Add a callback on this object to trigger when ``attr`` changes.

        Args:
            attr (str) : an attribute name on this object
            *callbacks (callable) : callback functions to register

        Returns:
            None

        Example:

        .. code-block:: python

            widget.on_change('value', callback1, callback2, ..., callback_n)

def on_change(self, attr, *callbacks):
        ''' Add a callback on this object to trigger when ``attr`` changes.

        Args:
            attr (str) : an attribute name on this object
            *callbacks (callable) : callback functions to register

        Returns:
            None

        Example:

        .. code-block:: python

            widget.on_change('value', callback1, callback2, ..., callback_n)

        '''
        if attr not in self.properties():
            raise ValueError("attempted to add a callback on nonexistent %s.%s property" % (self.__class__.__name__, attr))
        super(Model, self).on_change(attr, *callbacks)