Effective Use of 'nonlocal' in Python

The nonlocal keyword in Python is used in specific situations within nested functions:

1. In nested functions (a function inside another function).
2. When you need to modify a variable from the outer function within the inner function.
3. When the variable to be modified is not global, but belongs to the outer function's scope.

Example:

def outer_function():
    x = 10

    def inner_function():
        nonlocal x
        x = 20
        print(f"inner x: {x}")

    print(f"x before: {x}")
    inner_function()
    print(f"x after: {x}")

outer_function()

Output:

x before: 10
inner x: 20
x after: 20

nonlocal x indicates that x in the inner function refers to the x from the outer function. Without nonlocal, a new local variable x would be created in the inner function.

Use nonlocal with caution, as it can complicate code readability. Consider it only when it's truly necessary to modify a variable from an outer function.