Adding a test for rollback in case of rejection

IPython/utils/tests/test_traitlets.py

@@ -1333,11 +1333,20 b' def test_pickle_hastraits():'
 
 def test_hold_trait_notifications():
     changes = []
+
     class Test(HasTraits):
         a = Integer(0)
+        b = Integer(0)
+
         def _a_changed(self, name, old, new):
             changes.append((old, new))
-    
+
+        def _b_validate(self, value, trait):
+            if value != 0:
+                raise TraitError('Only 0 is a valid value')
+            return value
+
+    # Test context manager and nesting
     t = Test()
     with t.hold_trait_notifications():
         with t.hold_trait_notifications():
@@ -1356,8 +1365,16 b' def test_hold_trait_notifications():'
         t.a = 4
         nt.assert_equal(t.a, 4)
         nt.assert_equal(changes, [])
-    nt.assert_equal(changes, [(3,4)])
 
+    nt.assert_equal(changes, [(3,4)])
+    # Test roll-back
+    try:
+         with t.hold_trait_notifications():
+             t.b = 1  # raises a Trait error
+    except:
+        pass
+    nt.assert_equal(t.b, 0)
+    
 
 class OrderTraits(HasTraits):
     notified = Dict()

IPython/utils/traitlets.py

@@ -594,6 +594,7 b' class HasTraits(py3compat.with_metaclass(MetaHasTraits, object)):'
         """
         if self._cross_validation_lock is True:
             yield
+            return
         else:
             self._cross_validation_lock = True
             cache = {}
@@ -628,8 +629,8 b' class HasTraits(py3compat.with_metaclass(MetaHasTraits, object)):'
                     notifications = {}
                     raise e
                 finally:
-                    self._cross_validation_lock = False
                     self._notify_trait = _notify_trait
+                    self._cross_validation_lock = False
                     if isinstance(_notify_trait, types.MethodType):
                         # FIXME: remove when support is bumped to 3.4.
                         # when original method is restored,