def is_cols(cols):
    """Check variable type fall into the column name category.
    Args:
        cols (str | list{str} | None):
            Column names candidate.
    Returns:
        bool:
            if `col` is either str, list{str} or None
    """
    ok = isinstance(cols, (str, list)) or cols is None
    if isinstance(cols, list):
        ok = ok and all([isinstance(col, str)
                         for col in cols])
    return ok