Understanding how does a decorator really work

Question

I'm starting studying decorators and I already hit an obstacle. First here is my code.

 def deco (f):

    def coucou():
        print("this is function{}".format(f))
        return f()
    return coucou

@deco
def salut():
    print("salut")


def hi():
    return salut()

I will try to explain my problem as well as I can with my bad English. If I understand it this is how things should happen: I execute my hi() function which returns salut() and because salut is modified by the decorator coucou will be executed and coucou returns ....... salut(), what i mean is that I expect an infinite loop but that doesn't happen and I don't understand why. Can anyone explain practically how decorators work?

https://realpython.com/primer-on-python-decorators/ This may help in understanding how decorators work — Pro Q, Jan 19 '19 at 00:20
You may also want to have a look at the documentation: [PEP-318](https://www.python.org/dev/peps/pep-0318/#current-implementation-history) and [PEP-3129](https://www.python.org/dev/peps/pep-3129/#implementation). — Ondrej K., Jan 19 '19 at 00:29

score 2 · Accepted Answer · answered Jan 19 '19 at 00:10

2

The f in coucou is the undecorated (original) version of salut.

answered Jan 19 '19 at 00:10

Scott Hunter

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yeah but why and how? again if i got it, the decorator modify the function in its definition and makes it point permanently to the function returned by the decorator, am i wrong? – Newgate Ace Jan 19 '19 at 00:13
1

@NewgateAce it *doesn't* modify the original function. It creates a new function, and returns a new function. So the *global name* `salut` now refers to the function returned by the decorator, i.e. `coucou`, but `coucou`'s *local name* `f`, which is a free variable closed over by `coucou`, still refers to the original function – juanpa.arrivillaga Jan 19 '19 at 00:16
@NewgateAce in other words, there is nothing magical about the decorator syntax. It is equivalent to `def my_function(): ...; my_func = deco(my_func)` – juanpa.arrivillaga Jan 19 '19 at 00:24
@juanpa.arrivillaga i see now, f = deco(f) makes it clearer for me. thanks for your reply. – Newgate Ace Jan 19 '19 at 00:30
@juanpa.arrivillaga what im about to say might be stupid, but when i print "salut" and "deco(salut)", they dont have the same reference ... – Newgate Ace Jan 19 '19 at 00:46
Um, because `deco(salut)` will *create a new object each time it is called*, but what, *exactly* are you doing? – juanpa.arrivillaga Jan 19 '19 at 00:49

Understanding how does a decorator really work

1 Answers1