def metric(fn):
    @functools.wraps(fn)
    def wrapper(*args, **kw):
        start = time.time()
        fn(*args, **kw) #运行slow fast函数
        end = time.time() - start
        print('%s executed in %.3f ms' % (fn.__name__, end * 1000))
        return fn(*args, **kw)
    return wrapper