TEST: Target timezone DST edge cases

[vyasr]

Description

Keep ordinary timezone localization exhaustive over every compatible zone. Run ambiguous and nonexistent-time handling only for zones whose offsets differ for the exact timestamps under test; non-transition zones otherwise duplicate the ordinary-localization coverage.

Coverage accounting

• Ordinary localization remains exhaustive over all zones.
• Each DST-edge fixture includes every zone with a transition in its tested timestamp set, plus UTC as a non-transition control.
• America/Metlakatla and America/Grand_Turk remain in their respective fixtures so the existing expected failures stay covered.

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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📥 Commits

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📒 Files selected for processing (2)

• python/cudf/cudf/tests/conftest.py
• python/cudf/cudf/tests/series/accessors/test_dt.py

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📝 Summary

Summary by CodeRabbit

• Tests
  • Expanded timezone localization coverage for ambiguous and nonexistent local times.
  • Added transition-aware timezone scenarios, including consistent coverage for key time zones and UTC.
  • Improved expected-result handling across timezone implementations.

Walkthrough

Changes

Timezone localization coverage

Layer / File(s)	Summary
Transition-aware timezone fixtures python/cudf/cudf/tests/conftest.py	Added detection for timezone offset transitions and fixtures for ambiguous and nonexistent local times. Updated the SPDX year range.
Datetime localization test integration python/cudf/cudf/tests/series/accessors/test_dt.py	Updated localization tests to use dedicated fixtures and construct pandas expectations with zoneinfo.ZoneInfo.

Priority: ⬇️ Low — Defer the timezone test fixture change because it is a narrow update to DST edge-case coverage without elevated product or external urgency.

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

Merge Risk: ⚪ Minimal · up to 27893

Timezone localization tests now focus ambiguous and nonexistent-time cases on relevant transition zones while retaining required fallback and UTC coverage. 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 8 functions across 2 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 main change: testing timezone daylight-saving-time edge cases.
Description check	✅ Passed	The description directly explains the timezone fixture changes, coverage goals, and retained expected failures.
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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[vyasr]

/merge