def _keys(true, false, default):
    """
    Format keys used for yes/no with the default in upper case

    eg with default=False and y/n for the keys, this should return '[y/N]'
    """
    true = default and true.upper() or true.lower()
    false = not default and false.upper() or false.lower()
    return ' [{}/{}]'.format(true, false)
