gh-95388: Deprecate creating immutable types with mutable bases (GH-95533)

Lib/test/test_capi.py

@@ -15,6 +15,7 @@ import textwrap
 import threading
 import time
 import unittest
+import warnings
 import weakref
 from test import support
 from test.support import MISSING_C_DOCSTRINGS
@@ -644,6 +645,34 @@ class CAPITest(unittest.TestCase):
                 with self.assertRaises(SystemError):
                     _testcapi.create_type_from_repeated_slots(variant)
 
+    def test_immutable_type_with_mutable_base(self):
+        # Add deprecation warning here so it's removed in 3.14
+        warnings._deprecated(
+            'creating immutable classes with mutable bases', remove=(3, 14))
+
+        class MutableBase:
+            def meth(self):
+                return 'original'
+
+        with self.assertWarns(DeprecationWarning):
+            ImmutableSubclass = _testcapi.make_immutable_type_with_base(
+                MutableBase)
+        instance = ImmutableSubclass()
+
+        self.assertEqual(instance.meth(), 'original')
+
+        # Cannot override the static type's method
+        with self.assertRaisesRegex(
+                TypeError,
+                "cannot set 'meth' attribute of immutable type"):
+            ImmutableSubclass.meth = lambda self: 'overridden'
+        self.assertEqual(instance.meth(), 'original')
+
+        # Can change the method on the mutable base
+        MutableBase.meth = lambda self: 'changed'
+        self.assertEqual(instance.meth(), 'changed')
+
+
     def test_pynumber_tobase(self):
         from _testcapi import pynumber_tobase
         self.assertEqual(pynumber_tobase(123, 2), '0b1111011')