scope.md

Python by Example: Scope

Python looks up names in four scopes in order: local (inside the function), enclosing (outer functions), global (module level), built-in. Use global to assign to a module-level variable from inside a function; use nonlocal to assign to a variable in an enclosing function.

What you'll learn:

• LEGB scope lookup order
• global to modify a module-level variable
• nonlocal to modify a variable in an enclosing scope

x = "global"


def outer():
    x = "enclosing"

    def inner():
        x = "local"
        print(x)

    inner()
    print(x)


outer()
print(x)


count = 0


def increment():
    global count
    count += 1


increment()
increment()
print(count)


def make_counter():
    n = 0

    def inc():
        nonlocal n
        n += 1
        return n

    return inc


counter = make_counter()
print(counter())
print(counter())

Each scope gets its own x—assigning inside a function never touches the outer x unless you use global or nonlocal. nonlocal is how closures maintain mutable state across calls.

To run this program:

$ python source/scope.py
local
enclosing
global
2
1
2

Tip: Prefer returning values over using global—it makes data flow explicit and functions easier to test.

Try it: Rewrite increment() to take and return count instead of using global.

Source: scope.py

Next: Recursion