code: def GetMetadataAttribute(self, attribute_name):
      
      table_name = 'metadata'
  
      has_table = self._database_file.HasTable(table_name)
      if not has_table:
        return None
  
      column_names = ['value']
      condition = 'name == "{0:s}"'.format(attribute_name)
  
      values = list(self._database_file.GetValues(
          [table_name], column_names, condition))
  
      number_of_values = len(values)
      if number_of_values == 0:
        return None
  
      if number_of_values == 1:
        return values[0]['value']
  
      raise RuntimeError('More than one value found in database.')
docs: Retrieves the metadata attribute.
  
      Args:
        attribute_name (str): name of the metadata attribute.
  
      Returns:
        str: the metadata attribute or None.
  
      Raises:
        RuntimeError: if more than one value is found in the database.