Python: recursive method rollbacks changes?

Question

So I got stuck on something like this (a simplified version)

class Node:
    def __init__(self):
        self.left= None
        self.cost = 0

    def change(self):
        if self.left is not None:
            self.left.cost=self.cost+1
            self.left.change

data=[]
for i in range(10):
    data.append(Node())
    if i>0:
        data[i].left = data[i-1]

data[8].change()
print(data[2].cost) #0

I want data[2].cost to have changed, but it rollbacks. Can I make it works without skipping recursion? (In full version I actually keep a two-dimensional array of nodes that have four pointers, so making an iteration suck.)

`self.left.change` does _not_ recurse. It doesn't call the function — Jean-François Fabre, Nov 14 '18 at 15:27
Is this `self.left.change` supposed to be `self.left.change()`? — Silver, Nov 14 '18 at 15:27
I'm not sure this question will be of much benefit for future readers. Maybe OP may delete it ? — kriss, Nov 14 '18 at 15:35
@rrter123 No, it just means you're human. I once spent more time than I'd like to admit searching for a bug where I had looped on `i` but indexed with `j`. — pjs, Nov 14 '18 at 15:35
@pjs and in my actual problem I did x instead of self.x. The happy life of the programmer. Do you think I should delete this ask? — rrter123, Nov 14 '18 at 15:43
@rrter123 I'm not sure you could delete it at this point, given that there is an upvoted answer. Meanwhile, you got a small bump from it. Even if you can delete it, it's a judgement call on your part. I don't think it's compelling either way. — pjs, Nov 14 '18 at 19:40

score 1 · Accepted Answer · answered Nov 14 '18 at 15:30

1

You forgot () when you call your change method.

def change(self):
    if self.left is not None:
        self.left.cost=self.cost+1
        self.left.change()

output:

6

answered Nov 14 '18 at 15:30

iElden

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score 0 · Answer 2 · answered Nov 14 '18 at 15:33

0

Obviously you want to call change but you didn't. You just refered to the function object and did nothing with it.

Just change self.left.change to self.left.change() to call it

answered Nov 14 '18 at 15:33

kriss

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Python: recursive method rollbacks changes?

2 Answers2