def metric(fn):
    @functools.wraps(fn)
    def wrapper(*args,**kw):
        start = time.time()
        fn(*args,**kw)
        end = time.time()
        print('%s executed in %s ms' % (fn.__name__, str(end - start)))
        return fn(*args,**kw)
    return wrapper

首先调用metric(fast)返回一个wrapper,再运行wrapper,运行了fast(x,y)