ENH: Copy attrs on join (possibly depending on left, right, etc.)

[rommeswi]

Feature Type

• Adding new functionality to pandas

• Changing existing functionality in pandas

• Removing existing functionality in pandas

Problem Description

df.join() does not retain the attrs of the dataset. Given that the attrs manual states that "many operations that create new datasets will retain attrs", this seems like an omission.

Feature Description

Join is different from concat because there is a clear dataframe that the operation is on. Therefore, it would seem natural if df.join() would retain the attrs of the initial dataframe.

Alternative Solutions

It would also be possible to make the attrs dependent on "how" but this would only be natural for "left" and "right".

Additional Context

No response

[timhoffm]

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,

pandas/pandas/core/generic.py

Line 6056 in 7fe270c

if method == "concat":

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.

[rhshadrach]

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

[timhoffm]

#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.