Variables scopes in python, the for case why it works?

Question

In the python example below, methods and attributes seem to be out of scope but they still work, what is happening?

for module in output:
    a = 1
    attributes=[]
    methods=[]
    for branch in module[2]:

        for leaf in branch[2]:
            if leaf[0]=="method":
                methods.append(leaf[1]) 
            if leaf[0]=="attribute":
                attributes.append(leaf[1])
print methods
print attributes
print module[0]
print a

but if I outdent one more level it stops working

for filename in os.listdir("."):
    print filename
    fName, fExtension = os.path.splitext(filename)
    print fName, fExtension
    if fExtension == ".idl":
        f = open(filename)
        idl = f.read()
        f.close()
        output = parse(idl)
        pprint.pprint(output)
        root={}
        for module in output:
            a = 1
            attributes=[]
            methods=[]
            for branch in module[2]:
                for leaf in branch[2]:
                    if leaf[0]=="method":
                        methods.append(leaf[1]) 
                    if leaf[0]=="attribute":
                        attributes.append(leaf[1])
    print methods
    print module[0]

it says: NameError: name 'methods' is not defined I'm using python 2.7

Only functions, classes, methods and modules create new scopes. — Martijn Pieters, Aug 14 '12 at 12:49
Please see this [Short Description of Python Scoping Rules](http://stackoverflow.com/questions/291978/short-description-of-python-scoping-rules) — kojiro, Aug 14 '12 at 12:50
You could have also gone for `leaf`, try `for z in (1,2,3): print z`, then `z`. `z` is assigned and in scope! i.e. will return 3. List comprehensions on the other hand... http://stackoverflow.com/a/541958/201648 — Aaron Newton, Aug 14 '12 at 13:01
@AaronNewton -- List comprehensions behave differently in this regard depending on python version (2.x "leaks" the loop variable into the current scope where apparently python 3.x doesn't). — mgilson, Aug 14 '12 at 13:04
@mgilson true. I've just tested it under 2.7.3 and sure enough I get '9' `>>> i = 0 >>> [i+1 for i in range(10)] [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>> i 9` — Aaron Newton, Aug 14 '12 at 13:08

mgilson · Accepted Answer · 2012-08-14T13:23:09.383

4

As indicated in the comments, for loops, while loops, if statements, etc. do not create a new scope. In fact, the only things that create a new scopes are functions, classes, modules and methods. Therefore, when you create a new variable inside a for loop, it is available outside of that loop because they share the same scope.

edited Aug 14 '12 at 13:23

answered Aug 14 '12 at 13:01

mgilson

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`print module[0]` isn't surprising if you take into account a few lines up: `for branch in module[2]`. It seems quite likely that `output` yields things supporting the sequence protocol (I'd put my money on tuples). – lvc Aug 14 '12 at 13:15
I edited with more code example were it brakes – Eduard Florinescu Aug 14 '12 at 13:16
1

@EduardFlorinescu -- This probably breaks because `fExtension == ".idl":` probably evaluates to `False`. This means that the code in the `if` statement is never executed and that variable is never added to the scope (resulting in a `NameError`). – mgilson Aug 14 '12 at 13:21
True. That's what happened. So we nee to be careful where the variable is defined, so practically there are other cases of scope by defining the variable in a specific condition or not. This I will keep in mind as a good practice even if the scope is overall local, don't use a variable out of the scope of a specific conditional evaluation if it was defined there. – Eduard Florinescu Aug 14 '12 at 13:29
@EduardFlorinescu -- Yeah. You'll sometimes see things like: `if condition: x=3 ; else: x = None` (that example is better accomplished using the ternary operator, but hopefully you get the idea). – mgilson Aug 14 '12 at 13:35
Yes is better with ternary because is an one-liner. Now I understood this particular situation, in case is not obvious:D I am new with python. – Eduard Florinescu Aug 14 '12 at 13:43
1

@EduardFlorinescu -- No problem. We're happy to help. Hopefully you stick with Python long enough to love it as much as I do :^) – mgilson Aug 14 '12 at 13:46
Its already my favourite but then there are people that say that I didn't met ruby. – Eduard Florinescu Aug 14 '12 at 13:55

Variables scopes in python, the for case why it works?

1 Answers1