Decorated function returns “None”

Question

I'm extremely new to python, and i just encountered decorators. I'm still kinda confused by them but i am learning

i was trying to make a decorator that tells me how much time my function took to finish, but apparently when i try to use it on a function that should return something, it just returns "None"

I've seen only a couple of questions talking about this problem but none of them actually helped

Here's my code

import time


def time_it(func):  # Here i make a simple decorator function that should time my decorated function
    def wrapper(*args, **kwargs):
        t1 = time.time()
        func(*args)
        t2 = time.time()
        total = t2 - t1
        print("The function '" + func.__name__ + "' took", str(total)[0:5], "seconds to complete")

    return wrapper


@time_it
def square(nums):  # I make a function that squares every number in a list
    new_list = []
    for n in nums:
        new_list.append(n ** 2)
    return new_list


lis = [f for f in range(200000)]  # i make a list with a range of 200000
print(square(lis))  

sorry for any grammatical errors, i'm not a native english speaker

Answer 1

The decorator replaces square with wrapper and wrapper does not return anything. It should return the value returned by the wrapped function.

This is the correct way to do it:

def time_it(func):
    def wrapper(*args, **kwargs):
        t1 = time.time()
        try:
            return func(*args, **kwargs)
        finally:
            t2 = time.time()
            total = t2 - t1
            print("The function '" + func.__name__ + "' took", str(total)[0:5], "seconds to complete")

    return wrapper

I changed 3 things:

• added return , so that the value is returned from decorated function
• added **kwargs to func calls, because it may be needed if used differently
• added try / finally block, so that the printout happens even in case of an exception, plus this makes it easier to return the value.

Answer 2

The problem is that your inner function return value isn't being returned. The change is noted below:

from functools import wraps

def time_it(func):  # Here i make a simple decorator function that should time my decorated function
    @wraps(func)
    def wrapper(*args, **kwargs):
        t1 = time.time()
        ## Note the change on this line -- I now store the return result from the called function 
        result = func(*args, **kwargs)
        t2 = time.time()
        total = t2 - t1
        print("The function '" + func.__name__ + "' took", str(total)[0:5], "seconds to complete")

        ## And then explicitly return the result
        return result

    return wrapper

For the decorator, you need to remember that it's just a closure, with some fancy syntax. You still need to deal with the function return parameters yourself.

A couple of additions:

• from functools import wraps and @wraps(func)
  • this will create wrap the inner function with some details that exist in the wrapping function. There's a small example in the python docs here: https://docs.python.org/3/library/functools.html

Answer 3

Your decorated function doesn't return anything explicitely - so, by default, it returns None .

You can capture the output before printing the time, and return at the end:

def time_it(func):  # Here i make a simple decorator function that should time my decorated function
    def wrapper(*args, **kwargs):
        t1 = time.time()
        out = func(*args)
        t2 = time.time()
        total = t2 - t1
        print("The function '" + func.__name__ + "' took", str(total)[0:5], "seconds to complete")
        return out
    return wrapper