Use closure to prevent traitlet callbacks from firing twice.

IPython/utils/tests/test_traitlets.py

@@ -993,6 +993,8 b' class TestBind(TestCase):'
         # Change one of the values to make sure they stay in sync.
         a.value = 5
         self.assertEqual(a.value, b.value)
+        b.value = 6
+        self.assertEqual(a.value, b.value)
 
     def test_bind_different(self):
         """Verify two traitlets of different types can be bound together using bind."""
@@ -1014,6 +1016,8 b' class TestBind(TestCase):'
         # Change one of the values to make sure they stay in sync.
         a.value = 5
         self.assertEqual(a.value, b.count)
+        b.count = 4
+        self.assertEqual(a.value, b.count)
 
     def test_unbind(self):
         """Verify two binded traitlets can be unbinded."""
@@ -1032,3 +1036,40 b' class TestBind(TestCase):'
         # Change one of the values to make sure they stay in sync.
         a.value = 5
         self.assertNotEqual(a.value, b.value)
+
+    def test_callbacks(self):
+        """Verify two binded traitlets have their callbacks called once."""
+
+        # Create two simple classes with Int traitlets.
+        class A(HasTraits):
+            value = Int()
+        class B(HasTraits):
+            count = Int()
+        a = A(value=9)
+        b = B(count=8)
+        
+        # Register callbacks that count.
+        callback_count = []
+        def a_callback(name, old, new):
+            callback_count.append('a')
+        a.on_trait_change(a_callback, 'value')
+        def b_callback(name, old, new):
+            callback_count.append('b')
+        b.on_trait_change(b_callback, 'count')
+
+        # Conenct the two classes.
+        c = bind((a, 'value'), (b, 'count'))
+
+        # Make sure b's count was set to a's value once.
+        self.assertEqual(''.join(callback_count), 'b')
+        del callback_count[:]
+
+        # Make sure a's value was set to b's count once.
+        b.count = 5
+        self.assertEqual(''.join(callback_count), 'ba')
+        del callback_count[:]
+
+        # Make sure b's count was set to a's value once.
+        a.value = 4
+        self.assertEqual(''.join(callback_count), 'ab')
+        del callback_count[:]

IPython/utils/traitlets.py

@@ -202,13 +202,15 b' class bind(object):'
         if len(args) < 2:
             raise TypeError('At least two traitlets must be provided.')
 
-        self.objects = args
+        self.objects = {}
+        initial = getattr(args[0][0], args[0][1])
         for obj,attr in args:
-            obj.on_trait_change(self._update, attr)
+            if getattr(obj, attr) != initial:
+                setattr(obj, attr, initial)
 
-        # Syncronize the traitlets initially.
-        initial = getattr(args[0][0], args[0][1])
-        self._update(args[0][1], initial, initial)
+            callback = self._make_closure(obj,attr)
+            obj.on_trait_change(callback, attr)
+            self.objects[(obj,attr)] = callback
 
     @contextlib.contextmanager
     def _busy_updating(self):
@@ -218,16 +220,23 b' class bind(object):'
         finally:
             self.updating = False
 
-    def _update(self, name, old, new):
+    def _make_closure(self, sending_obj, sending_attr):
+        def update(name, old, new):
+            self._update(sending_obj, sending_attr, new)
+        return update
+
+    def _update(self, sending_obj, sending_attr, new):
         if self.updating:
             return
         with self._busy_updating():
-            for obj,attr in self.objects:
-                setattr(obj, attr, new)
+            for obj,attr in self.objects.keys():
+                if obj is not sending_obj or attr != sending_attr:
+                    setattr(obj, attr, new)
     
     def unbind(self):
-        for obj,attr in self.objects:
-            obj.on_trait_change(self._update, attr, remove=True)
+        for key, callback in self.objects.items():
+            (obj,attr) = key
+            obj.on_trait_change(callback, attr, remove=True)
 
 #-----------------------------------------------------------------------------
 # Base TraitType for all traits