This should be handled the same way as concat: Propagate only if all inputs have the same attrs.

concat is currently a hard-coded special case,

but we may want to delegate the attrs combination back to the operation instead, i.e.

if hasattr(other, "__combined_attrs__"):
      self.attrs = other.__combined_attrs__()

Note that other is the "combination object" for there calls, i.e. _MergeOperation, _Concatenator etc, which will have to grow the logic for combining attrs.

Alternatively, one could leave the combination logic in __finalize__ but provide a uniform interface on all "combination objects" to give their inputs. Currently, thats non-uniform _Concatenator.objs, but _MergeOperation.left/_MergeOperation.right.

Thanks for the report! @timhoffm - can you post a reproducible example.

#60357 should fix this. I've choosen the somewhat smaller refactoring and not pushed the combination logic back into the "combination objects". In fact #59141 removed _Concatenator in favor of simple functions. Therefore, I've now choosen the common API to be "provides the inputs via input_objs parameter".

Note that join() is implemented via concat() or merge() depending on the case. I've only added explicit tests for these fundamental operations, not for join(), but could add that if desired.