def _sdefault_(sec, key):
    """Get default string or None"""
    default = sec.get(key, None)
    if isinstance(default, (list, tuple)):
        default = ",".join(map(str, default))
    else:
        default = str(default).strip("()")
    default = default.replace("%", "%%")
    if default == "None":
        default = None
    return default