bpo-36026: make descr error message consistent (GH-11930)

set.add(0) and set.add.__get__(0) now raise TypeError
with same error message.

Lib/test/test_descr.py

@@ -1614,10 +1614,14 @@ order (MRO) for bases """
 
         with self.assertRaises(TypeError) as cm:
             spam_cm(list)
-        self.assertEqual(
-            str(cm.exception),
+        expected_errmsg = (
             "descriptor 'classmeth' requires a subtype of 'xxsubtype.spamlist' "
             "but received 'list'")
+        self.assertEqual(str(cm.exception), expected_errmsg)
+
+        with self.assertRaises(TypeError) as cm:
+            spam_cm.__get__(None, list)
+        self.assertEqual(str(cm.exception), expected_errmsg)
 
     def test_staticmethods(self):
         # Testing static methods...
@@ -1952,6 +1956,29 @@ order (MRO) for bases """
         self.assertEqual(E().foo.__func__, C.foo) # i.e., unbound
         self.assertTrue(repr(C.foo.__get__(C(1))).startswith("<bound method "))
 
+    @support.impl_detail("testing error message from implementation")
+    def test_methods_in_c(self):
+        # This test checks error messages in builtin method descriptor.
+        # It is allowed that other Python implementations use
+        # different error messages.
+        set_add = set.add
+
+        expected_errmsg = "descriptor 'add' of 'set' object needs an argument"
+
+        with self.assertRaises(TypeError) as cm:
+            set_add()
+        self.assertEqual(cm.exception.args[0], expected_errmsg)
+
+        expected_errmsg = "descriptor 'add' for 'set' objects doesn't apply to a 'int' object"
+
+        with self.assertRaises(TypeError) as cm:
+            set_add(0)
+        self.assertEqual(cm.exception.args[0], expected_errmsg)
+
+        with self.assertRaises(TypeError) as cm:
+            set_add.__get__(0)
+        self.assertEqual(cm.exception.args[0], expected_errmsg)
+
     def test_special_method_lookup(self):
         # The lookup of special methods bypasses __getattr__ and
         # __getattribute__, but they still can be descriptors.

Objects/descrobject.c

@@ -78,8 +78,8 @@ descr_check(PyDescrObject *descr, PyObject *obj, PyObject **pres)
     }
     if (!PyObject_TypeCheck(obj, descr->d_type)) {
         PyErr_Format(PyExc_TypeError,
-                     "descriptor '%V' for '%s' objects "
-                     "doesn't apply to '%s' object",
+                     "descriptor '%V' for '%.100s' objects "
+                     "doesn't apply to a '%.100s' object",
                      descr_name((PyDescrObject *)descr), "?",
                      descr->d_type->tp_name,
                      obj->ob_type->tp_name);
@@ -99,7 +99,7 @@ classmethod_get(PyMethodDescrObject *descr, PyObject *obj, PyObject *type)
         else {
             /* Wot - no type?! */
             PyErr_Format(PyExc_TypeError,
-                         "descriptor '%V' for type '%s' "
+                         "descriptor '%V' for type '%.100s' "
                          "needs either an object or a type",
                          descr_name((PyDescrObject *)descr), "?",
                          PyDescr_TYPE(descr)->tp_name);
@@ -108,8 +108,8 @@ classmethod_get(PyMethodDescrObject *descr, PyObject *obj, PyObject *type)
     }
     if (!PyType_Check(type)) {
         PyErr_Format(PyExc_TypeError,
-                     "descriptor '%V' for type '%s' "
-                     "needs a type, not a '%s' as arg 2",
+                     "descriptor '%V' for type '%.100s' "
+                     "needs a type, not a '%.100s' as arg 2",
                      descr_name((PyDescrObject *)descr), "?",
                      PyDescr_TYPE(descr)->tp_name,
                      type->ob_type->tp_name);
@@ -117,8 +117,8 @@ classmethod_get(PyMethodDescrObject *descr, PyObject *obj, PyObject *type)
     }
     if (!PyType_IsSubtype((PyTypeObject *)type, PyDescr_TYPE(descr))) {
         PyErr_Format(PyExc_TypeError,
-                     "descriptor '%V' for type '%s' "
-                     "doesn't apply to type '%s'",
+                     "descriptor '%V' requires a subtype of '%.100s' "
+                     "but received '%.100s'",
                      descr_name((PyDescrObject *)descr), "?",
                      PyDescr_TYPE(descr)->tp_name,
                      ((PyTypeObject *)type)->tp_name);
@@ -181,7 +181,7 @@ descr_setcheck(PyDescrObject *descr, PyObject *obj, PyObject *value,
     if (!PyObject_TypeCheck(obj, descr->d_type)) {
         PyErr_Format(PyExc_TypeError,
                      "descriptor '%V' for '%.100s' objects "
-                     "doesn't apply to '%.100s' object",
+                     "doesn't apply to a '%.100s' object",
                      descr_name(descr), "?",
                      descr->d_type->tp_name,
                      obj->ob_type->tp_name);
@@ -239,9 +239,8 @@ methoddescr_call(PyMethodDescrObject *descr, PyObject *args, PyObject *kwargs)
     if (!_PyObject_RealIsSubclass((PyObject *)Py_TYPE(self),
                                   (PyObject *)PyDescr_TYPE(descr))) {
         PyErr_Format(PyExc_TypeError,
-                     "descriptor '%V' "
-                     "requires a '%.100s' object "
-                     "but received a '%.100s'",
+                     "descriptor '%V' for '%.100s' objects "
+                     "doesn't apply to a '%.100s' object",
                      descr_name((PyDescrObject *)descr), "?",
                      PyDescr_TYPE(descr)->tp_name,
                      self->ob_type->tp_name);
@@ -278,9 +277,8 @@ _PyMethodDescr_FastCallKeywords(PyObject *descrobj,
     if (!_PyObject_RealIsSubclass((PyObject *)Py_TYPE(self),
                                   (PyObject *)PyDescr_TYPE(descr))) {
         PyErr_Format(PyExc_TypeError,
-                     "descriptor '%V' "
-                     "requires a '%.100s' object "
-                     "but received a '%.100s'",
+                     "descriptor '%V' for '%.100s' objects "
+                     "doesn't apply to a '%.100s' object",
                      descr_name((PyDescrObject *)descr), "?",
                      PyDescr_TYPE(descr)->tp_name,
                      self->ob_type->tp_name);