gh-142781: Fix type confusion in zoneinfo weak cache

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -1577,6 +1577,37 @@ class EvilZoneInfo(self.klass):
 class CZoneInfoCacheTest(ZoneInfoCacheTest):
     module = c_zoneinfo
 
+    def test_inconsistent_weak_cache_get(self):
+        class Cache:
+            def get(self, key, default=None):
+                return 1337
+
+        class ZI(self.klass):
+            pass
+        # Class attribute must be set after class creation
+        # to override zoneinfo.Zoneinfo.__init_subclass__.
+        ZI._weak_cache = Cache()
+
+        with self.assertRaises(TypeError) as te:
+            ZI("America/Los_Angeles")
+        self.assertEqual(str(te.exception), "expected ZI, got int")
+
+    def test_inconsistent_weak_cache_setdefault(self):
+        class Cache:
+            def get(self, key, default=None):
+                return default
+            def setdefault(self, key, value):
+                return 1337
+
+        class ZI(self.klass):
+            pass
+        # Class attribute must be set after class creation
+        # to override zoneinfo.Zoneinfo.__init_subclass__.
+        ZI._weak_cache = Cache()
+
+        with self.assertRaises(TypeError) as te:
+            ZI("America/Los_Angeles")
+        self.assertEqual(str(te.exception), "expected ZI, got int")
 
 class ZoneInfoPickleTest(TzPathUserMixin, ZoneInfoTestBase):
     module = py_zoneinfo

Misc/NEWS.d/next/Library/2025-12-18-00-14-16.gh-issue-142781.gcOeYF.rst

@@ -0,0 +1,2 @@
+:mod:`zoneinfo`: fix a crash when instantiating :class:`~zoneinfo.ZoneInfo`
+objects for which the internal class-level cache is inconsistent.

Modules/_zoneinfo.c

@@ -342,8 +342,24 @@ zoneinfo_ZoneInfo_impl(PyTypeObject *type, PyObject *key)
             Py_DECREF(weak_cache);
             return NULL;
         }
+
+        if (!PyObject_TypeCheck(instance, type)) {
+            PyErr_Format(PyExc_TypeError, "expected %s, got %s",
+                         type->tp_name, Py_TYPE(instance)->tp_name);
+            Py_DECREF(instance);
+            Py_DECREF(weak_cache);
+            return NULL;
+        }
+
         ((PyZoneInfo_ZoneInfo *)instance)->source = SOURCE_CACHE;
     }
+    else if (!PyObject_TypeCheck(instance, type)) {
+        PyErr_Format(PyExc_TypeError, "expected %s, got %s",
+                     type->tp_name, Py_TYPE(instance)->tp_name);
+        Py_DECREF(instance);
+        Py_DECREF(weak_cache);
+        return NULL;
+    }
 
     update_strong_cache(state, type, key, instance);
     Py_DECREF(weak_cache);