def force_list(data):
    """Wrap data in a tuple.

    Wraps its argument in a tuple so that it can be iterated over, unless
    it was a list, a tuple, or a set (therefore already iterable).

    Note that if ``data`` is ``None`` this method will return ``None``, which
    is not iterable.
    """
    if data is not None and not isinstance(data, (list, tuple, set)):
        return data,
    return data
