code: def update_mode(arg_namespace):
      
      try:
          updater.update(custom_sources=arg_namespace.custom)
      except (PermissionError, FileNotFoundError) as exception:
          if isinstance(exception, PermissionError):
              print('No write permission for current working directory.')
          if isinstance(exception, FileNotFoundError):
              print('Necessary resources for updating not found in current '
                    'working directory.')
docs: Check command line arguments and run update function.