Question

Full disclosure, this is part of a homework assignment (though a small snippet, the project itself is a game playing AI).

I have this function built into a tree node class:

    def recursive_score_calc(self):
        current_score = self.board
        for c in self.children:
            child_score = c.recursive_score_calc()
            if(turn_color == 1):
                if(child_score > current_score):
                    current_score = child_score
            else:
                if(child_score < current_score):
                    current_score = child_score
        self.recursive_score = current_score
        return current_score

On a tree of depth 1 (a root and some children), it hits the Python recursion limit already. The function is designed to use dynamic programming to build a min-max tree from the bottom up. To be honest, I have no idea why this isn’t working as intended, but I am fairly new to Python as well.

Good people of Stack Overflow: Why does this code give me a stack overflow?

The Entire Class in question:

from Numeric import *

class TreeNode:
    children = []
    numChildren = 0
    board = zeros([8,8], Int)
    turn_color = 0 # signifies NEXT to act
    board_score = 0 # tally together board items
    recursive_score = 0 # set when the recursive score function is called

def __init__(self, board, turn_color):
    self.board = copy.deepcopy(board)
    self.turn_color = turn_color
    for x in range (0,7):
        for y in range (0,7):
            self.board_score = self.board_score + self.board[x][y]

def add_child(self, child):
    self.children.append(child)
    self.numChildren = self.numChildren + 1

def recursive_score_calc(self):
    current_score = self.board # if no valid moves, we are the board. no move will make our score worse
    for c in self.children:
        child_score = c.recursive_score_calc()
        if(turn_color == 1):
            if(child_score > current_score):
                current_score = child_score
        else:
            if(child_score < current_score):
                current_score = child_score
    self.recursive_score = current_score
    return current_score

The function that interacts with this (Please Note, this is bordering on the edge of what is appropriate to post here, I’ll remove this part after I accept an answer): [It didn’t turn out to be the critical part anyways]

Answer 1

This bit of your code:

class TreeNode:
    children = []

means that every instance of the class shares the same children list. So, in this bit:

def add_child(self, child):
    self.children.append(child)

you’re appending to the “class-global” list. So, of course, every node is a child of every other node, and disaster is guaranteed.

Fix: change your class to

class TreeNode(object):
    numChildren = 0
    board = zeros([8,8], Int)
    turn_color = 0 # signifies NEXT to act
    board_score = 0 # tally together board items
    recursive_score = 0 # set when the recursive score function is called

def __init__(self, board, turn_color):
    self.children = []
    self.board = copy.deepcopy(board)
    self.turn_color = turn_color
... etc, etc ...

the rest doesn’t need to change to fix this bug (though there may be opportunities to improve it or fix other bugs, I have not inspected it deeply), but failing to assign self.children in __init__ is causing your current bug, and failing to inherit from object (unless you’re using Python 3, but I’d hope you would mention this little detail if so;-) is just a bug waiting to happen.