python - Why do we need wrapper function in decorators?

Question

If I create a decorator like following:

def my_decorator(some_fun):
    def wrapper():
        print("before some_function() is called.")
        some_fun()
        print("after some_function() is called.")
    return wrapper

@my_decorator
def just_some_function():
    print("Wheee!")

Another decorator can be defined as:

def my_decorator(some_fun):
    print("before some_function() is called.")
    some_fun()
    print("after some_function() is called.")

@my_decorator
def just_some_fun():
    print("some fun")

Both decorators will work the same. What is the benefit of using "wrapper" function inside decorator. I didn't get the purpose.

Answer 1

The purpose of having a wrapper function is that a function decorator receives a function object to decorate, and it must return the decorated function.

Your 2nd version of my_decorator doesn't have an explicit return statement, so it returns None. When my_decorator is called via the @ decorator syntax

before some_function() is called.
some fun
after some_function() is called.

gets printed, and then None gets assigned to the name just_some_fun. So if you add print(just_some_fun) to the end of that code it will print None.

It may be easier to understand what's going on if we get rid of the @ syntactic sugar and re-write your code using normal function calling syntax:

def my_decorator(some_fun):
    print("before some_function() is called.")
    some_fun()
    print("after some_function() is called.")

def just_some_fun():
    print("some fun")

just_some_fun = my_decorator(just_some_fun)