def _parse_expected_tuple(arg, default=tuple()):
    """ Parse the argument into an expected tuple.

    The argument can be None (i.e., using the default), a single element (i.e.,
    a length-1 tuple), or a tuple
    """
    try:
        _ = iter(arg)
    except TypeError:
        tpl = tuple(default) if arg is None else (arg,)
    else:
        tpl = tuple(arg)
    return tpl