def format_label(sl, fmt=None):
    """
    Combine a list of strings to a single str, joined by sep.
    Passes through single strings.
    :param sl:
    :return:
    """
    if isinstance(sl, str):
        # Already is a string.
        return sl

    if fmt:
        return fmt.format(*sl)

    return ' '.join(str(s) for s in sl)
Combine a list of strings to a single str, joined by sep.
    Passes through single strings.
    :param sl:
    :return: