My recursive function seems to be using the original list instead of creating a new one in python

Question

I'm pretty new to python and I've been trying to write a minimax algorithm for tic-tac-toe. But I can't get my program to return the expected value and I'm not sure what's going on. My minimax method should be returning 1, but instead pretty much broken. I believe it's using the original list instead of creating a new one. I tried using copy() but it didn't work, unless I used it wrong. Here is my code:

import copy

def checkWin(board):
    if board == [['y', 'x', 'y'], ['x', 'y', 'x'], ['x', 'y', 'x']]:
        return 't'
    if board == [['y', 'x', 'y'], ['x', 'y', 'x'], ['x', 'x', 'y']]:
        return 'y'
    return '0'
    
def minimax(board, player):
    status = checkWin(board)
    if status == 't':
        return 0
    if status == 'y':
        return 1

    if player == 'y':
        value = -100000
        for i in range(3):
            for k in range(3):
                if board[i][k] == '':
                    board2 = copy.deepcopy(board)
                    board2[i][k] == 'y'
                    bestMove = minimax(board2, 'x')
                    if bestMove > value:
                        value = bestMove
        return value
                    
    else:
        value = 100000
        for i in range(3):
            for k in range(3):  
                if board[i][k] == '':
                    board2 = copy.deepcopy(board)
                    board2[i][k] == 'x'
                    bestMove = minimax(board2, 'y')         
                    if bestMove < value:
                        value = bestMove

        return value

board = [
            ['y', 'x', 'y'],
            ['x', 'y', 'x'],
            ['x', 'x', '']
        ]               
bestMove = minimax(board, 'y')
print(bestMove)

Any help is appreciated, thanks.

Edit. Thanks to the people who helped, I'm sorry I'm such an idiot. But I will make sure to learn how to debug my programs more carefully and thoroughly.

Answer 1

You want to assign value not compare board2[i][k] == 'x' should be board2[i][k] = 'x' same with y.

import copy

def checkWin(board):
    if board == [['y', 'x', 'y'], ['x', 'y', 'x'], ['x', 'y', 'x']]:
        return 't'
    if board == [['y', 'x', 'y'], ['x', 'y', 'x'], ['x', 'x', 'y']]:
        return 'y'
    return '0'
    
def minimax(board, player):
    status = checkWin(board)
    if status == 't':
        return 0
    if status == 'y':
        return 1

    if player == 'y':
        value = -100000
        for i in range(3):
            for k in range(3):
                if board[i][k] == '':
                    board2 = copy.deepcopy(board)
                    board2[i][k] = 'y'
                    bestMove = minimax(board2, 'x')
                    if bestMove > value:
                        value = bestMove
        return value
                    
    else:
        value = 100000
        for i in range(3):
            for k in range(3):  
                if board[i][k] == '':
                    board2 = copy.deepcopy(board)
                    board2[i][k] = 'x'
                    bestMove = minimax(board2, 'y')         
                    if bestMove < value:
                        value = bestMove

        return value

board = [
            ['y', 'x', 'y'],
            ['x', 'y', 'x'],
            ['x', 'x', '']
        ]               
bestMove = minimax(board, 'y')
print(bestMove)