Question

Closures are an incredibly useful language feature. They let us do clever things that would otherwise take a lot of code, and often enable us to write code that is more elegant and more clear. In Python 2.x, closures variable names cannot be rebound; that is, a function defined inside another lexical scope cannot do something like some_var = 'changed!' for variables outside of its local scope. Can someone explain why that is? There have been situations in which I would like to create a closure that rebinds variables in the outer scope, but it wasn’t possible. I realize that in almost all cases (if not all of them), this behavior can be achieved with classes, but it is often not as clean or as elegant. Why can’t I do it with a closure?

Here is an example of a rebinding closure:

def counter():
    count = 0
    def c():
        count += 1
        return count
    return c

This is the current behavior when you call it:

>>> c()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in c
UnboundLocalError: local variable 'count' referenced before assignment

What I’d like it to do instead is this:

>>> c()
1
>>> c()
2
>>> c()
3

Answer 1

To expand on Ignacio’s answer:

def counter():
    count = 0
    def c():
        nonlocal count
        count += 1
        return count
    return c

x = counter()
print([x(),x(),x()])

gives [1,2,3] in Python 3; invocations of counter() give independent counters. Other solutions – especially using itertools/yield are more idiomatic.