Closures in Python

Not everyone programmer knows what is closures, but it is quite interesting, so I’m going to try explain it in this article.

What is closure?

Let’s take a look in this example:

def say_name(name):
    def say_goodbye():
        print(f"Don't say me goodbye, {name}!")
    return say_goodbye


f = say_name('Andrei')
f()  # Don't say me goodbye, Andrei!

We define one func in another func and return internal func as object. Then we call external func, and then call internal. Looks OK, right? Or no? When you think about this code, you’ll fount some strange thing: how the f() calling knows the name variable? f is pointed to internal func, which doesn’t accept any params. In this call we don’t set any names.

Okay, let’s took another example, more strange:

def counter(value=0):
    def plus_one():
        nonlocal value
        value += 1
        return value
    return plus_one


c = counter()

print(c())  # 1
print(c())  # 2

Here we add nonlocal construction to be able to modify value in enclosing scope from local scope. So, what happens here? When I first time found this example I was really surprised. Somehow value remember its value (yes, it was bad idea to set name variable as value) between c() calls. There is no any global variable which stores value. Where is it stores? And what a hell is going on here? :)

Okay, okay :) Let me explain. Wen we call internal function in global scope, interpreter create snapshot of the used variables in enclosing scope (regardless it’s nesting level). Python interpreter collect all links to all objects are used in internal function and save it in tuple named closure. You can se this tuple when ask dunder method __closure__ of internal function. You can see how variable is closures in our example:

def counter(value=0):
    def plus_one():
        nonlocal value
        value += 1
        return value
    return plus_one


c = counter()
print(c.__closure__)  # (<cell at 0x1017da4c0: int object at 0x10156b910>,)
print(c.__closure__[0].cell_contents)  # 0
c()
print(c.__closure__[0].cell_contents)  # 1

So, the answer on question “where is value stores?” is: in magic method __closure__ of internal function. Note, you can’t change this variables, because there is no direct mapping name - value. And you can’t change order of the stored variables, because __closure__ is tuple.