How to modify local variables from a nested closure?

Question

Consider the following code example:

def testClosure():
    count = 0

    def increment():
        count += 1

    for i in range(10):
        increment()

    print(count)

Calling this results in:

Traceback (most recent call last):
  File "/Users/cls/workspace/PLPcython/src/sandbox.py", line 23, in <module>
    testClosure()
  File "/Users/cls/workspace/PLPcython/src/sandbox.py", line 18, in testClosure
    increment()
  File "/Users/cls/workspace/PLPcython/src/sandbox.py", line 15, in increment
    count += 1
UnboundLocalError: local variable 'count' referenced before assignment

I am used to code like this working in C++:

void testClosure() {
    int count = 0

    auto increment = [&](){
        count += 1;
    };

    for (int i = 0; i < 10; ++i) {
        increment();
    }

}

What am I doing wrong? Is it not possible to modify local variables of the outer function from an inner function? Are these different types of closures (Python vs C++)?

Sorry bit lost, is there a realworld example where this is actually useful? — Jakob Bowyer, Jul 18 '13 at 14:36
@JakobBowyer Yes, but this toy example makes it easier to understand and solve the actual problem. — clstaudt, Jul 18 '13 at 14:37
My question was basically answered by a mention of the `nonlocal` keyword of Python 3, which I did not know. — clstaudt, Jul 18 '13 at 14:46
@JakobBowyer real world example: https://github.com/kennethreitz/httpbin/blob/master/httpbin/core.py#L217 — woozyking, Jul 18 '13 at 14:59

score 2 · Accepted Answer · answered Jul 18 '13 at 14:47

I got it working if you do this:

def testClosure():
    count = 0

    def increment():
        nonlocal count
        count += 1

    for i in range(10):
        increment()

    print(count)
testClosure()

Note this will only work in Python 3.x, but you are clearly using that so it isn't a problem.

How to modify local variables from a nested closure?

1 Answers1