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

Lib/calendar.py

@@ -150,13 +150,20 @@ def __len__(self):
     standalone_month_name = month_name
     standalone_month_abbr = month_abbr
 else:
-    # 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
+        # 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. The strftime() calls
+        # inside set() are evaluated lazily here, so on platforms that
+        # accept '%OB' at construction time but reject it later (e.g.
+        # under Wine), this can still raise ValueError.
+        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
 
 
 def isleap(year):

Lib/test/test_calendar.py

@@ -644,6 +644,33 @@ 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_name_survives_lazy_OB_failure(self):
+        # gh-155245: some platforms accept '%OB'/'%Ob' when a
+        # _localized_month object is constructed, but the underlying
+        # strftime() call -- which only happens lazily, inside set() --
+        # can still raise ValueError once it actually runs (e.g. under
+        # Wine). Reloading the calendar module used to let that
+        # ValueError propagate instead of falling back to
+        # month_name/month_abbr.
+        import importlib
+
+        real_date = datetime.date
+
+        class FakeDate(real_date):
+            def strftime(self, fmt):
+                if '%O' in fmt:
+                    raise ValueError('Invalid format string')
+                return super().strftime(fmt)
+
+        datetime.date = FakeDate
+        try:
+            importlib.reload(calendar)
+        finally:
+            datetime.date = real_date
+            importlib.reload(calendar)
+
+        self.assertEqual(calendar.standalone_month_name[1], calendar.month_name[1])
+
     def test_locale_text_calendar(self):
         try:
             cal = calendar.LocaleTextCalendar(locale='')

Misc/NEWS.d/next/Library/2026-08-07-12-00-00.gh-issue-155245.aB3xY9.rst

@@ -0,0 +1 @@
+Fixed a bug where :mod:`calendar` could raise an uncaught :exc:`ValueError` on import when falling back from :data:`calendar.standalone_month_name`/:data:`calendar.standalone_month_abbr` to :data:`calendar.month_name`/:data:`calendar.month_abbr`, on platforms where ``strftime()`` accepts ``%OB``/``%Ob`` when a name object is constructed but rejects them once the value is actually formatted (e.g. under Wine).