为什么我的装饰器按此顺序执行代码？

Question

I have the following code and I have a hard time understanding why it prints the statements in the order it does.

def main():
    print('1')
    registry=[]

def register(func):
    print('2')
    registry.append(func)
    return func

@register
def f1():
    print('3')
print('4')
f1()
main()

This code prints:

1
2
4
3

But I'm wondering why it doesn't print:

1
2
3
4

when the @register is called I understand it that register(f1) is called, it prints 2 and then f1 is returned. To me it seems like 3 should be printed next since f1 is returned. But instead f1 is not called until the very end f1() statement. Doesn't return func run the function it returns?

Answer 1

Consider the equivalent code that doesn't use decorator syntax. Also, we make registry a pre-defined global so that the code actually runs.

registry = []

def main():
    print('1')
    #registry=[]

def register(func):
    print('2')
    registry.append(func)
    return func

def f1():
    print('3')

f1 = register(f1)
print('4')
f1()
main()

The first function that gets called is register , so the first value output is 2 . Next, print('4') outputs 4 . Third, f1 is called and outputs 3 . Finally, main is called and outputs 1 .

register never calls f1 ; it simply adds it to the list registry and returns it.