bpo-36433: fix confusing error messages in classmethoddescr_call (GH-12556)

https://bugs.python.org/issue36433

Lib/test/test_descr.py

@@ -1597,12 +1597,27 @@ order (MRO) for bases """
         self.assertEqual(x2, SubSpam)
         self.assertEqual(a2, a1)
         self.assertEqual(d2, d1)
-        with self.assertRaises(TypeError):
+
+        with self.assertRaises(TypeError) as cm:
             spam_cm()
-        with self.assertRaises(TypeError):
+        self.assertEqual(
+            str(cm.exception),
+            "descriptor 'classmeth' of 'xxsubtype.spamlist' "
+            "object needs an argument")
+
+        with self.assertRaises(TypeError) as cm:
             spam_cm(spam.spamlist())
-        with self.assertRaises(TypeError):
+        self.assertEqual(
+            str(cm.exception),
+            "descriptor 'classmeth' requires a type "
+            "but received a 'xxsubtype.spamlist' instance")
+
+        with self.assertRaises(TypeError) as cm:
             spam_cm(list)
+        self.assertEqual(
+            str(cm.exception),
+            "descriptor 'classmeth' requires a subtype of 'xxsubtype.spamlist' "
+            "but received 'list'")
 
     def test_staticmethods(self):
         # Testing static methods...

Misc/NEWS.d/next/Core and Builtins/2019-03-26-17-23-02.bpo-36433.-8XzZf.rst

+Fixed TypeError message in classmethoddescr_call.

Objects/descrobject.c

@@ -315,20 +315,18 @@ classmethoddescr_call(PyMethodDescrObject *descr, PyObject *args,
     if (!PyType_Check(self)) {
         PyErr_Format(PyExc_TypeError,
                      "descriptor '%V' requires a type "
-                     "but received a '%.100s'",
+                     "but received a '%.100s' instance",
                      descr_name((PyDescrObject *)descr), "?",
-                     PyDescr_TYPE(descr)->tp_name,
                      self->ob_type->tp_name);
         return NULL;
     }
     if (!PyType_IsSubtype((PyTypeObject *)self, PyDescr_TYPE(descr))) {
         PyErr_Format(PyExc_TypeError,
-                     "descriptor '%V' "
-                     "requires a subtype of '%.100s' "
-                     "but received '%.100s",
+                     "descriptor '%V' requires a subtype of '%.100s' "
+                     "but received '%.100s'",
                      descr_name((PyDescrObject *)descr), "?",
                      PyDescr_TYPE(descr)->tp_name,
-                     self->ob_type->tp_name);
+                     ((PyTypeObject*)self)->tp_name);
         return NULL;
     }