Question

I have a question about global scope in python.

I make this script and works but I don’t know why:

#! /bin/python3
# -*- coding: UTF-8 -*-

data = []
stats = {'white':0, }

def main():
    global data

    with open(args.finput, 'r') as f:
        data = f.readlines()

    rwhitespaces()

    with open(foutput, 'w') as f:
        for line in data:
            f.write(line)

    print(stats)

def rwhitespaces():
    cnt = 0
    for line in data:
        if line == '\n':
            data.pop(cnt) # Modifing data var without global keywork and works, why??
            stats['fistro'] = 1 # Modifing stats var without global keywork and works why??
        cnt += 1

if __name__ == "__main__":
    main()

So I’m misunderstanding something with global scope can someone explain me???
Thanks and sorry my bad english

Answer 1

This works because you are not changing the reference, but rather accessing a mutable object, and getting it to perform changes upon itself.

global is required to assign a new value to a globally scoped variable (x = blah), but it isn’t needed to merely access one. As when you do data.pop(cnt) all you are doing is accessing the variable, you can do it without using the global keyword.

In short, it’s not about changing properties of the object, it’s about assignment of an object to a variable.