Understanding Python Global, Local, and Nonlocal Variables

Understanding Python Global, Local, and Nonlocal Variables

Master the use of global, local, and nonlocal variables in Python, and know when to apply each for clean, maintainable code.

Video: Python Local and Global Variables

Global Variables

In Python, a variable declared outside any function—at module level—is a global variable. It can be read from anywhere in the module, but must be declared with the global keyword if you want to modify it inside a function.

Example: creating and reading a global variable

x = "global"

def foo():
    print("x inside:", x)

foo()
print("x outside:", x)

Output

x inside: global
x outside: global

Attempting to reassign x without declaring it global results in an UnboundLocalError:

x = "global"

def foo():
    x = x * 2
    print(x)

foo()

Output

UnboundLocalError: local variable 'x' referenced before assignment

To modify a global variable inside a function, use the global keyword:

x = "global"

def foo():
    global x
    x = x * 2
    print(x)

foo()

Output

globalglobal

Local Variables

A local variable is defined within a function’s body and exists only during that function’s execution. It cannot be accessed outside the function.

Example: accessing a local variable outside its scope

def foo():
    y = "local"

foo()
print(y)

Output

NameError: name 'y' is not defined

Defining and using a local variable correctly:

def foo():
    y = "local"
    print(y)

foo()

Output

local

Combining Global and Local Variables

It’s common to use both scopes in the same module. The global keyword ensures that a variable refers to the module-level instance.

x = "global "

def foo():
    global x
    y = "local"
    x = x * 2
    print(x)
    print(y)

foo()

Output

global global 
local

When a local variable shares a name with a global one, they are independent:

x = 5

def foo():
    x = 10
    print("local x:", x)

foo()
print("global x:", x)

Output

local x: 10
global x: 5

Nonlocal Variables

Nested functions can access and modify variables from their enclosing (but not global) scope using the nonlocal keyword.

def outer():
    x = "local"

    def inner():
        nonlocal x
        x = "nonlocal"
        print("inner:", x)

    inner()
    print("outer:", x)

outer()

Output

inner: nonlocal
outer: nonlocal

Note: Updating a nonlocal variable inside the inner function updates the variable in the outer function’s scope.