gh-98963: Restore the ability to have a dict-less property. (#105262)

Ignore doc string assignment failures in `property` as has been the
behavior of all past Python releases.

Author: gpshead

Lib/test/test_property.py

@@ -246,16 +246,67 @@ class PropertySubSlots(property):
 class PropertySubclassTests(unittest.TestCase):
 
     def test_slots_docstring_copy_exception(self):
-        try:
+        # A special case error that we preserve despite the GH-98963 behavior
+        # that would otherwise silently ignore this error.
+        # This came from commit b18500d39d791c879e9904ebac293402b4a7cd34
+        # as part of https://bugs.python.org/issue5890 which allowed docs to
+        # be set via property subclasses in the first place.
+        with self.assertRaises(AttributeError):
             class Foo(object):
                 @PropertySubSlots
                 def spam(self):
                     """Trying to copy this docstring will raise an exception"""
                     return 1
-        except AttributeError:
-            pass
-        else:
-            raise Exception("AttributeError not raised")
+
+    def test_property_with_slots_no_docstring(self):
+        # https://github.com/python/cpython/issues/98963#issuecomment-1574413319
+        class slotted_prop(property):
+            __slots__ = ("foo",)
+
+        p = slotted_prop()  # no AttributeError
+        self.assertIsNone(getattr(p, "__doc__", None))
+
+        def undocumented_getter():
+            return 4
+
+        p = slotted_prop(undocumented_getter)  # New in 3.12: no AttributeError
+        self.assertIsNone(getattr(p, "__doc__", None))
+
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_property_with_slots_docstring_silently_dropped(self):
+        # https://github.com/python/cpython/issues/98963#issuecomment-1574413319
+        class slotted_prop(property):
+            __slots__ = ("foo",)
+
+        p = slotted_prop(doc="what's up")  # no AttributeError
+        self.assertIsNone(p.__doc__)
+
+        def documented_getter():
+            """getter doc."""
+            return 4
+
+        # Historical behavior: A docstring from a getter always raises.
+        # (matches test_slots_docstring_copy_exception above).
+        with self.assertRaises(AttributeError):
+            p = slotted_prop(documented_getter)
+
+    @unittest.skipIf(sys.flags.optimize >= 2,
+                     "Docstrings are omitted with -O2 and above")
+    def test_property_with_slots_and_doc_slot_docstring_present(self):
+        # https://github.com/python/cpython/issues/98963#issuecomment-1574413319
+        class slotted_prop(property):
+            __slots__ = ("foo", "__doc__")
+
+        p = slotted_prop(doc="what's up")
+        self.assertEqual("what's up", p.__doc__)  # new in 3.12: This gets set.
+
+        def documented_getter():
+            """what's up getter doc?"""
+            return 4
+
+        p = slotted_prop(documented_getter)
+        self.assertEqual("what's up getter doc?", p.__doc__)
 
     @unittest.skipIf(sys.flags.optimize >= 2,
                      "Docstrings are omitted with -O2 and above")

Misc/NEWS.d/next/Core and Builtins/2023-06-02-17-39-19.gh-issue-98963.J4wJgk.rst

@@ -0,0 +1,4 @@
+Restore the ability for a subclass of :class:`property` to define ``__slots__``
+or otherwise be dict-less by ignoring failures to set a docstring on such a
+class.  This behavior had regressed in 3.12beta1.  An :exc:`AttributeError`
+where there had not previously been one was disruptive to existing code.

Objects/descrobject.c

@@ -1485,7 +1485,10 @@ class property(object):
         self.__get = fget
         self.__set = fset
         self.__del = fdel
-        self.__doc__ = doc
+        try:
+            self.__doc__ = doc
+        except AttributeError:  # read-only or dict-less class
+            pass
 
     def __get__(self, inst, type=None):
         if inst is None:
@@ -1791,6 +1794,19 @@ property_init_impl(propertyobject *self, PyObject *fget, PyObject *fset,
         if (rc <= 0) {
             return rc;
         }
+        if (!Py_IS_TYPE(self, &PyProperty_Type) &&
+            prop_doc != NULL && prop_doc != Py_None) {
+            // This oddity preserves the long existing behavior of surfacing
+            // an AttributeError when using a dict-less (__slots__) property
+            // subclass as a decorator on a getter method with a docstring.
+            // See PropertySubclassTest.test_slots_docstring_copy_exception.
+            int err = PyObject_SetAttr(
+                        (PyObject *)self, &_Py_ID(__doc__), prop_doc);
+            if (err < 0) {
+                Py_DECREF(prop_doc);  // release our new reference.
+                return -1;
+            }
+        }
         if (prop_doc == Py_None) {
             prop_doc = NULL;
             Py_DECREF(Py_None);
@@ -1806,19 +1822,32 @@ property_init_impl(propertyobject *self, PyObject *fget, PyObject *fset,
     if (Py_IS_TYPE(self, &PyProperty_Type)) {
         Py_XSETREF(self->prop_doc, prop_doc);
     } else {
-        /* If this is a property subclass, put __doc__
-           in dict of the subclass instance instead,
-           otherwise it gets shadowed by __doc__ in the
-           class's dict. */
+        /* If this is a property subclass, put __doc__ in the dict
+           or designated slot of the subclass instance instead, otherwise
+           it gets shadowed by __doc__ in the class's dict. */
 
         if (prop_doc == NULL) {
             prop_doc = Py_NewRef(Py_None);
         }
         int err = PyObject_SetAttr(
                     (PyObject *)self, &_Py_ID(__doc__), prop_doc);
-        Py_XDECREF(prop_doc);
-        if (err < 0)
-            return -1;
+        Py_DECREF(prop_doc);
+        if (err < 0) {
+            assert(PyErr_Occurred());
+            if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
+                PyErr_Clear();
+                // https://github.com/python/cpython/issues/98963#issuecomment-1574413319
+                // Python silently dropped this doc assignment through 3.11.
+                // We preserve that behavior for backwards compatibility.
+                //
+                // If we ever want to deprecate this behavior, only raise a
+                // warning or error when proc_doc is not None so that
+                // property without a specific doc= still works.
+                return 0;
+            } else {
+                return -1;
+            }
+        }
     }
 
     return 0;