Python函数装饰器之谜

Question

I recently leant the subject of python decorators, and found something strange for the below two different programs:

Example1:

def deco(func):
    def inner():
        print("Running inner")
    return func

@deco
def target():
    print("Running target")

def main():
    print("Running main()")
    target()

if __name__ == "__main__":
    main()

Example2:

def deco(func):
    print("Running inner")
    return func

@deco
def target():
    print("Running target")

def main():
    print("Running main()")
    target()

if __name__ == "__main__":
    main()

When running Example1 and Example2, it will get 2 different results:

Running main()
Running target

in Example1, and

Running inner
Running main()
Running target

in Example2.

It seems "Running inner" was lost in Example1. Why?

Answer 1

Running Example 2 will give that result because deco is executed at definition time, and in that example it prints inner immediately.

Example 1 won't print inner because you never do anything with the inner func. Your decorator there should be returning inner , not func ; and inner itself should call func:

def deco(func):
    def inner():
        print("Running inner")
        func()
    return inner   # note change here

When you do this, you will get this result:

Running main()
Running inner
Running target

which is what you would expect from a decorator.