def listify(x):
    """Turn argument into a list.

    This is a convenience function that allows strings
    to be used as a shorthand for [string] in some arguments.

    Returns None for None.
    Returns a list for a list or tuple.
    Returns [x] for anything else.

    :param x: value to be listified.

    """
    if x is None:
        return None
    elif isinstance(x, (list, tuple)):
        return x
    else:
        return [x]