Variable Scope and `nonlocal` Keyword

1. Understanding Python Variable Scope

In Python, scope refers to the region of a program where a variable is recognized. Python follows the LEGB rule for scope resolution:

Scope Level	Meaning	Example
L: Local	Inside the current function	Variable in a function
E: Enclosing	Inside any enclosing function(s)	Nested function variables
G: Global	At the top-level of the script	Declared outside all functions
B: Built-in	Python's built-in names	`len`, `sum`, etc.

Python Variable Scope Examples

1. Local Scope2. Global Scope3. Enclosing Scope (Nested Functions)4. Built-in Scope

def func():
    x = 10  # Local variable
    print(x)

func()
# Output: 10

x = 5  # Global variable

def show():
    print(x)

show()
# Output: 5

def outer():
    y = 20  # Enclosing variable

    def inner():
        print(y)  # Accessing enclosing variable

    inner()

outer()
# Output: 20

print(len("hello"))  # 'len' is a built-in function
# Output: 5

Variable Lookup Order (LEGB)

If a variable is referenced inside a function, Python checks in the following order:
1. Local
2. Enclosing
3. Global
4. Built-in

2. `global` Keyword

Use global to modify a global variable inside a function.

x = 10

def change():
    global x
    x = 20

change()
print(x)
# Output: 20

3. `nonlocal` Keyword

The nonlocal keyword is used inside a nested function to modify a variable in the enclosing (non-global) scope.

Examples

Example 1Example 2

def outer():
    x = "outer variable"

    def inner():
        nonlocal x
        x = "modified by inner"

    inner()
    print(x)

outer()
# Output: modified by inner

def counter():
    count = 0

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

    return increment

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

4. `nonlocal` vs `global`

Feature	`global`	`nonlocal`
Scope	Refers to global scope	Refers to nearest enclosing scope
Use case	Modify global variable	Modify enclosing function variable
Used in	Any function	Nested functions

5. Summary

Concept	Description
Scope	Region where a variable is accessible
LEGB Rule	Lookup order: Local → Enclosing → Global → Built-in
global	Refers to and modifies global variable
nonlocal	Refers to and modifies enclosing variable in nested function

Variable Scope and nonlocal Keyword