My guess (as snack.py is really bad documented):
class Widget:
  def setCallback(self, obj, data = None):
    if data:
      self.w.setCallback(obj, data)
    self.w.setCallback(obj)
should be:
class Widget:
  def setCallback(self, obj, data = None):
    if data:
      self.w.setCallback(obj, data)
    else:
      self.w.setCallback(obj)