How to properly import a shared variable?

Question

I am having the following 3 Python files:

a.py :

myvar = 1
   
def increment():
    global myvar
    myvar += 1

b.py :

import a
    
a.increment()
print(a.myvar)

c.py :

from a import increment, myvar
    
increment()
print(myvar)

Now when I run b.py and c.py independently, I get:

python3 ./b.py
2
python3 ./c.py
1

Can you explain the difference?

Thanks!

This is Python 3.7.3 on the latest Debian GNU/Linux (stable).

Answer 1

The overall reason is that integers are immutable .

Let me explain. In your c.py script, a variable myvar is imported from the module a , and then, the increment() method is called on a.myvar doing myvar += 1 in module a . Since we have the reference to myvar already, and integers are immutable, Python can't set that reference to the new value. The reassignment happened only in module a .

To get the updated value after increment() , you should also import your module a , and try to access the reference directly via a.myvar

NOTE that c.py will work correctly if myvar was any of list,dict,set , or other mutable objects.

Here's the updated c.py , which works correctly on integers.

import a
from a import increment, myvar

increment()
print(myvar)

print(a.myvar)

And here's an a.py example with mutable ojbects. Try it with the same c.py and see how it works.

# a.py
myvar = ["some"]

def increment():
    global myvar
    myvar.append("thing")