CODEWITHSUNDEEP
pythonglobal-variables
Yannis
Yannis

Reputation: 205

Using global variables in Python modules

I have these 4 modules

globals.py

globvara = "a"

mod1.py

from globals import *

print globvara

output:

a

mod2.py

from mod1 import *

def changegv(newval1):
    #global globvara
    globvara = newval1

def usechangegv(newval2):
    changegv(newval2)

and mod3.py

from mod2 import *

usechangegv("b")

print globvara

output:

a
a

I am wondering why the global var does not change in module 2. I am missing something in global variables. Even if I uncomment the global globvara line I get the same result. Where is the error?

Upvotes: 0

Views: 88

Answers (2)

Saish
Saish

Reputation: 521

Don't use the

from <module> import <variable>

As it creates a copy of the variable.

Do a simple:

import <module>

And all accesses to the global variable should use the "variable" within "module":

<module>.<variable> = ...

or

print <module>.<variable>

Upvotes: 1

kindall
kindall

Reputation: 184091

Python global variables are global only to modules. When you import a variable from another module (e.g. from mod1 import *), Python creates duplicate references to the value in the importing module. So you now have two names, mod1.globvara and mod2.globvara, which initially point to the same value, but which are not in any way connected. If you change globvara in mod2.py, you are changing mod2.globvara and mod1.globvara is not affected.

To avoid this problem, import the module, not the individual names defined in it. For example, import globals. Then always refer to globals.globvara (or better yet, globals.a). Since you are always accessing and assigning the same name, it will work the way you expect.

Upvotes: 5

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