docs: def _check_directory(directory):
      """Raise exception if directory does not exist."""
      if directory is not None:
          if not exists(directory):
              raise CommandError(
                  "Cannot run command - directory {0} does not exist".format(directory)
              )
  
          if not isdir(directory):
              raise CommandError(
                  "Cannot run command - specified directory {0} is not a directory.".format(
                      directory
                  )
              )