minor fixes on custom serialization, and traitlet hold_traits

IPython/core/application.py

@@ -215,7 +215,7 b' class BaseIPythonApplication(Application):'
             return crashhandler.crash_handler_lite(etype, evalue, tb)
     
     def _ipython_dir_changed(self, name, old, new):
-        if old is not None and old is not Undefined:
+        if old is not Undefined:
             str_old = py3compat.cast_bytes_py2(os.path.abspath(old),
                 sys.getfilesystemencoding()
             )

IPython/html/static/widgets/js/widget.js

@@ -369,7 +369,7 b' define(["widgets/js/manager",'
             // being the value or the promise of the serialized value
             var serializers = this.constructor.serializers;
             if (serializers) {
-                for (k in attrs) {
+                for (var k in attrs) {
                     if (serializers[k] && serializers[k].serialize) {
                         attrs[k] = (serializers[k].serialize)(attrs[k], this);
                     }

traitlets/traitlets.py

@@ -624,7 +624,7 b' class HasTraits(py3compat.with_metaclass(MetaHasTraits, object)):'
                     if cache[name][1] is not Undefined:
                         setattr(self, name, cache[name][1])
                     else:
-                        delattr(self, name)
+                        self._trait_values.pop(name)
                 cache = {}
                 raise e
             finally: