code: def update(self, observable, handlers):
          
          addedreaders, removedreaders = handlers
          for reader in addedreaders:
              item = self.Append(str(reader))
              self.SetClientData(item, reader)
          for reader in removedreaders:
              item = self.FindString(str(reader))
              if wx.NOT_FOUND != item:
                  self.Delete(item)
          selection = self.GetSelection()
docs: Toolbar ReaderObserver callback that is notified when
          readers are added or removed.