def parse_line(line_data):
    """
    Return the (line number, left hand side, right hand side) of a stripped
    postfix config line.

    Lines are like:
    smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
    """
    num,line = line_data
    left, sep, right = line.partition("=")
    if not sep:
        return None
    return (num, left.strip(), right.strip())