Understanding global variable in Python

Question

I came across a strange issue in Python when using global variables.

I have two modules(files):mod1.py and mod2.py

mod1 tries to modify the global variable var defined in mod2. But the var in mod2 and var in mod seems to be two different things. Thus, the result shows that such modification does not work.

Here is the code:

#code for mod2.py
global var  
var = 1 
def fun_of_mod2():
    print var

#code for mod1.py
from mod2 import var,fun_of_mod2    
global var #commenting out this line yields the same result
var = 2 #I want to modify the value of var defined in mod2
fun_of_mod2() #but it prints: 1 instead of 2. Modification failed :-(

Any hint on why this happens? And how can I modify the value of val defined in mod2 in mod1?

Thanks

kindall · Accepted Answer · 2012-06-03 07:22:47Z

12

When you import var into mod1:

from mod2 import var,fun_of_mod2

You are giving it the name var in mod1's namespace. It is as if you did this:

import mod2
var = mod2.var
fun_of_mod2 = mod2.fun_of_mod2
del mod2

In other words, there are now two names for the value, mod1.var and mod2.var. They are the same at first, but when you reassign mod1.var, mod2.var still points to the same thing.

What you want to do is just:

import mod2

Then access and assign the variable as mod2.var.

It's important to note that global variables in Python are not truly global. They are global only to the module they're declared in. To access global variables inside another module, you use the module.variable syntax. The global statement can be used inside a function to allow a module-global name to be assigned to (without it, assigning to a variable makes it a local variable in that function). It has no other effect.

edited Jun 3, 2012 at 7:22

answered Jun 3, 2012 at 7:10

kindall

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1 Comment

lvc Over a year ago

+1, but to fully answer the question you should mention what global does. The OP's major problem seems to be misunderstanding where it is useful.

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