def _tolist(arg):
    """
    Assure that *arg* is a list, e.g. if string or None are given.

    Parameters
    ----------
    arg :
        Argument to make list

    Returns
    -------
    list
        list(arg)

    Examples
    --------
    >>> _tolist('string')
    ['string']
    >>> _tolist([1,2,3])
    [1, 2, 3]
    >>> _tolist(None)
    [None]

    """
    if isinstance(arg, str):
        return [arg]
    try:
        return list(arg)
    except TypeError:
        return [arg]