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?
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 comes to the rescue.
def f():
a=0
def g():
nonlocal a
a+=1
print a
return g
g=f()
g()