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calendar module fails to import on Wine due to incomplete %OB error handling #155245
Description
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:
- Run Python 3.15 under Wine (any version 9.0-11.14)
- 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
Activity
Sudeep-A-Kulkarni commentedon Aug 6, 2026
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.
Sudeep-A-Kulkarni commentedon Aug 6, 2026
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.
tekinertekin commentedon Aug 6, 2026
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.
picnixz commentedon Aug 6, 2026
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.
Sudeep-A-Kulkarni commentedon Aug 7, 2026
I've reproduced the root cause locally (without needing an actual Wine environment) by subclassing
datetime.datesostrftimeraisesValueErrorwhenever%Oappears in the format string, then modeling the same lazy-evaluation shape used by_localized_month. This confirms the issue: thetry/except ValueErroraround the_localized_month('%OB')construction only guards the constructor call, but the actualstrftimeinvocation happens lazily later, whenset()iterates over the object in theelseblock — which has no exception handling of its own. That's why Wine's lack of%Osupport isn't caught.I also verified a fix locally: wrapping the
set()comparisons inside theelseblock in their own nestedtry/except ValueError(falling back tomonth_name/month_abbron failure) resolves it cleanly. I'll follow up with a PR that applies this fix along with a regression test that mocksstrftimeto simulate the%O-unsupported case, so it doesn't require Wine to run in CI.pythongh-155245: Fix calendar failing to import when strftime rejects…
pythongh-155245: Add regression test for lazy %OB strftime failure
pythongh-155245: Add NEWS entry for calendar %OB fix
Merge branch 'main' into pythongh-155245-fix-calendar-OB
pythongh-155245: Remove trailing whitespace in test_calendar.py
Merge branch 'main' into pythongh-155245-fix-calendar-OB
Merge branch 'main' into pythongh-155245-fix-calendar-OB
Merge branch 'main' into pythongh-155245-fix-calendar-OB
pythongh-155245: Use mock.patch and import_fresh_module instead of im…