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

Doc/library/calendar.rst

@@ -493,25 +493,68 @@ The :mod:`calendar` module exports the following data attributes:
 
 .. data:: month_name
 
-   A sequence that represents the months of the year in the current locale.  This
-   follows normal convention of January being month number 1, so it has a length of
-   13 and ``month_name[0]`` is the empty string.
+   A sequence that represents the months of the year in the current locale.
+   This follows normal convention of January being month number 1, so it has
+   a length of 13 and ``month_name[0]`` is the empty string.
 
        >>> import calendar
        >>> list(calendar.month_name)
        ['', 'January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
 
+   .. caution::
+
+      In locales with alternative month names forms, the :data:`!month_name` sequence
+      may not be suitable when a month name stands by itself and not as part of a date.
+      For instance, in Greek and in many Slavic and Baltic languages, :data:`!month_name`
+      will produce the month in genitive case. Use :data:`standalone_month_name` for a form
+      suitable for standalone use.
+
 
 .. data:: month_abbr
 
    A sequence that represents the abbreviated months of the year in the current
-   locale.  This follows normal convention of January being month number 1, so it
-   has a length of 13 and  ``month_abbr[0]`` is the empty string.
+   locale. This follows normal convention of January being month number 1, so
+   it has a length of 13 and  ``month_abbr[0]`` is the empty string.
 
        >>> import calendar
        >>> list(calendar.month_abbr)
        ['', 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
 
+   .. caution::
+
+      In locales with alternative month names forms, the :data:`!month_abbr` sequence
+      may not be suitable when a month name stands by itself and not as part of a date.
+      Use :data:`standalone_month_abbr` for a form suitable for standalone use.
+
+
+.. data:: standalone_month_name
+
+   A sequence that represents the months of the year in the current locale
+   in the grammatical form used when a month name stands by itself if
+   the locale provides one. If the locale does not supply a standalone form,
+   it is equal to :data:`month_name`.
+
+       >>> import calendar
+       >>> list(calendar.standalone_month_name)
+       ['', 'January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
+
+   .. versionadded:: next
+
+
+.. data:: standalone_month_abbr
+
+   A sequence that represents the abbreviated months of the year in the current
+   locale in the grammatical form used when a month name stands by itself if
+   the locale provides one. If the locale does not supply a standalone form
+   it is equal to :data:`month_abbr`.
+
+       >>> import calendar
+       >>> list(calendar.standalone_month_abbr)
+       ['', 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
+
+   .. versionadded:: next
+
+
 .. data:: JANUARY
           FEBRUARY
           MARCH

Lib/calendar.py

@@ -14,8 +14,9 @@
 __all__ = ["IllegalMonthError", "IllegalWeekdayError", "setfirstweekday",
            "firstweekday", "isleap", "leapdays", "weekday", "monthrange",
            "monthcalendar", "prmonth", "month", "prcal", "calendar",
-           "timegm", "month_name", "month_abbr", "day_name", "day_abbr",
-           "Calendar", "TextCalendar", "HTMLCalendar", "LocaleTextCalendar",
+           "timegm", "month_name", "month_abbr", "standalone_month_name",
+           "standalone_month_abbr", "day_name", "day_abbr", "Calendar",
+           "TextCalendar", "HTMLCalendar", "LocaleTextCalendar",
            "LocaleHTMLCalendar", "weekheader",
            "Day", "Month", "JANUARY", "FEBRUARY", "MARCH",
            "APRIL", "MAY", "JUNE", "JULY",
@@ -139,6 +140,17 @@ def __len__(self):
 month_name = _localized_month('%B')
 month_abbr = _localized_month('%b')
 
+# On platforms that support the %OB and %Ob specifiers, they are used
+# to get the standalone form of the month name. This is required for
+# some languages such as Greek, Slavic, and Baltic languages.
+try:
+    standalone_month_name = _localized_month('%OB')
+    standalone_month_abbr = _localized_month('%Ob')
+except ValueError:
+    # The platform does not support the %OB and %Ob specifiers.
+    standalone_month_name = month_name
+    standalone_month_abbr = month_abbr
+
 
 def isleap(year):
     """Return True for leap years, False for non-leap years."""
@@ -377,7 +389,7 @@ def formatmonthname(self, theyear, themonth, width, withyear=True):
         """
         _validate_month(themonth)
 
-        s = month_name[themonth]
+        s = standalone_month_name[themonth]
         if withyear:
             s = "%s %r" % (s, theyear)
         return s.center(width)
@@ -510,9 +522,9 @@ def formatmonthname(self, theyear, themonth, withyear=True):
         """
         _validate_month(themonth)
         if withyear:
-            s = '%s %s' % (month_name[themonth], theyear)
+            s = '%s %s' % (standalone_month_name[themonth], theyear)
         else:
-            s = '%s' % month_name[themonth]
+            s = standalone_month_name[themonth]
         return '<tr><th colspan="7" class="%s">%s</th></tr>' % (
             self.cssclass_month_head, s)
 

Lib/test/test_calendar.py

@@ -546,7 +546,8 @@ def test_days(self):
             self.assertEqual(value[::-1], list(reversed(value)))
 
     def test_months(self):
-        for attr in "month_name", "month_abbr":
+        for attr in ("month_name", "month_abbr", "standalone_month_name",
+                     "standalone_month_abbr"):
             value = getattr(calendar, attr)
             self.assertEqual(len(value), 13)
             self.assertEqual(len(value[:]), 13)
@@ -556,6 +557,16 @@ def test_months(self):
             # verify it "acts like a sequence" in two forms of iteration
             self.assertEqual(value[::-1], list(reversed(value)))
 
+    def test_standalone_month_name_and_abbr(self):
+        # Ensure that the standalone month names and abbreviations are
+        # equal to the regular month names and abbreviations for
+        # the "C" locale.
+        with calendar.different_locale("C"):
+            self.assertListEqual(list(calendar.month_name),
+                                 list(calendar.standalone_month_name))
+            self.assertListEqual(list(calendar.month_abbr),
+                                 list(calendar.standalone_month_abbr))
+
     def test_locale_text_calendar(self):
         try:
             cal = calendar.LocaleTextCalendar(locale='')

Misc/NEWS.d/next/Library/2025-03-17-21-15-04.gh-issue-131146.P9Cdx0.rst

@@ -0,0 +1,3 @@
+Add :data:`calendar.standalone_month_name` and :data:`calendar.standalone_month_abbr` to
+provide month names and abbreviations in a grammatical form used when a month name stands
+by itself if the locale provides one.

Misc/NEWS.d/next/Library/2025-03-17-21-21-06.gh-issue-131146.A5Obgv.rst

@@ -0,0 +1,3 @@
+Fix :class:`calendar.TextCalendar` and :class:`calendar.HTMLCalendar` and
+:mod:`calendar` CLI to display month names in the nominative case if
+provided by the locale.