undo failed changes

before raising

otherwise, invalid values were still accepted

IPython/html/widgets/tests/test_interaction.py

@@ -298,7 +298,7 b' def test_default_values():'
     )
 
 def test_default_out_of_bounds():
-    @annotate(f=(0, 10.), h={'a': 1, 'b': 2}, j=['hi', 'there'])
+    @annotate(f=(0, 10.), h={'a': 1}, j=['hi', 'there'])
     def f(f='hi', h=5, j='other'):
         pass
     
@@ -310,13 +310,13 b' def test_default_out_of_bounds():'
         ),
         h=dict(
             cls=widgets.DropdownWidget,
-            values={'a': 1, 'b': 2},
-            value=1
+            values={'a': 1},
+            value=1,
         ),
         j=dict(
             cls=widgets.DropdownWidget,
             values={'hi':'hi', 'there':'there'},
-            value='hi'
+            value='hi',
         ),
     )
 

IPython/html/widgets/widget_selection.py

@@ -90,6 +90,8 b' class _SelectionWidget(DOMWidget):'
                         # set the selected value name
                         self.value_name = k
                         return
+                # undo the change, and raise KeyError
+                self.value = old
                 raise KeyError(new)
             finally:
                 self.value_lock.release()