[BUG] Fix _get_class_flags dropping Mixin flags under multiple inheritance

[vedant21-oss]

Summary

Fixes #539 - _FlagManager._get_class_flags dropped flags from user-defined Mixin classes under multiple inheritance due to a hardcoded [:-2] MRO slice.

Root Cause

The previous code assumed the last two MRO entries are always BaseObject and object:

for parent_class in reversed(inspect.getmro(cls)[:-2]):

Under Python's C3 linearisation, class MyObj(BaseObject, MyMixin) produces:

MyObj -> BaseObject -> _FlagManager -> MyMixin -> object

MyMixin sits at position [-2], so the slice permanently drops it and all its flags - silently, with no error.

Fix

Iterate the full MRO and skip only object via an explicit identity check:

for parent_class in reversed(inspect.getmro(cls)):
    if parent_class is object:
        continue

object is always the last entry and can never carry user-defined flags, making it the only safe sentinel to exclude.

Changes

• skbase/base/_tagmanager.py: Replace [:-2] slice with if parent_class is object: continue. Add detailed comment explaining the root cause so this is never accidentally reverted.
• • skbase/tests/test_meta.py: Add missing _FlagManager import + add test_get_class_flags_preserves_mixin_tags regression test that pins this fix and will fail loudly if the slice returns.

Testing

11 passed in 0.02s

[fkiraly]

if we want to make this general, do we not want to drop it entirely?

Also, please do not add comments that explain the previous situation, they do not make sense to a reader of the code base. AIs often add comments that reference a change.

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.03%. Comparing base (306958d) to head (559cc36).
⚠️ Report is 197 commits behind head on main.

Files with missing lines	Patch %	Lines
skbase/testing/test_all_objects.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #540      +/-   ##
==========================================
- Coverage   85.07%   84.03%   -1.04%     
==========================================
  Files          45       52       +7     
  Lines        3015     3934     +919     
==========================================
+ Hits         2565     3306     +741     
- Misses        450      628     +178     

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[fkiraly]

I fixed it.