DEV: Support default_value in `ForwardDeclaredMixin`.

IPython/utils/tests/test_traitlets.py

@@ -1326,11 +1326,11 b' class TestEventful(TestCase):'
 ###
 class ForwardDeclaredInstanceTrait(HasTraits):
 
-    value = ForwardDeclaredInstance(klass='ForwardDeclaredBar')
+    value = ForwardDeclaredInstance('ForwardDeclaredBar')
 
 class ForwardDeclaredTypeTrait(HasTraits):
 
-    value = ForwardDeclaredType(klass='ForwardDeclaredBar')
+    value = ForwardDeclaredType('ForwardDeclaredBar')
 
 class ForwardDeclaredInstanceListTrait(HasTraits):
 

IPython/utils/traitlets.py

@@ -750,6 +750,12 b' class HasTraits(py3compat.with_metaclass(MetaHasTraits, object)):'
 class ClassBasedTraitType(TraitType):
     """A trait with error reporting for Type, Instance and This."""
 
+    def _resolve_string(self, string):
+        """
+        Resolve a string supplied for a type into an actual object.
+        """
+        return import_item(string)
+
     def error(self, obj, value):
         kind = type(value)
         if (not py3compat.PY3) and kind is InstanceType:
@@ -842,9 +848,9 b' class Type(ClassBasedTraitType):'
 
     def _resolve_classes(self):
         if isinstance(self.klass, py3compat.string_types):
-            self.klass = import_item(self.klass)
+            self.klass = self._resolve_string(self.klass)
         if isinstance(self.default_value, py3compat.string_types):
-            self.default_value = import_item(self.default_value)
+            self.default_value = self._resolve_string(self.default_value)
 
     def get_default_value(self):
         return self.default_value
@@ -951,7 +957,7 b' class Instance(ClassBasedTraitType):'
 
     def _resolve_classes(self):
         if isinstance(self.klass, py3compat.string_types):
-            self.klass = import_item(self.klass)
+            self.klass = self._resolve_string(self.klass)
 
     def get_default_value(self):
         """Instantiate a default value instance.
@@ -971,17 +977,17 b' class ForwardDeclaredMixin(object):'
     """
     Mixin for forward-declared versions of Instance and Type.
     """
-    def _resolve_classes(self):
+    def _resolve_string(self, string):
         """
         Find the specified class name by looking for it in the module in which
         our this_class attribute was defined.
         """
         try:
             modname = self.this_class.__module__
-            self.klass = import_item('.'.join([modname, self.klass]))
+            return import_item('.'.join([modname, string]))
         except AttributeError:
             raise ImportError(
-                "Module {} has no attribute {}".format(modname, self.klass)
+                "Module {} has no attribute {}".format(modname, string)
             )
 
 
@@ -1388,8 +1394,8 b' class Container(Instance):'
     def instance_init(self, obj):
         if isinstance(self._trait, TraitType):
             self._trait.this_class = self.this_class
-        if hasattr(self._trait, '_resolve_classes'):
-            self._trait._resolve_classes()
+        if hasattr(self._trait, 'instance_init'):
+            self._trait.instance_init(obj)
         super(Container, self).instance_init(obj)