TEST: Focus MultiIndex sort_index coverage

[vyasr]

Description

Replace the broad MultiIndex sort_index Cartesian matrix with focused cases that separate sort semantics from lifecycle behavior.

Coverage accounting

• The selected sort cases retain every previous level spelling: numeric positions, level names, single-item lists, multi-level lists, None, and the invalid level selection.
• They exercise ascending and descending order and both null placements where those choices contribute distinct behavior.
• A dedicated lifecycle matrix retains every inplace and ignore_index combination, and a separate case retains the axis="index" alias.

Checklist

• I am familiar with the CONTRIBUTING.md guidelines.
• New or existing tests cover these test-only changes.
• Documentation updates are not needed.

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[vyasr]

/ok to test

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Reviewing files that changed from the base of the PR and between 4f7f5a9 and e15d8d3.

📒 Files selected for processing (1)

• python/cudf/cudf/tests/dataframe/methods/test_sort_index.py

🚧 Files skipped from review as they are similar to previous changes (1)

• python/cudf/cudf/tests/dataframe/methods/test_sort_index.py

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📝 Summary

Summary by CodeRabbit

• Tests
  • Expanded MultiIndex sorting coverage across standard and in-place operations.
  • Added validation for return values and resulting DataFrame state when sorting in place.
  • Added coverage for the "index" axis alias and related sorting scenarios.
  • Streamlined reusable test setup while preserving existing behavior combinations.
• User Impact
  • No user-facing functionality changes.

Walkthrough

The MultiIndex sort_index tests now use a shared assertion helper. Parameterized cases cover sorting options, lifecycle settings, return values for inplace=True, and the "index" axis alias.

Changes

MultiIndex sort_index test coverage

Layer / File(s)	Summary
Core MultiIndex sorting coverage python/cudf/cudf/tests/dataframe/methods/test_sort_index.py	A reusable helper supports parameterized tests for levels, ordering, null placement, and return-value behavior during row-index sorting.
Lifecycle and axis alias coverage python/cudf/cudf/tests/dataframe/methods/test_sort_index.py	Tests cover inplace, ignore_index, and the "index" axis alias.

Priority: ⬇️ Low — Defer this test-only change because it narrowly expands MultiIndex sort_index coverage without altering public behavior.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to e15d8

This change focuses MultiIndex sort_index test coverage without changing runtime behavior. No current merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the test-only change and the focused MultiIndex sort_index coverage.
Description check	✅ Passed	The description directly explains the changes to MultiIndex sort_index test coverage, including retained sorting, lifecycle, and alias cases.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@python/cudf/cudf/tests/dataframe/methods/test_sort_index.py`:
- Around line 137-139: Update test_dataframe_multiindex_sort_index_lifecycle to
assert that both expected and got are None when inplace is True, then compare
the mutated DataFrames; preserve the existing returned-DataFrame comparison for
inplace=False.

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Reviewing files that changed from the base of the PR and between a8ad204 and 4f7f5a9.

📒 Files selected for processing (1)

• python/cudf/cudf/tests/dataframe/methods/test_sort_index.py

Included review availability: Your plan provides up to 12 included reviews per hour; 9 remain after this review.