code: def category_changed_cb(self, selection, model):
          
          (model, iter) = selection.get_selected()
          id = 0
          if iter is None:
              self.activity_store.clear()
          else:
              self.prev_selected_activity = None
  
              id = model[iter][0]
              self.activity_store.load(model[iter][0])
  
          #start with nothing
          self.get_widget('activity_edit').set_sensitive(False)
          self.get_widget('activity_remove').set_sensitive(False)
  
          return True
docs: enables and disables action buttons depending on selected item