def _get_switch_str(opt):
    """
    Output just the '-r, --rev [VAL]' part of the option string.
    """
    if opt[2] is None or opt[2] is True or opt[2] is False:
        default = ''
    else:
        default = '[VAL]'
    if opt[0]:
        return '-%s, --%s %s' % (opt[0], opt[1], default)
    else:
        return '--%s %s' % (opt[1], default)
