docs: def add_logging_handler(handler, format_='file'):
      """
      mostly for util_logging internals
      """
      global __UTOOL_ROOT_LOGGER__
      if __UTOOL_ROOT_LOGGER__ is None:
          builtins.print('[WARNING] logger not started, cannot add handler')
          return
      # create formatter and add it to the handlers
      #logformat = '%Y-%m-%d %H:%M:%S'
      #logformat = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
      timeformat = '%H:%M:%S'
      if format_ == 'file':
          logformat = '[%(asctime)s]%(message)s'
      elif format_ == 'stdout':
          logformat = '%(message)s'
      else:
          raise AssertionError('unknown logging format_: %r' % format_)
      # Create formatter for handlers
      formatter = logging.Formatter(logformat, timeformat)
      handler.setLevel(logging.DEBUG)
      handler.setFormatter(formatter)
      __UTOOL_ROOT_LOGGER__.addHandler(handler)