python装饰器修饰的函数的return值是否只能是Nonetype

Question

我写了一个装饰器来获取程序的运行时间，但是函数return值变成了Nonetype

def gettime(func):
    def wrapper(*args, **kw):
        t1 = time.time()
        func(*args, **kw)
        t2 = time.time()
        t = (t2-t1)*1000
        print("%s run time is: %.5f ms"%(func.__name__, t))

    return wrapper

如果我不使用装饰器，return 值是正确的。

A = np.random.randint(0,100,size=(100, 100))
B = np.random.randint(0,100,size=(100, 100))
def contrast(a, b):
    res = np.sum(np.equal(a, b))/(A.size)
    return res

res = contrast(A, B)
print("The correct rate is: %f"%res)

结果是：The correct rate is: 0.012400

如果我使用装饰器：

@gettime
def contrast(a, b):
    res = np.sum(np.equal(a, b))/len(a)
    return res

res = contrast(A, B)
print("The correct rate is: %f"%res)

会有报错：

contrast run time is: 0.00000 ms

TypeError: must be real number, not NoneType

当然，如果我删除 print 语句，我可以获得正确的运行时间，但是 res 接受 Nonetype。

Answer 1

由于包装器替换了修饰的函数，它还需要传递return值：

def wrapper(*args, **kw):
    t1 = time.time()
    ret = func(*args, **kw)  # save it here
    t2 = time.time()
    t = (t2-t1)*1000
    print("%s run time is: %.5f ms"%(func.__name__, t))
    return ret  # return it here

Answer 2

或者你可以这样做：

def gettime(func):
    def wrapper(*args, **kw):
        t1 = time.time()
        func(*args, **kw)
        t2 = time.time()
        t = (t2-t1)*1000
        print("%s run time is: %.5f ms"%(func.__name__, t))
        print("The correct rate is: %f"%func(*args,**kw))
    return wrapper


@gettime
def contrast(a, b):
    res = np.sum(np.equal(a, b))/a.size
    return res
contrast(A,B)

python装饰器修饰的函数的return值是否只能是Nonetype

Whether the return value of a function modified by python decorator can only be Nonetype

python

decorator

nonetype