Python nonlocal Keyword

Usage

If a variable is declared in an enclosing (outer) function, it is nonlocal to nested (inner) function.

The nonlocal keyword is used to update these variables inside a nested function. The usage of nonlocal is very similar to that of global, except that the former is primarily used in nested functions.

Syntax

nonlocal var1,var2,…

Basic Example

Here’s a basic example that tries to reassign enclosing function’s local variable inside a nested function.

# enclosing function
def f1():
    x = 42
    
    # nested function
    def f2():
        x = 0
        print(x)    # x is 0
    f2()
    print(x)        # x is still 42
    
f1()

Here, the value of existing variable x didn’t change. Because, Python created a new local variable named x that shadows the variable in the outer scope.

Preventing that behavior is where the nonlocal keyword comes in.

# enclosing function
def f1():
    x = 42
    
    # nested function
    def f2():
        nonlocal x
        x = 0
        print(x)    # x is now 0
    f2()
    print(x)        # x remains 0
    
f1()

The x inside the nested function now refers to the x outside the function, so changing x inside the function changes the x outside it.