def validate_boolean(setting, value, option_parser,
                     config_parser=None, config_section=None):
    """Check/normalize boolean settings:
         True:  '1', 'on', 'yes', 'true'
         False: '0', 'off', 'no','false', ''
    """
    if isinstance(value, bool):
        return value
    try:
        return option_parser.booleans[value.strip().lower()]
    except KeyError:
        raise LookupError('unknown boolean value: "%s"' % value)