Accessing global function's variables in a local function

Question

Here's my test script:

def main(): #global
    n = 1
    z = None

    def addone():  #local
        if not z:
            n = n+1
    addone()
    print n

main()

I step into the addone() function once it hits the calling line. At this point I can only see the variable z, but cannot see n.

Now, if n is referenced before assignment, shouldn't z be too?

Similarly, if I change n=n+1 to z='hi', I can no longer see z! This is contrary to all my previous beliefs about local/global functions! The more you know, the more you know you don't know about Python.

Question(s):

Why can I see one but not the other?
Do I want to be prepending global to these variables I want to reassign?

Perhaps this might be helpful: http://stackoverflow.com/a/21978980/748858 — mgilson, Jan 15 '15 at 02:47
ah ha, useful indeed. thanks. I would have thought I had to use `global` outside of the function, but nice that i can use it within. — tenwest, Jan 15 '15 at 03:06
wait...maybe it is as I thought. doesn't `global` have to be defined **outside** of the function? I am getting a `NameError: global name 'x' is not defined`. calling it `global` outside of the function as well causes it to work. — tenwest, Jan 15 '15 at 03:13

score 1 · Accepted Answer · answered Jan 15 '15 at 03:05

1

The best solution is to upgrade to Python 3 and use in the inner function nonlocal n. The second-best, if you absolutely have to stick with Python 2:

def main(): #global
    n = [1]
    z = None

    def addone():  #local
        if not z:
            n[0] += 1
    addone()
    print n[0]

main()

As usual, "there is no problem in computer science that cannot be solved with an extra level of indirection". By making n a list (and always using and assigning n[0]) you are in a sense introducing exactly that life-saving "extra level of indirection".

answered Jan 15 '15 at 03:05

Alex Martelli

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Unfortunately I am constrained to python2...this is a very interesting workaround, I may use it for now as I don't think we want to be declaring the variable in question as a `global`. – tenwest Jan 15 '15 at 03:14
@tenwest, yep, `global` should be avoided whenever possible! – Alex Martelli Jan 15 '15 at 03:15

score 0 · Answer 2 · edited Jan 15 '15 at 02:44

Okay, after some testing, I realised that it all has to do with the reassignment of variables.

for example:

def main(): #global
    n = 1
    z = None

    def addone():  #local
        if not z:
            x = n+1
    addone()
    print n

main()

Now shows both n and z when I am inside the addone() function. This is because I am no longer trying to reassign n, makes sense to me so as to protect global variables from manipulation if one so happens to use similar names in local functions.

Accessing global function's variables in a local function

2 Answers2