Set trait values from static defaults at instance_init

Should fix the problem introduced in ipythongh-31 for the widgets.

One small behavioural change: traitlets with no default value will raise
TraitError when accessed instead of returning Undefined.

Author: takluyver

traitlets/tests/test_traitlets.py

@@ -49,7 +49,8 @@ def test_get_undefined(self):
         class A(HasTraits):
             a = TraitType
         a = A()
-        self.assertEqual(a.a, Undefined)
+        with self.assertRaises(TraitError):
+            a.a
 
     def test_set(self):
         class A(HasTraitsStub):

traitlets/traitlets.py

@@ -414,7 +414,9 @@ def instance_init(self, obj):
         # use provides a static default, transfer that to obj._trait_values.
         if (self._dynamic_default_callable(obj) is None) \
                 and (self.default_value is not Undefined):
-            self.validate_default_value(obj)
+            v = self.validate_default_value(obj)
+            if self.name is not None:
+                obj._trait_values[self.name] = v
 
     def __get__(self, obj, cls=None):
         """Get the value of the trait by self.name for the instance.
@@ -432,10 +434,10 @@ def __get__(self, obj, cls=None):
             except KeyError:
                 # Check for a dynamic initializer.
                 dynamic_default = self._dynamic_default_callable(obj)
-                if dynamic_default is not None:
-                    value = self._validate(obj, dynamic_default())
-                else:
-                    value = self.validate_default_value(obj)
+                if dynamic_default is None:
+                    raise TraitError("No default value found for %s trait of %r"
+                                     % (self.name, obj))
+                value = self._validate(obj, dynamic_default())
                 obj._trait_values[self.name] = value
                 return value
             except Exception: