Function within function to change a variable value

Question

Why doesn't the variable url change?

def test():
    url = "Start"

    def change():
        global url
        url = "End"

    change()
    print(url)

    return url

test()

And yes, it needs to be a function within a function. That way the rest of my code is more simple. Thanks for your help!

Answer 1

Global variables are those defined at the top-level scope of a module, not just any variable defined outside the scope of a function. The variable defined in test is a nonlocal variable from change 's perspective, not a global variable.

Use the nonlocal keyword instead:

def test():
    url = "Start"

    def change():
        nonlocal url
        url = "End"

    change()
    print(url)

    return url

nonlocal causes the assignment to take place in the closest enclosing scope where url is defined. (If no such local scope exists, the nonlocal declaration is a syntax error. A definition in the global scope does not count, in contrast with free variables who are looked up in the closest enclosing scope, whether local or global.)

---

In your original function, change really does set (or even create) a global variable named url . For example:

def test():
    url = "Start"

    def change():
        global url
        url = "End"

    change()
    print(url)

    return url

try:
    print(url)
except NameError:
    print("'url' not yet defined")

test()  # Outputs "Start"
print(url)  # Outputs "End"

Answer 2

The solution is to use the nonlocal keyword

def test():
    url = "Start"
    def change():
        nonlocal url
        url = "End"

    change()
    print(url)

    return url