Sync change of variables between files python

Question

I would like to share a variable between files.

B.py

b_var = 'start'

def change_var():
    global b_var
    b_var = 'changed'

test.py

from proj.B import change_var, b_var

print(b_var)
change_var()
print(b_var)

The output is:

start
start

How can I get the output to be:

start
changed

does [this](https://stackoverflow.com/questions/13034496/using-global-variables-between-files) answer your question. — Ceres, Feb 22 '21 at 12:31
@Ceres Actually no because a list is mutable so it can work this way but a string isn't... — lordcommander, Feb 22 '21 at 12:32
@OlvinRoght Nice. But is there a way that i could write b_var instead of B.b_var and it will still work? — lordcommander, Feb 22 '21 at 12:35

score 2 · Accepted Answer · answered Feb 22 '21 at 12:44

The short answer is that as soon as you use a simple variable, you have lost. Because when you use (whatever value can be):

var = value

you do not try to change the object that var previouly refered to, but just make var point to a different object. So any other variables that pointed to the original object are not affected.

So you will have to use an attribute on a mutable object (here the module), or an element of a container. That way, the object itself will the changed and all variables pointing to that object will reflect the change. Of course if at any point of your program you have a simple variable pointing to the attribute, this one will not reflect the change.

So this would be ok:

from proj.B import change_var   # change_var will not change: we directly import it
import B

print(B.b_var)      # but we use b_var (expected to change) through another object
change_var()
print(B.b_var)

Ceres · Answer 2 · 2021-02-22T12:53:03.487

1

A solution similar to the python docs

B.py

b_var = 'start'

mod.py

import B

def change_var():
    B.b_var = 'changed'

test.py

import B
import mod
print(B.b_var)
mod.change_var()
print(B.b_var)

output

start
changed

edited Feb 22 '21 at 12:53

answered Feb 22 '21 at 12:43

Ceres

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score 1 · Answer 3 · answered Feb 22 '21 at 12:52

1

Because b_var is not a variable, is a reference to an immutable string.

The import sentence is copying the reference, and when change_var() runs it creates a new string and change only the reference on B module but not on test one.

answered Feb 22 '21 at 12:52

pbacterio

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Sync change of variables between files python

3 Answers3