Why does the decorator return the type None?

Question

Help me understand why this decorator does not add a value to the end of the list that is formed in the do_list () function.

I get the result of the function working with the decorator None

def dec_do_list(func):
    def wrapper(arg: int):
        result = func(arg).append(4)
        return result
    return wrapper
@dec_do_list
def do_list(arg: int):
    import random
    result = []
    for i in range(arg):
        result.append(random.random())
    return result

print(do_list(4))

Thank you in advance!

P.S.
I do this for educational purposes, in order to better understand the Decorator pattern

Asked By: Виталий Агарев

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Source

Answer 1

result = func(arg).append(4)

append doesn’t return a new list. It modifies the list in-place. You are getting the result as the return value of append and then returning it so at the end the returned value is None.

You can change it to:

result = func(arg)
result.append(4)
return result

or, in one line:

return func(arg) + [4]

Answered By: AKS

Why does the decorator return the type None?

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