TEST: Reuse rolling GroupBy test inputs

[vyasr]

Description

Reuses module-scoped, read-only pandas/cuDF GroupBy inputs in the rolling GroupBy tests. Every existing reduction, window size, input shape, and pandas comparison remains unchanged; the tests no longer recreate identical frames and grouping metadata for each reduction parameter.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📥 Commits

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• python/cudf/cudf/tests/window/test_rolling.py

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📝 Summary

Summary by CodeRabbit

• Tests
  • Refactored rolling groupby reduction tests to reuse shared pandas and cuDF input fixtures.
  • Expanded simple groupby coverage across multiple dataframe inputs.
  • Added dedicated test inputs for multi-key groupby and offset-window scenarios.
  • Preserved existing reduction and window-size coverage.

Walkthrough

The rolling groupby tests now use reusable module-scoped pandas and cuDF input fixtures. Simple, multi-key, and datetime-indexed groupby cases retain their existing reductions and window-size coverage.

Changes

Rolling groupby test fixtures

Layer / File(s)	Summary
Simple groupby input fixtures python/cudf/cudf/tests/window/test_rolling.py	Adds shared pandas and cuDF inputs for simple rolling groupby tests and updates the test to iterate over them.
Specialized groupby input fixtures python/cudf/cudf/tests/window/test_rolling.py	Adds dedicated fixtures for multi-column and datetime-indexed groupby tests and updates both tests to consume them.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Suggested reviewers: galipremsagar, mroeschke

Merge Risk: ⚪ Minimal · up to 2075b

This test-only refactor reuses read-only pandas/cuDF inputs without changing rolling GroupBy behavior or coverage, so it is ready to merge.

🚥 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 7 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains that rolling GroupBy tests reuse module-scoped pandas/cuDF inputs while preserving existing coverage.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: reusing rolling GroupBy test inputs.
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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🧪 Generate unit tests (beta)

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