def _normalize_list_argument(arg, name):
    """returns a list of strings from an argument that can be either
    list of strings, None (returns []), or a single string returns
    ([arg])"""
    if arg is None:
        return []
    elif isinstance(arg, str):
        return [arg]
    elif isinstance(arg, (list, tuple)):
        return arg
    raise TypeError(
        "Argument '%s' must be None, a string, or a sequence of strings, not %r"
         % (name, type(arg)))
