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

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.

Author: Sudeep-A-Kulkarni

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='')