def _check_list(str_or_list):
    """ If `str_or_list` is a list will return it as it is. If it is a str, will return a
    list with the str inside.

    Parameters
    ----------
    str_or_list: str or list

    Returns
    -------
    list
    """
    if str_or_list is None:
        return None
    if isinstance(str_or_list, list):
        return str_or_list
    if isinstance(str_or_list, str):
        return [str_or_list]
    raise ValueError('Expected a `str` or a `list`, got {}.'.format(type(
        str_or_list)))
