Make TraitListObject notify even if list values stay the same

traits/tests/test_trait_list_object.py

@@ -252,7 +252,7 @@ def test_setitem_converts(self):
             msg="Event contains non-integers for int-only list",
         )
 
-    def test_setitem_nochange(self):
+    def test_setitem_no_structural_change(self):
         tl = TraitList([1, 2, 3],
                        item_validator=int_item_validator,
                        notifiers=[self.notification_handler])
@@ -263,6 +263,39 @@ def test_setitem_nochange(self):
         self.assertIsNone(self.removed)
         self.assertIsNone(self.added)
 
+    def test_setitem_no_item_change(self):
+        tl = TraitList([1, 2, 3],
+                       item_validator=int_item_validator,
+                       notifiers=[self.notification_handler])
+
+        tl[0] = 1
+        self.assertEqual(tl, [1, 2, 3])
+        self.assertEqual(self.index, 0)
+        self.assertEqual(self.removed, [1])
+        self.assertEqual(self.added, [1])
+
+    def test_setitem_no_removed(self):
+        tl = TraitList([1, 2, 3],
+                       item_validator=int_item_validator,
+                       notifiers=[self.notification_handler])
+
+        tl[3:] = [4, 5, 6]
+        self.assertEqual(tl, [1, 2, 3, 4, 5, 6])
+        self.assertEqual(self.index, 3)
+        self.assertEqual(self.removed, [])
+        self.assertEqual(self.added, [4, 5, 6])
+
+    def test_setitem_no_added(self):
+        tl = TraitList([1, 2, 3],
+                       item_validator=int_item_validator,
+                       notifiers=[self.notification_handler])
+
+        tl[1:2] = []
+        self.assertEqual(tl, [1, 3])
+        self.assertEqual(self.index, 1)
+        self.assertEqual(self.removed, [2])
+        self.assertEqual(self.added, [])
+
     def test_setitem_iterable(self):
         tl = TraitList([1, 2, 3],
                        item_validator=int_item_validator,

traits/tests/test_traits.py

@@ -1089,9 +1089,12 @@ def test_trait_list_event(self):
         self.assertLastTraitListEventEqual(0, [6, 7], [4, 5])
         self.obj.alist[0:2:1] = [8, 9]
         self.assertLastTraitListEventEqual(0, [4, 5], [8, 9])
-        old_event = self.last_event
         self.obj.alist[0:2:1] = [8, 9]
-        # If no values changed, no new TraitListEvent will be generated.
+        # If list values stay the same, a new TraitListEvent will be generated.
+        self.assertLastTraitListEventEqual(0, [8, 9], [8, 9])
+        old_event = self.last_event
+        self.obj.alist[4:] = []
+        # If no structural change, NO new TraitListEvent will be generated.
         self.assertIs(self.last_event, old_event)
         self.obj.alist[0:4:2] = [10, 11]
         self.assertLastTraitListEventEqual(

traits/trait_list_object.py

@@ -285,7 +285,7 @@ def __setitem__(self, key, value):
                 pass
         super().__setitem__(key, value)
 
-        if added != removed:
+        if added or removed:
             self.notify(normalized_index, removed, added)
 
     def append(self, object):