gh-89519: Emit a warning when classmethod() wraps a non-callable descriptor

Lib/test/test_decorators.py

@@ -318,12 +318,13 @@ def inner(cls):
                 # This should already work.
                 return 'spam'
 
-            @classmethod
-            @decorator
-            def outer(cls):
-                # Raised TypeError with a message saying that the 'Wrapper'
-                # object is not callable.
-                return 'eggs'
+            with self.assertWarns(FutureWarning):
+                @classmethod
+                @decorator
+                def outer(cls):
+                    # Raised TypeError with a message saying that the 'Wrapper'
+                    # object is not callable.
+                    return 'eggs'
 
         self.assertEqual(Class.inner(), 'spam')
         self.assertEqual(Class.outer(), 'eggs')
@@ -385,10 +386,11 @@ def f2(cls):
                 def f3(cls):
                     return cls
 
-                @classmethod
-                @classmethod
-                def f4(cls):
-                    return cls
+                with self.assertWarns(FutureWarning):
+                    @classmethod
+                    @classmethod
+                    def f4(cls):
+                        return cls
 
                 @myclassmethod
                 @MyClassMethod1

Lib/test/test_doctest.py

@@ -17,6 +17,7 @@
 import shutil
 import types
 import contextlib
+import warnings
 
 
 if not support.has_subprocess_support:
@@ -103,14 +104,16 @@ def a_classmethod(cls, v):
 
     a_class_attribute = 42
 
-    @classmethod
-    @property
-    def a_classmethod_property(cls):
-        """
-        >>> print(SampleClass.a_classmethod_property)
-        42
-        """
-        return cls.a_class_attribute
+    with warnings.catch_warnings():
+        warnings.simplefilter('ignore', FutureWarning)
+        @classmethod
+        @property
+        def a_classmethod_property(cls):
+            """
+            >>> print(SampleClass.a_classmethod_property)
+            42
+            """
+            return cls.a_class_attribute
 
     class NestedClass:
         """
@@ -460,7 +463,7 @@ def basics(): r"""
     >>> tests = finder.find(sample_func)
 
     >>> print(tests)  # doctest: +ELLIPSIS
-    [<DocTest sample_func from test_doctest.py:33 (1 example)>]
+    [<DocTest sample_func from test_doctest.py:34 (1 example)>]
 
 The exact name depends on how test_doctest was invoked, so allow for
 leading path components.

Lib/test/test_property.py

@@ -187,21 +187,23 @@ def test_refleaks_in___init__(self):
                      "Docstrings are omitted with -O2 and above")
     def test_class_property(self):
         class A:
-            @classmethod
-            @property
-            def __doc__(cls):
-                return 'A doc for %r' % cls.__name__
+            with self.assertWarns(FutureWarning):
+                @classmethod
+                @property
+                def __doc__(cls):
+                    return 'A doc for %r' % cls.__name__
         self.assertEqual(A.__doc__, "A doc for 'A'")
 
     @unittest.skipIf(sys.flags.optimize >= 2,
                      "Docstrings are omitted with -O2 and above")
     def test_class_property_override(self):
         class A:
             """First"""
-            @classmethod
-            @property
-            def __doc__(cls):
-                return 'Second'
+            with self.assertWarns(FutureWarning):
+                @classmethod
+                @property
+                def __doc__(cls):
+                    return 'Second'
         self.assertEqual(A.__doc__, 'Second')
 
     def test_property_set_name_incorrect_args(self):

Misc/NEWS.d/next/Core and Builtins/2022-05-07-16-13-15.gh-issue-89519.hRQ7Xx.rst

@@ -0,0 +1,2 @@
+Emit :exc:`FutureWarning` when :func:`classmethod` wraps a non-callable
+descriptor (such as :class:`property`).

Objects/funcobject.c

@@ -896,7 +896,9 @@ cm_descr_get(PyObject *self, PyObject *obj, PyObject *type)
     }
     if (type == NULL)
         type = (PyObject *)(Py_TYPE(obj));
-    if (Py_TYPE(cm->cm_callable)->tp_descr_get != NULL) {
+    if (Py_TYPE(cm->cm_callable)->tp_descr_get != NULL &&
+            !PyCallable_Check(cm->cm_callable))
+    {
         return Py_TYPE(cm->cm_callable)->tp_descr_get(cm->cm_callable, type,
                                                       type);
     }
@@ -913,6 +915,14 @@ cm_init(PyObject *self, PyObject *args, PyObject *kwds)
         return -1;
     if (!PyArg_UnpackTuple(args, "classmethod", 1, 1, &callable))
         return -1;
+    if (Py_TYPE(callable)->tp_descr_get != NULL && !PyCallable_Check(callable)) {
+        if (PyErr_WarnFormat(PyExc_FutureWarning, 1,
+                             "chaining classmethod and %.200s is deprecated",
+                             Py_TYPE(callable)->tp_name))
+        {
+            return -1;
+        }
+    }
     Py_INCREF(callable);
     Py_XSETREF(cm->cm_callable, callable);