Python Language Nonlocal Variables


Example

Python 3.x3.0

Python 3 added a new keyword called nonlocal. The nonlocal keyword adds a scope override to the inner scope. You can read all about it in PEP 3104. This is best illustrated with a couple of code examples. One of the most common examples is to create function that can increment:

def counter():
    num = 0
    def incrementer():
        num += 1
        return num
    return incrementer

If you try running this code, you will receive an UnboundLocalError because the num variable is referenced before it is assigned in the innermost function. Let's add nonlocal to the mix:

def counter():
    num = 0
    def incrementer():
        nonlocal num
        num += 1
        return num
    return incrementer

c = counter()
c() # = 1
c() # = 2
c() # = 3

Basically nonlocal will allow you to assign to variables in an outer scope, but not a global scope. So you can't use nonlocal in our counter function because then it would try to assign to a global scope. Give it a try and you will quickly get a SyntaxError. Instead you must use nonlocal in a nested function.

(Note that the functionality presented here is better implemented using generators.)