gh-148663: Document that IllegalMonthError inherits from both ValueEr…

…ror and IndexError

Doc/library/calendar.rst

@@ -580,8 +580,16 @@ The :mod:`!calendar` module defines the following exceptions:
 
 .. exception:: IllegalMonthError(month)
 
-   A subclass of :exc:`ValueError`,
+   A subclass of both :exc:`ValueError` and :exc:`IndexError`,
    raised when the given month number is outside of the range 1-12 (inclusive).
+   The :exc:`IndexError` base class is preserved for backwards compatibility
+   with code that caught :exc:`IndexError` for bad month numbers prior to
+   Python 3.13.
+
+   .. versionchanged:: 3.12
+      :exc:`IllegalMonthError` is now also a subclass of
+      :exc:`ValueError`. Newer code should avoid catching
+      :exc:`IndexError`.
 
    .. attribute:: month
 

Lib/test/test_calendar.py

@@ -495,12 +495,17 @@ def test_formatmonth(self):
             calendar.TextCalendar().formatmonth(0, 2),
             result_0_02_text
         )
+
     def test_formatmonth_with_invalid_month(self):
         with self.assertRaises(calendar.IllegalMonthError):
             calendar.TextCalendar().formatmonth(2017, 13)
         with self.assertRaises(calendar.IllegalMonthError):
             calendar.TextCalendar().formatmonth(2017, -1)
 
+    def test_illegal_month_error_bases(self):
+        self.assertIsSubclass(calendar.IllegalMonthError, ValueError)
+        self.assertIsSubclass(calendar.IllegalMonthError, IndexError)
+
     def test_formatmonthname_with_year(self):
         self.assertEqual(
             calendar.HTMLCalendar().formatmonthname(2004, 1, withyear=True),

Misc/NEWS.d/next/Documentation/2026-04-17-02-28-55.gh-issue-148663.MHIbRB.rst

@@ -0,0 +1,2 @@
+Document that :class:`calendar.IllegalMonthError` is a subclass of both
+:exc:`ValueError` and :exc:`IndexError` since Python 3.12.