code: def _parse_block(self):
          
          if self._iter_rows is not None:
              return
  
          rows = _avro_rows(self._block, self._avro_schema)
          self._num_items = self._block.avro_rows.row_count
          self._remaining = self._block.avro_rows.row_count
          self._iter_rows = iter(rows)
docs: Parse metadata and rows from the block only once.