Python nonlocal

The nonlocal keyword in Python is used inside nested functions to indicate that a variable is not local to the current function but belongs to an enclosing (non-global) scope. It allows the modification of variables defined in the nearest enclosing function scope.

Example

def outer_function():

    count = 0  # Defined in the enclosing scope

    def inner_function():

        nonlocal count  # Modify the variable in the enclosing scope

        count += 1

        print(f"Count inside inner_function: {count}")

    inner_function()

    print(f"Count inside outer_function: {count}")

outer_function()

Output:

Count inside inner_function: 1

Count inside outer_function: 1

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Syntax

nonlocal variable_name

• variable_name: The name of the variable from the enclosing scope that you want to modify.

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Why Use nonlocal?

1. Modify Enclosing Scope Variables: Allows a nested function to change variables in the enclosing (but non-global) scope.
2. Encapsulation: Useful for managing state or counters within nested functions while keeping them private to the enclosing function.
3. Alternative to Globals: Helps avoid using global variables when working with nested functions.

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Common Examples

1. Using nonlocal for State Management

def outer():

    message = "Hello"  # Enclosing scope variable

    def inner():

        nonlocal message

        message = "Hi"  # Modify the enclosing variable

        print("Inside inner:", message)

    inner()

    print("Inside outer:", message)

outer()

Output:

Inside inner: Hi

Inside outer: Hi

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2. Counter with nonlocal

def counter():

    count = 0  # Enclosing scope variable

    def increment():

        nonlocal count

        count += 1

        return count

    return increment

counter_instance = counter()

print(counter_instance())  # Output: 1

print(counter_instance())  # Output: 2

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3. Without nonlocal (For Comparison)

def outer():

    x = 5

    def inner():

        x = 10  # Creates a new local variable, doesn't modify the enclosing one

        print("Inner x:", x)

    inner()

    print("Outer x:", x)

outer()

Output:

Inner x: 10

Outer x: 5