Description

MaurizioGian

opened on Aug 5, 2026

Issue body actions

Title: calendar module fails to import on Wine due to incomplete %OB error handling

Component: Library (Lib)
Type: bug
Python Version: 3.15

Summary:

The calendar module added in Python 3.15 (gh-131146) fails to import when running
under Wine because the error handling for unsupported %OB/%Ob format specifiers
is incomplete.

The try/except block catches ValueError during _localized_month object construction,
but the actual strftime call happens lazily in getitem when set() iterates
over the sequence in the else block.

Steps to Reproduce:

1. Run Python 3.15 under Wine (any version 9.0-11.14)
2. Execute: import calendar

Expected Result:

calendar module imports successfully, falling back to month_name/month_abbr
when %OB/%Ob are not supported

Actual Result:

Traceback (most recent call last):
File "", line 1, in
File "calendar.py", line 156, in
File "calendar.py", line 110, in getitem
ValueError: Invalid format string

Analysis:

In Lib/calendar.py lines 143-156:

try:
standalone_month_name = _localized_month('%OB')
standalone_month_abbr = _localized_month('%Ob')
except ValueError:
standalone_month_name = month_name
standalone_month_abbr = month_abbr
else:
# This calls getitem which triggers the actual strftime call
if len(set(standalone_month_name)) != len(set(month_name)): # <-- ValueError here
standalone_month_name = month_name

The _localized_month constructor just stores the format string; the actual
datetime.strftime() call happens in getitem when set() iterates. Wine's
strftime doesn't support %O and raises ValueError at this point, which is
not caught.

Suggested Fix:

Wrap the set() comparison in the else block with try/except:

try:
standalone_month_name = _localized_month('%OB')
standalone_month_abbr = _localized_month('%Ob')
except ValueError:
standalone_month_name = month_name
standalone_month_abbr = month_abbr
else:
try:
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
except ValueError:
standalone_month_name = month_name
standalone_month_abbr = month_abbr

Impact:

This breaks Python 3.15 compatibility with Wine, affecting users who build
Windows executables using PyInstaller under Wine (a common CI/CD pattern).

Related:

• Original feature: calendar displays month names in a genitive case for some locales #131146
• Wine bug (to be filed): strftime missing %O modifier support

Linked PRs

• gh-155245: Fix calendar failing to import when strftime rejects %OB #155269
• gh-155245: Fix calendar failing to import when strftime rejects %OB #155317

Activity

1. aisk

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   OS-unsupported

   on Aug 5, 2026
2. github-project-automation

   added this to Unsupported platformson Aug 5, 2026
3. Sudeep-A-Kulkarni commented on Aug 6, 2026

   

   Sudeep-A-Kulkarni

   on Aug 6, 2026

   More actions

   Hi, I'd like to work on this issue. I'll submit a PR with the suggested fix (wrapping the set() comparison in a try/except ValueError) along with a regression test and NEWS entry.
4. Sudeep-A-Kulkarni commented on Aug 6, 2026

   

   Sudeep-A-Kulkarni

   on Aug 6, 2026

   More actions

   Thanks for the quick fix, @tekinertekin! Since a PR is already up and looks solid, I'll hold off on submitting a separate one and look for another issue to work on instead.
5. tekinertekin commented on Aug 6, 2026

   

   tekinertekin

   on Aug 6, 2026

   More actions

   Thanks for the quick fix, @tekinertekin! Since a PR is already up and looks solid, I'll hold off on submitting a separate one and look for another issue to work on instead.

   Thanks for being so gracious about this, @Sudeep-A-Kulkarni and sorry for the collision. You claimed this an hour before I opened the PR and I genuinely missed your comment; I only checked for a linked PR, not for someone having called it. Your description of the fix was the same one I ended up with.

   If you'd rather take this one yourself, say so and I'll close mine — you were here first. Otherwise I'm happy for you to review it.
6. picnixz commented on Aug 6, 2026

   

   picnixz

   on Aug 6, 2026

   Member

   More actions

   I closed @tekinertekin's PR because (1) it's fully generated by an agent and we don't want to encourage such work just to be able to claim a "contributor" badge for CPython as it's not entirely fair to those who made the work by themselves instead of just letting an agent do the job (2) @Sudeep-A-Kulkarni wanted to work on that first.

   @Sudeep-A-Kulkarni Please go ahead, but avoid using an LLM to just write the PR. Try to first find a locale for which we can exercise the issue and otherwise. In addition, make sure that you don't just trust your agent's output. We don't want a fully-LLM generated PR anyway.
7. Sudeep-A-Kulkarni commented on Aug 7, 2026

   

   Sudeep-A-Kulkarni

   on Aug 7, 2026

   More actions

   I've reproduced the root cause locally (without needing an actual Wine environment) by subclassing datetime.date so strftime raises ValueError whenever %O appears in the format string, then modeling the same lazy-evaluation shape used by _localized_month. This confirms the issue: the try/except ValueError around the _localized_month('%OB') construction only guards the constructor call, but the actual strftime invocation happens lazily later, when set() iterates over the object in the else block — which has no exception handling of its own. That's why Wine's lack of %O support isn't caught.

   I also verified a fix locally: wrapping the set() comparisons inside the else block in their own nested try/except ValueError (falling back to month_name/month_abbr on failure) resolves it cleanly. I'll follow up with a PR that applies this fix along with a regression test that mocks strftime to simulate the %O-unsupported case, so it doesn't require Wine to run in CI.
8. Sudeep-A-Kulkarni

   added 6 commits that reference this issue on Aug 7, 2026

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

   b8dd4ab

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

   bcf985c

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

   db89848

   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
9. Sudeep-A-Kulkarni

   added 2 commits that reference this issue on Aug 17, 2026

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

   1653f52

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

   9660b8a
10. Sudeep-A-Kulkarni

    added a commit that references this issue on Sep 3, 2026

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

    84be097

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