众所周知，Python3 有一个符号，叫做 @ ，意为装饰器，具体过程如下：

@demo
def func():
    ...
# <=>
def func():
    ...
func = demo(func)

可以对 func 改进:

def demo(func):
    def new_func():
        func()
        print('func used')
    return new_func
@demo
def func():
    print('hello')

运行结果：

hello
func used

(@后面可以为任何可调用对象，被装饰的可以是函数或类，具体在这里不在详细赘述，感兴趣的可以上网查查看)

一、“立即”装饰器

这个名字是我自己取的，如有想法，欢迎在评论区留言！

# @lambda _: _()
def hello() -> str:
    return 'hello'

print(hello)
print(hello())

输出：

<function hello at 0xbalabala>
hello

这里 $\texttt{0xbalabala}$ 不是真的 balabala，是 hello 的地址

@lambda _: _()
def hello() -> str:
    return 'hello'

print(hello)
# print(hello()) # 报错，因为 hello 是个 str，而 str 对象不可调用

输出 hello

可以看出，被 @lambda _: _() 的函数/类变成了调用自己的结果

二、套娃装饰器

既然装饰完还是函数，那么，我们就可以再装饰一遍：

def demo(func):
    def new_func():
        return func() * 2
    return new_func
@demo
@demo
def func():
    return 1
print(func())

输出4
既然装饰器也是函数，那么，我们就可以给装饰器装饰两遍：

def dedemo(demo):
    def new_demo(func):
        return demo(demo(func))
    return new_demo
@dedemo
@dedemo
def demo(func):
    def new_func():
        return func() * 2
    return new_func
@demo
@demo
def func():
    return 1
print(func())

这样，输出$2^8$！
既然给装饰器装饰的装饰器也是函数，那么，手动dedemo = dedemo(dedemo)，不对，dedemo = dedemo(dedemo(dedemo))两次！

def dedemo(demo):
    def new_demo(func):
        return demo(demo(func))
    return new_demo
dedemo = dedemo(dedemo(dedemo))
dedemo = dedemo(dedemo(dedemo))
@dedemo
@dedemo
def demo(func):
    def new_func():
        return func() * 2
    return new_func
@demo
@demo
def func():
    return 1
print(func())

运行，输出————

Traceback (most recent call last):
  File "<pyshell#32>", line 1, in <module>
    main()
  File "<pyshell#31>", line 16, in main
    def func():
  File "<pyshell#31>", line 4, in new_demo
    return demo(demo(func))
  File "<pyshell#31>", line 4, in new_demo
    return demo(demo(func))
  File "<pyshell#31>", line 4, in new_demo
    return demo(demo(func))
  [Previous line repeated 1021 more times]
RecursionError: maximum recursion depth exceeded

你不要妄想修改递归最大深度，因为时 间 超 限
那么，到底会输出多少呢？
dedemo = dedemo(dedemo(dedemo)) 让demo的效果变成原来的两倍的两倍，也就是

def dedemo(demo):
    def new_demo(func):
        return demo(demo(demo(demo(demo(demo(demo(demo(func)))))))) # 8 个 demo
    return new_demo

dedemo = dedemo(dedemo(dedemo)) 让demo的效果变成原来的八倍的八倍，也就是

def dedemo(demo):
    def new_demo(func):
        return demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(demo(func)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))) # 2 ** 9 个 demo
    return new_demo

那么，@dedemo def demo就变成了func() * 2 ** 9，
两次就变成了func() * (2 ** 9) ** 2；
@demo def func就变成了(2 ** 9) ** 2，
两次就变成了((2 ** 9) ** 2) ** 2；也就是
$\left[\left(2^9\right)^2\right]^2=2^{9\times2\times2}=2^{36}$，
$2^{36}$个字节等于$64MB$
是一个编译成.exe的C++程序A+B problem的60+倍！