Nonlocal keyword in Python

Introduction

The nonlocal keyword lets you change a variable in an outer function from inside a nested function. It helps when you want to keep track of changes without using global variables.

When you have a function inside another function and want to update a variable from the outer function.

When you want to remember a value that changes each time the inner function runs.

When you want to avoid using global variables but still share data between nested functions.

Syntax

Python

def outer_function():
    variable = None
    def inner_function():
        nonlocal variable
        variable = 'new_value'
    inner_function()

You must declare nonlocal before changing the variable inside the inner function.

The variable must exist in the nearest outer function scope, not global or local to inner function.

Examples

This example shows how nonlocal lets the inner function update count from the outer function.

Python

def counter():
    count = 0
    def increment():
        nonlocal count
        count += 1
        return count
    return increment

count_up = counter()
print(count_up())  # 1
print(count_up())  # 2

The inner function changes the message variable from the outer function using nonlocal.

Python

def greet():
    message = 'Hello'
    def change_message():
        nonlocal message
        message = 'Hi'
    change_message()
    print(message)

greet()  # Output: Hi

Sample Program

This program uses nonlocal to increase factor each time the inner function runs, changing the multiplication result.

Python

def make_multiplier():
    factor = 2
    def multiply(number):
        nonlocal factor
        factor += 1
        return number * factor
    return multiply

multiplier = make_multiplier()
print(multiplier(5))  # First call
print(multiplier(5))  # Second call

Important Notes

If you forget nonlocal, Python treats the variable as local and you get an error if you try to assign it.

nonlocal only works with variables in the nearest outer function, not global variables.

Summary

nonlocal lets inner functions change variables from outer functions.

It helps keep track of changing data without using global variables.

Always declare nonlocal before assigning to the variable inside the inner function.