How Does Python's `nonlocal` Keyword Differ from `global` in Scope Management?

Understanding the Role of "nonlocal" in Python 3

In Python 3, "nonlocal" plays a crucial role in accessing variables defined in an enclosing scope, but outside the current one. Unlike "global," which references variables in the global scope, "nonlocal" allows you to interact with variables in a parent function's scope.

Consider this example without using "nonlocal":

x = 0
def outer():
    x = 1
    def inner():
        x = 2
        print("inner:", x)

    inner()
    print("outer:", x)

outer()
print("global:", x)

Output:

inner: 2
outer: 1
global: 0

As we can see, the variable "x" in the inner function is assigned a local value of 2, while the variable "x" in the outer function remains at 1. The global variable "x" retains its initial value of 0.

Now, let's rewrite this code using "nonlocal":

x = 0
def outer():
    x = 1
    def inner():
        nonlocal x
        x = 2
        print("inner:", x)

    inner()
    print("outer:", x)

outer()
print("global:", x)

Output:

inner: 2
outer: 2
global: 0

With "nonlocal," the variable "x" in the inner function is now bound to the variable "x" in the outer function. As a result, when "x" is modified within the inner function, it also affects its value in the outer function. The global variable "x" remains unchanged.

In contrast, "global" would bind the variable "x" in the inner function to the one in the global scope:

x = 0
def outer():
    x = 1
    def inner():
        global x
        x = 2
        print("inner:", x)
        
    inner()
    print("outer:", x)

outer()
print("global:", x)

Output:

inner: 2
outer: 1
global: 2

Understanding the subtle differences between "nonlocal" and "global" is crucial for effectively managing variables in Python code.

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