Python overwriting variables in nested functions

Question

Suppose I have the following python code:

def outer():     string = ""     def inner():         string = "String was changed by a nested function!"     inner()     return string 

I want a call to outer() to return "String was changed by a nested function!", but I get "". I conclude that Python thinks that the line string = "string was changed by a nested function!" is a declaration of a new variable local to inner(). My question is: how do I tell Python that it should use the outer() string? I can't use the global keyword, because the string isn't global, it just lives in an outer scope. Ideas?

Answer 1

In Python 3.x, you can use the nonlocal keyword:

def outer():     string = ""     def inner():         nonlocal string         string = "String was changed by a nested function!"     inner()     return string 

In Python 2.x, you could use a list with a single element and overwrite that single element:

def outer():     string = [""]     def inner():         string[0] = "String was changed by a nested function!"     inner()     return string[0]