code: def grid_items(self):
          
          if list(self.keys()) == []:  return {}
          cols = self._max_cols
          return {(idx // cols, idx % cols): (key, item)
                  for idx, (key, item) in enumerate(self.data.items())}
docs: Compute a dict of {(row,column): (key, value)} elements from the
          current set of items and specified number of columns.