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

_localized_month stores the format string and calls strftime lazily from
__getitem__, so constructing _localized_month('%OB') never raises. The
first strftime call happens further down, while comparing the standalone
names against the regular ones to detect systems that keep '%OB' as-is --
and that comparison sits in the else branch, outside the try block meant
to catch the failure. On a platform whose strftime rejects '%OB' the
ValueError therefore escaped and importing calendar failed outright.

Move the comparison into the try block so the intended fallback to
month_name/month_abbr applies.

Author: tekinertekin

Lib/calendar.py

@@ -146,17 +146,20 @@ def __len__(self):
 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:
+    # _localized_month only stores the format; strftime is called lazily when
+    # the names are first read, so systems that reject '%OB' raise ValueError
+    # here rather than above.
+    #
     # Some systems that do not support '%OB' will keep it as-is (i.e.,
     # we get [..., '%OB', '%OB', '%OB']), so for non-distinct names,
     # we fall back to month_name/month_abbr.
     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
 
 
 def isleap(year):

Lib/test/test_calendar.py

@@ -2,6 +2,7 @@
 import unittest
 
 from test import support
+from test.support import import_helper
 from test.support.script_helper import assert_python_ok, assert_python_failure
 import contextlib
 import datetime
@@ -11,6 +12,7 @@
 import platform
 import sys
 import time
+from unittest import mock
 
 # From https://en.wikipedia.org/wiki/Leap_year_starting_on_Saturday
 result_0_02_text = """\
@@ -644,6 +646,26 @@ def test_standalone_month_name_and_abbr_C_locale(self):
             self.assertListEqual(list(calendar.month_abbr),
                                  list(calendar.standalone_month_abbr))
 
+    def test_standalone_month_fallback_when_specifier_rejected(self):
+        # gh-155245: _localized_month stores the format string and only calls
+        # strftime when a name is first read, so a platform that rejects
+        # '%OB' raises ValueError while the fallback below is being computed,
+        # not while the object is being built. Importing calendar has to fall
+        # back to the regular names instead of failing outright.
+        class _RejectsStandalone(datetime.date):
+            def strftime(self, format):
+                if 'O' in format:
+                    raise ValueError(f'Invalid format string: {format}')
+                return super().strftime(format)
+
+        with mock.patch.object(datetime, 'date', _RejectsStandalone):
+            fresh_calendar = import_helper.import_fresh_module('calendar')
+
+        self.assertListEqual(list(fresh_calendar.standalone_month_name),
+                             list(fresh_calendar.month_name))
+        self.assertListEqual(list(fresh_calendar.standalone_month_abbr),
+                             list(fresh_calendar.month_abbr))
+
     def test_locale_text_calendar(self):
         try:
             cal = calendar.LocaleTextCalendar(locale='')

Misc/NEWS.d/next/Library/2026-08-06-13-05-42.gh-issue-155245.Rt4kQm.rst

@@ -0,0 +1,5 @@
+Fix :mod:`calendar` failing to import on platforms whose ``strftime``
+rejects the ``%OB`` and ``%Ob`` format specifiers.  The names are computed
+lazily, so the resulting :exc:`ValueError` was raised outside the ``try``
+block that was meant to catch it, instead of falling back to the regular
+month names.