def _ensure_tuple_or_list(arg_name, tuple_or_list):
    """Ensures an input is a tuple or list.

    This effectively reduces the iterable types allowed to a very short
    whitelist: list and tuple.

    :type arg_name: str
    :param arg_name: Name of argument to use in error message.

    :type tuple_or_list: sequence of str
    :param tuple_or_list: Sequence to be verified.

    :rtype: list of str
    :returns: The ``tuple_or_list`` passed in cast to a ``list``.
    :raises TypeError: if the ``tuple_or_list`` is not a tuple or list.
    """
    if not isinstance(tuple_or_list, (tuple, list)):
        raise TypeError('Expected %s to be a tuple or list. Received %r' %
            (arg_name, tuple_or_list))
    return list(tuple_or_list)
