling resolve in `__init__` works only when custom operator is doing:
```python
class CustomOp(BaseOperator):
template_fields = ("custom_field",)
def __init__(self, custom_field, *args, **kwargs):
self.custom_field = custom_field
super().__init__(*args, **kwargs) # resvole happens here
```
but not in case of (this is much more popular)
```python
class CustomOp(BaseOperator):
template_fields = ("custom_field",)
def __init__(self, custom_field, *args, **kwargs):
super().__init__(*args, **kwargs) # resvole happens here
self.custom_field = custom_field # this is not resolved
```
This is also checked in tests.
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