code: def AddTableColumns(self, table, columns):
      
      table_columns = self._table_columns.setdefault(table, [])
      for attr in columns:
        if attr not in table_columns:
          table_columns.append(attr)
docs: Add columns to table if they are not already there.
  
      Args:
        table: table name as a string
        columns: an iterable of column names