gh-155245: Fix calendar failing to import when strftime rejects %OB

[Sudeep-A-Kulkarni]

Summary

calendar.standalone_month_name/standalone_month_abbr are built by constructing a _localized_month-like object with the %OB/%Ob format codes. On some platforms (e.g. under Wine), strftime() accepts %OB at construction time but raises ValueError later, when the value is actually formatted. That later formatting only happens lazily, inside the set() calls in the else branch used to decide whether to fall back to month_name/month_abbr. Because that set() logic wasn't wrapped in a try/except, the ValueError propagated uncaught, causing import calendar itself to fail (gh-155245).

Fix

Wrap the set() comparison logic in a nested try/except ValueError, falling back to month_name/month_abbr if formatting fails at that point, same as the existing fallback for construction-time failures.

Testing

Added test_standalone_month_name_survives_lazy_OB_failure in Lib/test/test_calendar.py, which patches datetime.date.strftime (via a subclass, since datetime.date is immutable) to raise ValueError for any %O format code, reloads the calendar module, and asserts that standalone_month_name falls back to month_name instead of raising.

I reproduced the original failure locally first (confirmed the uncaught ValueError before the fix, and confirmed the fix resolves it) before writing this PR, per the guidance in the issue thread.

Fixes gh-155245

[python-cla-bot]

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[Sudeep-A-Kulkarni]

Hello, I wanted to follow up on this pull request, which has been awaiting review for some time. Please let me know if any changes are needed, or if there is additional information I can provide to help move it forward. Thank you for your time and consideration.

[simitana]

Logic looks right, but checked the indentation on the branch: the body of else: is indented 8 spaces instead of 4, inconsistent with the rest of the file and with the except ValueError: block right above it at the same nesting level. Still parses fine since it's just a deeper single block, but it reads like it's nested under something it isn't. Please re-indent to 4 spaces.

[picnixz]

Is there really no way to test this with real locales instead of fake ones like that? it's exactly the same as the PR i closed. As I said, don't let your agents run in automode and I believe this has not been followed. So confirm first whether we can use a true locale for this test.

[picnixz]

• Tell me (you as a human, without any LLM) whether we can have a test that doesn't rely on mocking.
• You don't need to change the comments.

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