Python: Sharing global variables between modules and classes therein

Question

I know that it's possible to share a global variable across modules in Python. However, I would like to know the extent to which this is possible and why. For example,

global_mod.py

x = None

mid_access_mod.py

from global_mod import *

class delta:
    def __init__(self):
        print x

bot_modif_mod.py

import mid_access_mod
import global_mod

class mew:
    def __init__(self):
        global_mod.x = 5

def main():
    m = mew()
    d = mid_access_mod.delta()

This prints None, even though all the modules are sharing the global variable x. Why is this the case? It seems like x is evaluated at mid_access_mod.py before it is assigned in bot_modif_mod.py by mew().

Answer 1

This happens because you are using immutable values (ints and None), and importing variables is like passing things by value, not passing things by reference.

If you made global_mod.x a list, and manipulated its first element, it would work as you expect.

When you do from global_mod import x, you are creating a name x in your module with the same value as x has in global_mod. For things like functions and classes, this works as you would expect, because people (generally) don't re-assign to those names later.

As Alex points out, if you use import global_mod, and then global_mod.x, you will avoid the problem. The name you define in your module will be global_mod, which always refers to the module you want, and then using attribute access to get at x will get you the latest value of x.