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GeezerVison is showing github.com — gh-155245: Fix calendar failing to import when strftime rejects %OB by Sudeep-A-Kulkarni · Pull Request #155317 · python/cpython

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Sudeep-A-Kulkarni commented Aug 7, 2026

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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

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Sudeep-A-Kulkarni added 3 commits

August 7, 2026 11:33


          pythongh-155245: Fix calendar failing to import when strftime rejects…

b8dd4ab

… %OB

The strftime() call for standalone month names ('%OB'/'%Ob') is evaluated lazily inside set(), outside the try/except that guards the initial _localized_month construction. On platforms where '%O' is accepted at construction but rejected when actually formatting (e.g. under Wine), this raised an uncaught ValueError. Wrap the set() comparisons in their own try/except so we fall back to month_name/month_abbr in that case too.


          pythongh-155245: Add regression test for lazy %OB strftime failure

bcf985c

Adds test_standalone_month_name_survives_lazy_OB_failure to OutputTestCase. It simulates a platform (like Wine) where strftime() accepts '%OB' when a _localized_month is constructed but raises ValueError once the lazy strftime call actually happens inside set(), by reloading the calendar module with a patched datetime.date. Without the fix in calendar.py this test fails with an uncaught ValueError.


          pythongh-155245: Add NEWS entry for calendar %OB fix

db89848

Sudeep-A-Kulkarni requested a review from AA-Turner as a code owner

August 7, 2026 06:57

python-cla-bot Bot commented Aug 7, 2026 •

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bedevere-app Bot mentioned this pull request

Aug 7, 2026

calendar module fails to import on Wine due to incomplete %OB error handling #155245

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Aug 7, 2026

Sudeep-A-Kulkarni added 3 commits

August 7, 2026 12:28


          Merge branch 'main' into pythongh-155245-fix-calendar-OB

2cbfa63


          pythongh-155245: Remove trailing whitespace in test_calendar.py

3c15175


          Merge branch 'main' into pythongh-155245-fix-calendar-OB

c1b6fe4

Sudeep-A-Kulkarni commented Aug 8, 2026

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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.

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picnixz reviewed

Aug 8, 2026

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Lib/test/test_calendar.py Resolved

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picnixz requested changes

Aug 8, 2026

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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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bedevere-app Bot added awaiting changes and removed awaiting review labels

Aug 8, 2026

bedevere-app Bot commented Aug 8, 2026

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A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

And if you don't make the requested changes, you will be poked with soft cushions!

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Sudeep-A-Kulkarni commented Aug 10, 2026

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Hi picnixz, sorry for the delay in answering this directly.

I looked into whether a real locale could exercise this instead of mocking, and I don't think one exists for this case. The root cause here isn't locale-specific — it's that Wine's strftime() doesn't support the %O modifier at all, on any locale. glibc, macOS's libc, and Windows's CRT all support %O, so on any environment CI actually runs on, %OB just works and never raises. The only way to get the real ValueError is to run under Wine itself, which isn't something we can do in standard CI.

That's why I went with subclassing datetime.date to make strftime raise on %O, rather than picking a locale — it's simulating Wine's missing feature, not a locale quirk. If there's a way to spin up a Wine-based CI job for this I'm open to it, but I don't think a locale swap alone gets us there.

I'll also fix the indentation simitana flagged.

I have made the requested changes; please review again.

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bedevere-app Bot added awaiting change review and removed awaiting changes labels

Aug 10, 2026

bedevere-app Bot commented Aug 10, 2026

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Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.

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bedevere-app Bot requested a review from picnixz

August 10, 2026 05:56

Sudeep-A-Kulkarni added 2 commits

August 17, 2026 11:49


          Merge branch 'main' into pythongh-155245-fix-calendar-OB

1653f52


          Merge branch 'main' into pythongh-155245-fix-calendar-OB

9660b8a

picnixz commented Aug 19, 2026

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Please don't hit the button "Update branch", it makes useless notifications: https://devguide.python.org/getting-started/pull-request-lifecycle/#update-branch-button. As for the PR: don't use importlib, use mocks and import_fresh_module helpers.

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          pythongh-155245: Use mock.patch and import_fresh_module instead of im…

84be097

…portlib.reload

Address review feedback from picnixz: rather than monkeypatching datetime.date directly and reloading the shared calendar module with importlib.reload() (which mutated the module for the rest of the test process), patch the 'datetime.date' name with mock.patch() and use test.support.import_helper.import_fresh_module() to get an isolated copy of calendar for the assertion.

encukou reviewed

Sep 14, 2026

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Thanks.
This test doesn't look very useful: it relies on the internals, so even with the test, changes to calendar's logic can still break things on Wine.

Per PEP 11 we can accept their patches that make CPython work on Wine, but testing/supporting/backporting is on whomever builds/runs there.

I'd delete the test from here.

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Comment thread

Lib/calendar.py

Comment on lines +160 to +163

		if len(set(standalone_month_name)) != len(set(month_name)):
		standalone_month_name = month_name
		if len(set(standalone_month_abbr)) != len(set(month_abbr)):
		standalone_month_abbr = month_abbr

encukou Sep 14, 2026

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AFAICS, you can put these lines in the try block, instead of adding a new block.

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Sudeep-A-Kulkarni commented Aug 7, 2026

Summary

Fix

Testing

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python-cla-bot Bot commented Aug 7, 2026 •

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