Merge pull request ipython#2311 from minrk/trait_change

always perform requested trait assignments

Some assignments do not change value, but do change the object,
and these should not be ignored.

For instance, container objects are currently impossible to assign
if they evaluate as equal, even if their types don't match
(e.g. assigning an empty defaultdict to a Dict trait, which prompted this PR).

IPython/utils/tests/test_traitlets.py

@@ -25,10 +25,11 @@
 import sys
 from unittest import TestCase
 
+import nose.tools as nt
 from nose import SkipTest
 
 from IPython.utils.traitlets import (
-    HasTraits, MetaHasTraits, TraitType, Any, CBytes,
+    HasTraits, MetaHasTraits, TraitType, Any, CBytes, Dict,
     Int, Long, Integer, Float, Complex, Bytes, Unicode, TraitError,
     Undefined, Type, This, Instance, TCPAddress, List, Tuple,
     ObjectName, DottedObjectName, CRegExp
@@ -906,3 +907,14 @@ def coerce(self, value):
     _default_value = re.compile(r'')
     _good_values = [r'\d+', re.compile(r'\d+')]
     _bad_values = [r'(', None, ()]
+
+class DictTrait(HasTraits):
+    value = Dict()
+
+def test_dict_assignment():
+    d = dict()
+    c = DictTrait()
+    c.value = d
+    d['a'] = 5
+    nt.assert_equal(d, c.value)
+    nt.assert_true(c.value is d)

IPython/utils/traitlets.py

@@ -309,8 +309,8 @@ def __get__(self, obj, cls=None):
     def __set__(self, obj, value):
         new_value = self._validate(obj, value)
         old_value = self.__get__(obj)
+        obj._trait_values[self.name] = new_value
         if old_value != new_value:
-            obj._trait_values[self.name] = new_value
             obj._notify_trait(self.name, old_value, new_value)
 
     def _validate(self, obj, value):