How do I change global variable in nested scope using python?

Question

My example below. I have tried adding global to the variable that I want to change but doesn't help. The error I'm getting is in the comment.

def getNumOfSwapsToSort(array):
    sorted = array.copy()
    sorted.sort()

    swaps = 0

    def swap():
        currentSwaps = 0
        for i, el in enumerate(array):
            if ((len(array) - 1) == i) and currentSwaps != 0:
                swap()

            elif el != sorted[i]:
                idx = sorted.index(el)
                temp = array[idx]
                array[idx] = el
                array[i] = temp
                # UnboundLocalError: local variable 'swaps' referenced before assignment
                # if I use global swaps, then NameError: name 'swaps' is not defined
                swaps += 1
                currentSwaps += 1

    swap()

    return swaps

An almost identical question: stackoverflow.com/questions/2609518/… — natka_m
– natka_m, Commented Feb 17, 2020 at 18:19
Possible duplicate of UnboundLocalError with nested function scopes — wjandrea
– wjandrea, Commented Feb 17, 2020 at 19:00

natka_m · Accepted Answer · 2020-02-17 18:07:54Z

3

The swaps variable in your code is not global, it's nonlocal. It was defined in the enclosing function getNumOfSwapsToSort(), not in the global namespace.

Your other function, swap(), has access to this nonlocal variable, but the variable is protected from being edited from the inner function. If you try to edit or overwrite it, it's going to be treated as a new, local variable - hence the UnboundLocalError you're getting.

Try using the nonlocal statement:

nonlocal swaps
swaps += 1

Here's an article with a few simple examples which explain the concept of closure in Python.

edited Feb 17, 2020 at 18:07

answered Feb 17, 2020 at 18:02

natka_m

1,69122 silver badges27 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

wjandrea · Accepted Answer · 2020-02-17 18:00:41Z

1

swaps isn't a global, it's in getNumOfSwapsToSort, which is the parent scope of swap. So use a nonlocal declaration.

def swap():
    nonlocal swaps
    ...

answered Feb 17, 2020 at 18:00

wjandrea

33.9k10 gold badges69 silver badges105 bronze badges

Collectives™ on Stack Overflow

How do I change global variable in nested scope using python?

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related