gh-96663: Add a better error message for __dict__-less classes setattr (#103232)

1 files changed, 17 insertions, 1 deletions

diff --git a/Lib/test/test_class.py b/Lib/test/test_class.py
index 894e0ca67de..bb35baea508 100644
--- a/Lib/test/test_class.py
+++ b/Lib/test/test_class.py
@@ -641,6 +641,14 @@ class ClassTests(unittest.TestCase):
         class B:
             y = 0
             __slots__ = ('z',)
+        class C:
+            __slots__ = ("y",)
+
+            def __setattr__(self, name, value) -> None:
+                if name == "z":
+                    super().__setattr__("y", 1)
+                else:
+                    super().__setattr__(name, value)
 
         error_msg = "'A' object has no attribute 'x'"
         with self.assertRaisesRegex(AttributeError, error_msg):
@@ -653,8 +661,16 @@ class ClassTests(unittest.TestCase):
             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,
+            "'C' object has no attribute 'x'"
+        ):
+            C().x = 0
 
         error_msg = "'B' object attribute 'y' is read-only"
         with self.assertRaisesRegex(AttributeError, error_msg):