Python Global Variables for Recursive Function

Question

In my math class, we have been told to write a program in python. In one of the parts, I want to test the convergence of a series. While writing the program, I realized I was fundamentally misunderstanding something about how python handles global variables. Take this code:

def main():

    global n
    n = 1

    def check():

        a = 10
        if n > a: print(n)
        else: n += 1

    check()

main()

This code fails because it says that n has not been defined yet. However, I can't define n inside of the check() function since that would just reset n to one upon every iteration! Is there some workaround for this problem?

n is not global inside of check() - you'd need to put the global n there, the one in main() isn't actually accomplishing anything since nothing outside of main is accessing it. — jasonharper
– jasonharper, Commented May 27, 2017 at 1:38
btw, I'm not sure there is something recursive there as the title suggests. — Paolo
– Paolo, Commented May 27, 2017 at 1:43
There's also no apparent reason for defining check at all, let alone inside main. — chepner
– chepner, Commented May 27, 2017 at 2:08

Taku · Accepted Answer · 2017-05-27 02:05:31Z

3

As already stated in the comments, n isn't in the global scope yet, since it's inside the nested function check. You will need to add global n to the check's scope, to access the global n value from the nested function:

def main():   
    global n
    n = 1

    def check():
        global n
        a = 10
        if n > a: print(n)
        else: n += 1

    check() 
main()

@PedrovonHertwig also pointed out you don't need global n in main (which is the case in your current context, ignore this if you want to use n elsewhere in the top-level scope) and thatn is perfectly fine staying in main's local scope. You can then replace the global keyword inside check to nonlocal n, telling python to use the n that is not in the local scope nor the global scope, but main's scope.

edited May 27, 2017 at 2:05

answered May 27, 2017 at 1:41

Taku

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4 Comments

Pedro von Hertwig Batista Over a year ago

Furthermore, you don't need the global keyword in the first n declaration, you can just do n = 1. This is because global just prevents a local variable from being created on assignment in the inner scope when there's a global with the same name.

Taku Over a year ago

@PedrovonHertwig I didn't know what the OP want, just followed his original code. Maybe he did want the n to be global.

Pedro von Hertwig Batista Over a year ago

Your answer is correct, I'm just adding an explanation and pointing out that unless n is defined in top-level scope, the first global n statement has no effect.

Taku Over a year ago

@PedrovonHertwig Thank you for your further explanation, I added some text regarding that topic:)

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