def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def _on_item_widget_size_allocate(self, item_widget, allocation, item):
    if item in self._items_allocations:
      self._update_width(allocation.width - self._items_allocations[item].width)
      self._update_height(allocation.height - self._items_allocations[item].height)
    else:
      self._update_width(allocation.width)
      self._update_height(allocation.height + self._item_spacing)