slice_arg

def slice_arg(string):
    try:
        if ":" in string:
            parts = string.split(":")
            args = []
            for part in parts:
                if not part.strip():
                    args.append(None)
                else:
                    args.append(int(part))
            return slice(*args)
        else:
            index = int(string)
            return slice(index, index+1)
    except ValueError:
        raise argparse.ArgumentTypeError("{} is not a valid slice".format(string))