Functions Scope

In Python, the interpreter searches four levels to find a variable name, following the LEGB rule: Local → Enclosing → Global → Built-in. By default, we can read outside variables, but we cannot modify them without explicit keywords.

The LEGB Scope Rule​

Python's scope rules might feel broader if we are coming from C.

x = "Global"

def outer():
    x = "Enclosing"

    def inner():
        x = "Local"
        print(f"Inner: {x}") # Prints "Local"

    inner()
    print(f"Outer: {x}") # Prints "Enclosing"

outer()
print(f"Global: {x}") # Prints "Global"

Output:

Inner: Local
Outer: Enclosing
Global: Global

Modifying Outside Variables​

To change a variable defined outside the current function's local scope, we must explicitly declare our intent using global or nonlocal.

Global Keyword​

Use global to modify a variable defined at the top level of the file.

count = 0

def increment():
    global count  # Tells Python to use the top-level 'count'
    count += 1

increment()
print(f"Global count: {count}")

Global count: 1

Nonlocal Keyword​

Use nonlocal to modify a variable in the parent (enclosing) function's scope.

def parent():
    name = "Alice"
    def child():
        nonlocal name  # Targets the variable in 'parent' scope
        name = "Bob"
    child()
    print(f"Parent name: {name}")

parent()

Parent name: Bob