human
I wrote the below code

    if (not field):
        return
    info_file = get_info_filename(target)
    LOG.info(_('Writing    stored    info    to    %s'), info_file)
    fileutils.ensure_tree(os.path.dirname(info_file))
    lock_name = ('info-%s' % os.path.split(target)[(-1)])
    lock_path = os.path.join(CONF.instances_path, 'locks')
    @lockutils.synchronized(lock_name, 'nova-', external=True, lock_path=lock_path)
    def write_file(info_file, field, value):
        d = {}
        if os.path.exists(info_file):
            with open(info_file, 'r') as f:
                d = _read_possible_json(f.read(), info_file)
        d[field] = value
        d[('%s-timestamp' % field)] = time.time()
        with open(info_file, 'w') as f:
            f.write(json.dumps(d))
    write_file(info_file, field, value)

What's a good function header?
assistant
#Here's a good function header.

def write_stored_info(target, field=None, value=None):