From 54beabc89abb49620527617c5e1a0d53811244b6 2015-03-22 23:47:14
From: Min RK <benjaminrk@gmail.com>
Date: 2015-03-22 23:47:14
Subject: [PATCH] add HasTraits.delay_trait_notifications

context manager for delaying trait notifications,
used to avoid race conditions in dict ordering
when loading from kwargs.
---

diff --git a/IPython/utils/tests/test_traitlets.py b/IPython/utils/tests/test_traitlets.py
index 30d82a9..83b70cd 100644
--- a/IPython/utils/tests/test_traitlets.py
+++ b/IPython/utils/tests/test_traitlets.py
@@ -1330,6 +1330,50 @@ def test_pickle_hastraits():
         nt.assert_equal(c2.i, c.i)
         nt.assert_equal(c2.j, c.j)
 
+
+class OrderTraits(HasTraits):
+    notified = Dict()
+    
+    a = Unicode()
+    b = Unicode()
+    c = Unicode()
+    d = Unicode()
+    e = Unicode()
+    f = Unicode()
+    g = Unicode()
+    h = Unicode()
+    i = Unicode()
+    j = Unicode()
+    k = Unicode()
+    l = Unicode()
+    
+    def _notify(self, name, old, new):
+        """check the value of all traits when each trait change is triggered
+        
+        This verifies that the values are not sensitive
+        to dict ordering when loaded from kwargs
+        """
+        # check the value of the other traits
+        # when a given trait change notification fires
+        self.notified[name] = {
+            c: getattr(self, c) for c in 'abcdefghijkl'
+        }
+    
+    def __init__(self, **kwargs):
+        self.on_trait_change(self._notify)
+        super(OrderTraits, self).__init__(**kwargs)
+
+def test_notification_order():
+    d = {c:c for c in 'abcdefghijkl'}
+    obj = OrderTraits()
+    nt.assert_equal(obj.notified, {})
+    obj = OrderTraits(**d)
+    notifications = {
+        c: d for c in 'abcdefghijkl'
+    }
+    nt.assert_equal(obj.notified, notifications)
+
+
 class TestEventful(TestCase):
 
     def test_list(self):
diff --git a/IPython/utils/traitlets.py b/IPython/utils/traitlets.py
index 7d3c5f1..d35e289 100644
--- a/IPython/utils/traitlets.py
+++ b/IPython/utils/traitlets.py
@@ -575,8 +575,31 @@ class HasTraits(py3compat.with_metaclass(MetaHasTraits, object)):
         # Allow trait values to be set using keyword arguments.
         # We need to use setattr for this to trigger validation and
         # notifications.
-        for key, value in iteritems(kw):
-            setattr(self, key, value)
+        
+        with self.delay_trait_notifications():
+            for key, value in iteritems(kw):
+                setattr(self, key, value)
+    
+    @contextlib.contextmanager
+    def delay_trait_notifications(self):
+        """Context manager for bundling trait change notifications
+        
+        Use this when doing multiple trait assignments (init, config),
+        to avoid race conditions in trait notifiers requesting other trait values.
+        All trait notifications will fire after all values have been assigned.
+        """
+        _notify_trait = self._notify_trait
+        notifications = []
+        self._notify_trait = lambda *a: notifications.append(a)
+        
+        try:
+            yield
+        finally:
+            self._notify_trait = _notify_trait
+        
+        # trigger delayed notifications
+        for args in notifications:
+            self._notify_trait(*args)
 
     def _notify_trait(self, name, old_value, new_value):