gh-96663: Add a better error message for __dict__-less classes setattr

Lib/test/test_class.py

@@ -640,22 +640,33 @@ class A:
             pass
         class B:
             y = 0
-            __slots__ = ('z',)
+            __slots__ = ('z', "foo")
 
         error_msg = "'A' object has no attribute 'x'"
         with self.assertRaisesRegex(AttributeError, error_msg):
             A().x
         with self.assertRaisesRegex(AttributeError, error_msg):
             del A().x
 
-        error_msg = "'B' object has no attribute 'x'"
+        error_msg = ("'B' object has no attribute 'x'")
         with self.assertRaisesRegex(AttributeError, error_msg):
             B().x
         with self.assertRaisesRegex(AttributeError, error_msg):
             del B().x
-        with self.assertRaisesRegex(AttributeError, error_msg):
+        with self.assertRaisesRegex(
+            AttributeError,
+            "'B' object has no attribute 'x' and no __dict__ for setting new attributes"
+        ):
             B().x = 0
 
+        with self.assertRaisesRegex(
+            AttributeError,
+            "'B' object has no attribute 'x' and no "
+            "__dict__ for setting new attributes. Did you mean: 'foo'"
+            ""
+        ):
+            B().fod = 1
+
         error_msg = "'B' object attribute 'y' is read-only"
         with self.assertRaisesRegex(AttributeError, error_msg):
             del B().y

Misc/NEWS.d/next/Core and Builtins/2023-04-04-00-40-04.gh-issue-96663.PdR9hK.rst

@@ -0,0 +1 @@
+Add a better, more introspect-able error message when setting attributes on classes without a ``__dict__`` and no slot member for the attribute.

Objects/object.c

@@ -1546,8 +1546,10 @@ _PyObject_GenericSetAttrWithDict(PyObject *obj, PyObject *name,
         if (dictptr == NULL) {
             if (descr == NULL) {
                 PyErr_Format(PyExc_AttributeError,
-                            "'%.100s' object has no attribute '%U'",
+                            "'%.100s' object has no attribute '%U' and no "
+                            "__dict__ for setting new attributes",
                             tp->tp_name, name);
+                set_attribute_error_context(obj, name);
             }
             else {
                 PyErr_Format(PyExc_AttributeError,
@@ -1577,9 +1579,11 @@ _PyObject_GenericSetAttrWithDict(PyObject *obj, PyObject *name,
         }
         else {
             PyErr_Format(PyExc_AttributeError,
-                         "'%.100s' object has no attribute '%U'",
+                         "'%.100s' object has no attribute '%U' and no "
+                         "__dict__ for setting new attributes",
                          tp->tp_name, name);
         }
+        set_attribute_error_context(obj, name);
     }
   done:
     Py_XDECREF(descr);