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()
a += 1 is the same as a = a + 1 for ints. It binds the name a to a new expression instead of modifying the object currently named by a, like a list operation does.
is an ugly hack, That's why Python3 adds nonlocal afterall. Usually it is better to design the code to not need this kind of hack, but sometimes the complication of the alternative could be works than using the hacky wayVersion for python 2:
def f():
a=0
def g():
g.a+=1
print g.a
g.a=a
return g
g=f()
g()
