Dynamic traits.

IPython/html/widgets/widget.py

@@ -292,6 +292,13 b' class Widget(LoggingConfigurable):'
             True if the callback should be unregistered."""
         self._display_callbacks.register_callback(callback, remove=remove)
 
+    def add_trait(self, traitname, trait):
+        """Dynamically add a trait attribute to the Widget."""
+        super(Widget, self).add_trait(traitname, trait)
+        if trait.get_metadata('sync'):
+             self.keys.append(traitname)
+             self.send_state(traitname)
+
     #-------------------------------------------------------------------------
     # Support methods
     #-------------------------------------------------------------------------

IPython/utils/traitlets.py

@@ -801,6 +801,12 b' class HasTraits(py3compat.with_metaclass(MetaHasTraits, object)):'
         else:
             return trait.get_metadata(key, default)
 
+    def add_trait(self, traitname, trait):
+        """Dynamically add a trait attribute to the HasTraits instance."""
+        self.__class__ = type(self.__class__.__name__, (self.__class__,),
+                              {traitname: trait})
+        trait.set_default_value(self)
+
 #-----------------------------------------------------------------------------
 # Actual TraitTypes implementations/subclasses
 #-----------------------------------------------------------------------------