python variable change scope

Question

I have a simple use case. I need to access variables that are outside the scope of a function. My problem is that, I am unable to see the changes being reflected.

I have:

def somefunc():
    print a # None
    print b # None

def main():
    a = 'whatever'
    b = 10
    module = modules[__name__]
    getattr(module, 'somefunc')

if __name__ == 'main':
    # NOTE: if and b are changed here, then somefunc can see those changes
    a = None
    b = None
    main()

NOTE: I cannot pass arguments to somefunc because of some other limitation. How do I see the changes to a and b in somefunc?

Thanks!

I didnt give you -1, but this is poor program design ... you really should be using data encapsulation and passing it around ... global variables are just asking for trouble — Joran Beasley
– Joran Beasley, Commented Apr 2, 2014 at 19:18
I totally understand. This should be in a class by itself. I declared something global probably after a couple of years, but sometimes shit needs to get done, whether or not it is architecturally right. #startupproblems — waka-waka-waka
– waka-waka-waka, Commented Apr 2, 2014 at 19:20
it will hurt alot more to fix later than just fixing it now with a better design ... — Joran Beasley
– Joran Beasley, Commented Apr 2, 2014 at 19:21
You use two terms erroneously: "variable", because this word is ambiguous in Python, and "scope", because all the people make a mistake concerning the sense of this word. But it's a too complex subject that doesn't interest people, and I have no time and energy to explain. — eyquem
– eyquem, Commented Apr 2, 2014 at 21:10

msvalkon · Accepted Answer · 2014-04-02 19:20:56Z

3

You need to declare them global using the global keyword. The global keyword allows you to change the value of a variable that is defined inside the module, outside of any function or class.

You can always print global variables without declaring them, and you can modify global mutable containres like lists and dicts without declaring them global, but you cannot assign values. Heres the official documentation, below is a working example.

a = None
b = None

def somefunc():
    # This works.
    print a # None
    print b # None

def main():
    global a, b
    a = 'whatever'
    b = 10
    # somefunc() exist in the local scope, you don't need the getattr.
    # just call it.
    somefunc()

if __name__ == '__main__': # You were missing the undescores surrounding main
    main()

Demo output:

msvalkon@Lunkwill:/tmp$ python t.py
whatever
10

It's a little bad practice to use globals. They have their use cases but usually there is a better way, like passing the data as arguments to functions or wrapping all functions inside a class which holds the value you'd normally make global. Globals are great for storing static information, like configuration info, a server address that doesn't change, a port number and so forth. You should try to avoid using them like normal variables.

edited Apr 2, 2014 at 19:20

answered Apr 2, 2014 at 19:04

msvalkon

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4 Comments

user1019517 Over a year ago

winner winner chicken dinner

waka-waka-waka Over a year ago

is it a bad approach using global?

Joran Beasley Over a year ago

yes ... but if thats the only way you can do it as you claim then thats the only way to do it

user1019517 Over a year ago

I think most people will tell you that globals are pretty much a bad idea. You could always be passing by reference instead, or create a class, or any number of things other than globals.

score 1 · Accepted Answer · 2014-04-03 21:28:13Z

1

You need to use the global keyword to prevent the scope of your function from overwriting the scope of a = None and b = None

def somefunc():
    print a # None
    print b # None

def main():
    global a, b
    a = 'whatever'
    b = 10
    module = modules[__name__]
    getattr(module, 'somefunc')

if __name__ == '__main__': # you need underscores around __main__ 
    a = None
    b = None
    main()

bam.

edited Apr 3, 2014 at 21:28

answered Apr 2, 2014 at 19:07

user1019517

2 Comments

Joran Beasley Over a year ago

although you do not need that when only referencing them (only when setting them)

user1019517 Over a year ago

guess I'm just a natural with extensibility, you know... nbd

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