modify a global variable among two files in python

Question

I have two python files: first test2.py :

a = 1


def func1():
    global a
    print(id(a))
    a += 1

then test1.py ：

from test2 import a, func1

def func0():
    global a
    print(id(a))
    a += 1

func0()

func1()

print(a)

It turns out if I run test1.py , the result is 2 rather than 3 which I thought it should be. I check the id of a in two functions and they are the same.

I have invoked two functions func0 and func1 , why global variable just did the addition once?

Answer 1

First, don't use global variables. Second, both modules test1 and test2 have separate namespaces. Use explicit reference to module test2 .

import test2
from test2 import func1

def func0():
    print(id(test2.a))
    test2.a += 1

func0()

func1()

print(test2.a)

Answer 2

The global keyword makes it global to your current module, in your case test1 and test2 . When doing the import it's effectively copying the value to your local module scope.

Perhaps this makes it clearer because that's effectively what happens:

import test2

a = test2.a
a = 123
# what is the value of test2.a? Unchanged, only the local a was changed