def listify(arg):
    """
    Simple utility method to ensure an argument provided is a list. If the
    provider argument is not an instance of `list`, then we return [arg], else
    arg is returned.

    :type arg: list
    :rtype: list
    """
    if isinstance(arg, (set, tuple)):
        # if it is a set or tuple make it a list
        return list(arg)
    if not isinstance(arg, list):
        return [arg]
    return arg