gh-131146: Fix month names in a genitive case in calendar module.

The calendar module displays month names in some locales using the genitive case.
This is grammatically incorrect, as the nominative case should be used when the month
is named by itself. To address this issue, this change introduces new lists
`alt_month_name` and `alt_month_abbr` that contain month names in the nominative case.
The module now uses `%OB` format specifier to get month names in the nominative case
where available.

Author: plashchynski

Lib/calendar.py

@@ -139,6 +139,24 @@ def __len__(self):
 month_name = _localized_month('%B')
 month_abbr = _localized_month('%b')
 
+# Check if the platform supports the %OB format code
+def _is_alt_mon_available():
+    try:
+        datetime.date(2001, 1, 1).strftime('%OB')
+        datetime.date(2001, 1, 1).strftime('%Ob')
+    except ValueError:
+        return False
+    return True
+
+# In Greek and in many Slavic and Baltic languages, "%OB" will produce
+# the month in nominative case.
+if _is_alt_mon_available():
+    alt_month_name = _localized_month('%OB')
+    alt_month_abbr = _localized_month('%Ob')
+else:
+    alt_month_name = month_name
+    alt_month_abbr = month_abbr
+
 
 def isleap(year):
     """Return True for leap years, False for non-leap years."""
@@ -377,7 +395,7 @@ def formatmonthname(self, theyear, themonth, width, withyear=True):
         """
         _validate_month(themonth)
 
-        s = month_name[themonth]
+        s = alt_month_name[themonth]
         if withyear:
             s = "%s %r" % (s, theyear)
         return s.center(width)
@@ -510,9 +528,9 @@ def formatmonthname(self, theyear, themonth, withyear=True):
         """
         _validate_month(themonth)
         if withyear:
-            s = '%s %s' % (month_name[themonth], theyear)
+            s = '%s %s' % (alt_month_name[themonth], theyear)
         else:
-            s = '%s' % month_name[themonth]
+            s = '%s' % alt_month_name[themonth]
         return '<tr><th colspan="7" class="%s">%s</th></tr>' % (
             self.cssclass_month_head, s)
 

Lib/test/test_calendar.py

@@ -1126,7 +1126,8 @@ def test__all__(self):
         not_exported = {
             'mdays', 'January', 'February', 'EPOCH',
             'different_locale', 'c', 'prweek', 'week', 'format',
-            'formatstring', 'main', 'monthlen', 'prevmonth', 'nextmonth', ""}
+            'formatstring', 'main', 'monthlen', 'prevmonth', 'nextmonth',
+            'alt_month_name', 'alt_month_abbr', ""}
         support.check__all__(self, calendar, not_exported=not_exported)