TEST: Reuse invariant Python test inputs

[vyasr]

Description

Move immutable, read-only test inputs to module-scoped fixtures so parameterized cases reuse construction work within each pytest worker. Every original dtype, size, slicing, display-option, UDF, indexing, and assertion case remains; the only mutating at/iat tests take a deep copy before assignment.

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

Reviewing files that changed from the base of the PR and between a8ad204 and c97e763.

📒 Files selected for processing (8)

• python/cudf/cudf/tests/dataframe/indexing/test_loc.py
• python/cudf/cudf/tests/dataframe/methods/test_apply.py
• python/cudf/cudf/tests/dataframe/test_at_iat.py
• python/cudf/cudf/tests/dataframe/test_repr.py
• python/cudf/cudf/tests/indexes/multiindex/methods/test_contains.py
• python/cudf/cudf/tests/private_objects/test_column_accessor.py
• python/cudf/cudf/tests/series/test_at_iat.py
• python/cudf/cudf/tests/testing/test_assert_column_equal.py

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📝 Summary

Summary by CodeRabbit

• Tests
  • Improved test efficiency by reusing shared, module-scoped fixtures across dataframe, index, series, column, and representation tests.
  • Added reusable fixtures for full and sliced dataframe representation scenarios.
  • Prevented scalar setter tests from mutating shared test data by copying inputs before modification.
  • Clarified read-only fixture usage and updated test metadata.

Walkthrough

Changes

Test fixture maintenance

Layer / File(s)	Summary
Shared fixture scope updates python/cudf/cudf/tests/dataframe/indexing/test_loc.py, python/cudf/cudf/tests/dataframe/methods/test_apply.py, python/cudf/cudf/tests/indexes/multiindex/methods/test_contains.py, python/cudf/cudf/tests/private_objects/test_column_accessor.py, python/cudf/cudf/tests/testing/test_assert_column_equal.py	Read-only fixtures now use module scope.
Mutation test isolation python/cudf/cudf/tests/dataframe/test_at_iat.py, python/cudf/cudf/tests/series/test_at_iat.py	Scalar setter tests deep-copy shared fixtures before mutation. Fixture comments document the read-only usage.
Representation test data setup python/cudf/cudf/tests/dataframe/test_repr.py	Supported dtypes and generated dataframe pairs are centralized in module-scoped fixtures. Full and sliced representation tests consume those fixtures.

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

Merge Risk: ⚪ Minimal · up to c97e7

This change improves test setup reuse without changing assertions or coverage. Shared inputs remain isolated from scalar mutations, and no current merge-blocking risk is identified.

Suggested reviewers: galipremsagar, mroeschke

🚥 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 20 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly describes the main change: reusing invariant Python test inputs across parameterized tests.
Description check	✅ Passed	The description accurately explains the module-scoped fixtures, reuse of immutable inputs, deep copies for mutating tests, and preserved test coverage.

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[bdice]

/merge