Difference between closures in python and javascript - Stack Overflow

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In JS, we can write closure like:

function f(){
var a=0;
function g(){
    alert(a++);
}
return g;
}
g=f()
g()

However, if I write following code in python

def f():
    a=0
    def g():
        a+=1
        print a
    return g
g=f()
g()

Then I get UnboundedLocalError.

Can anyone tell me the difference between closure in python and JS?

In JS, we can write closure like:

function f(){
var a=0;
function g(){
    alert(a++);
}
return g;
}
g=f()
g()

However, if I write following code in python

def f():
    a=0
    def g():
        a+=1
        print a
    return g
g=f()
g()

Then I get UnboundedLocalError.

Can anyone tell me the difference between closure in python and JS?

• javascript
• python
• closures

Share Improve this question Follow edited May 6, 2011 at 10:40 Ikke 101k2323 gold badges100100 silver badges120120 bronze badges asked May 6, 2011 at 10:37 user607722user607722 1,68622 gold badges1313 silver badges2222 bronze badges 2

• 3 Do you really believe that this difference will explain what you're seeing? – Ignacio Vazquez-Abrams Commented May 6, 2011 at 10:38
• I think so. Direct explanation of the observed result is also wele – user607722 Commented May 6, 2011 at 16:10

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 12

When you use a += 1 in Python it refers to a local (uninitialized) variable in scope of g function. Basically you can read variables from upper scopes, but if you try to write it will refer to a variable in most recent scope. To make it work like you want you have to use nonlocal keyword that is only present Python 3. In Python 2 you can't do that as far as I know, unless the variable you're trying to change is is global, then global keyword es to the rescue.

def f():
    a=0
    def g():
        nonlocal a
        a+=1
        print a
    return g
g=f()
g()

Version for python 2:

def f():
    a=0
    def g():
        g.a+=1
        print g.a

    g.a=a
    return g
g=f()
g()

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