def _split_colname(colname):
    """ split colname into varname and units

        :param str colname: original ldndc outputfile colname
        :return: varname and unit
        :rtype: tuple
    """
    out = colname, 'unknown'
    if '[' in colname:
        name, var_units = colname.split('[')
        units = var_units[:-1]
        out = name, units
    return out
