python module variable confusion

Question

Here is my code :

a module settings.py:

My_KEY = ""

and I am using this MY_KEY in another module

my_module.py

from settings import My_KEY


def function_a():
    MY_KEY = "abcd"


def function_b():
    function_a();
    print My_KEY

I expect to see a "abcd" in output when I call function_b(), but how come I get empty string. I am totally lost

If you want to explicitly change the module level variable it's clearer and more explicit to do it directly: import settings and then settings.MY_KEY = 'abcd' makes it clear that you wanted to change the variable. BTW it's a convention to use ALL_CAPS for constants that don't change, so it's doubly confusing here — theodox
– theodox, Commented Aug 20, 2013 at 23:30

unutbu · Accepted Answer · 2013-08-20 22:19:09Z

6

By default, when an assignment statement is encountered inside a function, Python takes the variable on the left-hand side to be a local variable.

If you want to modify a global variable from within a function, use the global statement to declare the variable to be a global:

def function_a():
    global My_KEY
    My_KEY = "abcd"

However, it is better to avoid modifying globals from within functions. In the long run it makes code harder to understand. You could instead pass My_KEY as a return value from function_a:

def function_a():
    My_KEY = "abcd"
    return My_KEY

def function_b():
    My_KEY = function_a()
    print My_KEY

edited Aug 20, 2013 at 22:19

answered Aug 20, 2013 at 22:02

unutbu

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alecxe · Accepted Answer · 2013-08-20 22:02:51Z

3

It's all about python scoping and namespacing.

You have actually 3 versions of My_KEY:

global variable My_KEY imported
My_KEY in function_a (I think you have a typo there: should be My_KEY instead of MY_KEY)
My_KEY in function_b

You can mark variable inside a function as global in order to modify it:

def function_a():
    global My_KEY
    My_KEY = "abcd"

answered Aug 20, 2013 at 22:02

alecxe

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Comments

tdelaney · Accepted Answer · 2013-08-20 22:41:54Z

Scoping is part of the issue but you also need to be careful about how you import variables. from settings import My_KEY creates a new variable in the local module's namespace that points to the original string. When you assign a different string to your module's My_KEY, that doesn't affect the My_KEY in the settings module. Its not a problem when you are modifying mutable objects, but any time you reassign a value, You are better off just importing the module the module itself and using the variable's dotted name.

settings.py:

My_KEY = ""

then a quick test in the shell

>>> from settings import My_KEY
>>> def function_a():
...     global My_KEY
...     My_KEY="abcd"
... 
>>> My_KEY
''
>>> function_a()
>>> My_KEY
'abcd'
>>> 
>>> # here's where it gets funky... the original settings.My_KEY didn't change 
>>> 
>>> import settings
>>> settings.My_KEY
''
>>>

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