gh-77757: replace exception wrapping by PEP-678 notes in typeobject's __set_name__

Doc/whatsnew/3.12.rst

@@ -192,6 +192,10 @@ Other Language Changes
 * :class:`slice` objects are now hashable, allowing them to be used as dict keys and
   set items. (Contributed by Will Bradshaw and Furkan Onder in :gh:`101264`.)
 
+* Exceptions raised in a typeobject's ``__set_name__`` method are no longer
+  wrapped by a :exc:`RuntimeError`. Context information is added to the
+  exception as a :pep:`678` note. (Contributed by Irit Katriel in :gh:`77757`.)
+
 New Modules
 ===========
 

Lib/test/test_functools.py

@@ -2980,7 +2980,7 @@ class MyClass(metaclass=MyMeta):
 
     def test_reuse_different_names(self):
         """Disallow this case because decorated function a would not be cached."""
-        with self.assertRaises(RuntimeError) as ctx:
+        with self.assertRaises(TypeError) as ctx:
             class ReusedCachedProperty:
                 @py_functools.cached_property
                 def a(self):
@@ -2989,7 +2989,7 @@ def a(self):
                 b = a
 
         self.assertEqual(
-            str(ctx.exception.__context__),
+            str(ctx.exception),
             str(TypeError("Cannot assign the same cached_property to two different names ('a' and 'b')."))
         )
 

Lib/test/test_subclassinit.py

@@ -134,30 +134,28 @@ class Descriptor:
             def __set_name__(self, owner, name):
                 1/0
 
-        with self.assertRaises(RuntimeError) as cm:
+        with self.assertRaises(ZeroDivisionError) as cm:
             class NotGoingToWork:
                 attr = Descriptor()
 
-        exc = cm.exception
-        self.assertRegex(str(exc), r'\bNotGoingToWork\b')
-        self.assertRegex(str(exc), r'\battr\b')
-        self.assertRegex(str(exc), r'\bDescriptor\b')
-        self.assertIsInstance(exc.__cause__, ZeroDivisionError)
+        notes = cm.exception.__notes__
+        self.assertRegex(str(notes), r'\bNotGoingToWork\b')
+        self.assertRegex(str(notes), r'\battr\b')
+        self.assertRegex(str(notes), r'\bDescriptor\b')
 
     def test_set_name_wrong(self):
         class Descriptor:
             def __set_name__(self):
                 pass
 
-        with self.assertRaises(RuntimeError) as cm:
+        with self.assertRaises(TypeError) as cm:
             class NotGoingToWork:
                 attr = Descriptor()
 
-        exc = cm.exception
-        self.assertRegex(str(exc), r'\bNotGoingToWork\b')
-        self.assertRegex(str(exc), r'\battr\b')
-        self.assertRegex(str(exc), r'\bDescriptor\b')
-        self.assertIsInstance(exc.__cause__, TypeError)
+        notes = cm.exception.__notes__
+        self.assertRegex(str(notes), r'\bNotGoingToWork\b')
+        self.assertRegex(str(notes), r'\battr\b')
+        self.assertRegex(str(notes), r'\bDescriptor\b')
 
     def test_set_name_lookup(self):
         resolved = []

Misc/NEWS.d/next/Core and Builtins/2023-04-09-22-21-57.gh-issue-77757._Ow-u2.rst

@@ -0,0 +1,3 @@
+Exceptions raised in a typeobject's ``__set_name__`` method are no longer
+wrapped by a :exc:`RuntimeError`. Context information is added to the
+exception as a :pep:`678` note.

Objects/typeobject.c

@@ -9137,13 +9137,28 @@ type_new_set_names(PyTypeObject *type)
         Py_DECREF(set_name);
 
         if (res == NULL) {
-            _PyErr_FormatFromCause(PyExc_RuntimeError,
+            /* Add note to the exception */
+            PyObject *exc = PyErr_GetRaisedException();
+            assert(exc != NULL);
+            PyObject *note = PyUnicode_FromFormat(
                 "Error calling __set_name__ on '%.100s' instance %R "
                 "in '%.100s'",
                 Py_TYPE(value)->tp_name, key, type->tp_name);
+
+            if (note) {
+                int res = _PyException_AddNote(exc, note);
+                Py_DECREF(note);
+                if (res == 0) {
+                    PyErr_SetRaisedException(exc);
+                    goto error;
+                }
+            }
+            _PyErr_ChainExceptions1(exc);
             goto error;
         }
-        Py_DECREF(res);
+        else {
+            Py_DECREF(res);
+        }
     }
 
     Py_DECREF(names_to_set);