What is the use of the "nonlocal" keyword in Python?

Question

Jayant Kumar · Accepted Answer

Table of Contents
Introduction
In Python, the nonlocal keyword is used within a nested function to refer to a variable in the enclosing (non-global) scope. This keyword allows you to modify variables that are defined in the nearest enclosing scope that is not global. It’s particularly useful in cases where you need to update a variable from an outer function within a nested function, without creating a new local variable.
Understanding the nonlocal Keyword
The nonlocal keyword is employed to work with variables in nested functions. Here’s what it accomplishes:

Accessing Enclosing Scope Variables: It allows a nested function to access and modify variables in its enclosing scope.
Avoiding Local Variable Creation: Without nonlocal, any assignment to a variable in a nested function would create a new local variable, not modify the outer one.

Syntax
def outer_function():
    variable = value
    
    def inner_function():
        nonlocal variable
        # Modify variable here

Examples of Using nonlocal
Basic Example
def outer():
    count = 0
    
    def inner():
        nonlocal count
        count += 1
        print("Count inside inner function:", count)
    
    inner()
    print("Count inside outer function:", count)

outer()
# Output:
# Count inside inner function: 1
# Count inside outer function: 1

In this example, nonlocal count in the inner function refers to the count variable defined in outer. It allows the inner function to modify the count variable in outer.
Use Case: Closures
Closures often use nonlocal to maintain state across multiple calls.
def make_counter():
    count = 0
    
    def counter():
        nonlocal count
        count += 1
        return count
    
    return counter

counter = make_counter()
print(counter())  # Output: 1
print(counter())  # Output: 2
print(counter())  # Output: 3

Here, counter is a closure that retains the state of count across multiple invocations. The nonlocal keyword ensures that each call to counter updates the same count variable.
Practical Use Cases

Maintaining State in Nested Functions: When you need to update a variable in an outer function from a nested function, nonlocal helps you achieve this.
Creating State in Closures: nonlocal is useful for creating function closures that maintain state across multiple function calls.

Conclusion
The nonlocal keyword is a powerful feature in Python that enables nested functions to modify variables in their enclosing scopes. It is essential for maintaining state in closures and for modifying variables in outer scopes without creating new local variables. Understanding and using nonlocal appropriately can lead to more flexible and maintainable code.

What is the use of the "nonlocal" keyword in Python?

Table of Contents

Introduction

Understanding the `nonlocal` Keyword

Syntax

Examples of Using `nonlocal`

Basic Example

Use Case: Closures

Practical Use Cases

Conclusion

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