Python Connect Four Minimax Algorithm Issue

Question

I recently created a Tic Tac Toe Minimax Algorithm that successfully worked. Shortly after, I modified the code to work for Connect 4. However, the algorithm keeps crashing with the error

local variable 'bestMove' referenced before assignment (Referring to the variable bestMove in the minimax function)

The only methods I changed from the Tic Tac Toe to Connect 4 were the showBoard() , getAvailableMoves() , and getWinner() functions.

I suspect the issue may be in the getAvailableMoves() because the amount of available moves on a board is more variable in Connect 4 than Tic Tac Toe because of the whole stacking idea. While I did implement that in the function, perhaps I shouldn't be doing it there, not sure though.

Here is my code below:

minimax.py

class ComputerBrain :

    def __init__(self, player = "x") :
        self._squares = {}

    def createBoard(self) :
        for i in range(6) :
            for j in range(7):
                self._squares[i, j] = " "
        self.showBoard()
        print("\n")

    def showBoard(self) :
        print ()
        print("------------------------------")
        print("|", self._squares[0, 0], "|", self._squares[0, 1], "|", self._squares[0, 2], "|", self._squares[0, 3],
              "|", self._squares[0, 4], "|", self._squares[0, 5], "|", self._squares[0, 6], "|")
        print("------------------------------")
        print("|", self._squares[1, 0], "|", self._squares[1, 1], "|", self._squares[1, 2], "|", self._squares[1, 3],
              "|", self._squares[1, 4], "|", self._squares[1, 5], "|", self._squares[1, 6], "|")
        print("------------------------------")
        print("|", self._squares[2, 0], "|", self._squares[2, 1], "|", self._squares[2, 2], "|", self._squares[2, 3],
              "|", self._squares[2, 4], "|", self._squares[2, 5], "|", self._squares[2, 6], "|")
        print("------------------------------")
        print("|", self._squares[3, 0], "|", self._squares[3, 1], "|", self._squares[3, 2], "|", self._squares[3, 3],
              "|", self._squares[3, 4], "|", self._squares[3, 5], "|", self._squares[3, 6], "|")
        print("------------------------------")
        print("|", self._squares[4, 0], "|", self._squares[4, 1], "|", self._squares[4, 2], "|", self._squares[4, 3],
              "|", self._squares[4, 4], "|", self._squares[4, 5], "|", self._squares[4, 6], "|")
        print("------------------------------")
        print("|", self._squares[5, 0], "|", self._squares[5, 1], "|", self._squares[5, 2], "|", self._squares[5, 3],
              "|", self._squares[5, 4], "|", self._squares[5, 5], "|", self._squares[5, 6], "|")
        print("------------------------------")

    def getAvailableMoves(self) :
        self._availableMoves = []
        for i in range(6) :
            for j in range(7):
                is_legal = False
                if i == 5:
                    is_legal = True
                else:
                    if self._squares[i + 1, j] != " ":
                        is_legal = True
                    else:
                        is_legal = False

                if self._squares[i, j] == " " and is_legal:
                    self._availableMoves.append((i, j))
        return self._availableMoves

    def makeMove(self, position, player) :
        x, y = position
        self._squares[x, y] = player

    def complete(self) :
        if " " not in self._squares.values() :
            return True
        if self.getWinner() != None :
            return True
        return False

    def getWinner(self) :
        for player in ("x", "o") :
            for i in range(6):
                for j in range(7):
                    if i < 3:
                        if self._squares[i, j] == player and self._squares[i + 1, j] == player and self._squares[i + 2, j] == player and self._squares[i + 3, j] == player:
                            return player
                        if j < 4:
                            if self._squares[i, j] == player and self._squares[i, j + 1] == player and self._squares[i, j + 2] == player and self._squares[i, j + 3] == player:
                                return player
                            if self._squares[i, j] == player and self._squares[i + 1, j + 1] == player and self._squares[i + 2, j + 2] == player and self._squares[i + 3, j + 3] == player:
                                return player
                        if j > 2:
                            if self._squares[i, j] == player and self._squares[i + 1, j - 1] == player and self._squares[i + 2, j - 2] == player and self._squares[i + 3, j - 3] == player:
                                return player
                    if i >= 3 and j < 4:
                        if self._squares[i, j] == player and self._squares[i, j + 1] == player and self._squares[i, j + 2] == player and self._squares[i, j + 3] == player:
                            return player

        if " " not in self._squares.values() :
            return "tie"

        return None

    def getEnemyPlayer(self, player) :
        if player == "x" :
            return "o"
        return "x"

    def minimax(self, player, depth = 0) :
        self.showBoard()
        if player == "o":
            best = -10
        else:
            best = 10
        if self.complete() :
            if self.getWinner() == "x" :
                return -10 + depth, None
            elif self.getWinner() == "tie" :
                return 0, None
            elif self.getWinner() == "o" :
                return 10 - depth, None

        for move in self.getAvailableMoves() :

            self.makeMove(move, player)
            val, _ = self.minimax(self.getEnemyPlayer(player), depth+1)
            self.makeMove(move, " ")

            if player == "o" :
                if val > best :
                    best, bestMove = val, move
            else :
                if val < best :
                    best, bestMove = val, move

        return best, bestMove

run.py

from minimax import ComputerBrain

def findPlace(available, value):
    for i in range(7):
        x, y = available[i]
        if value == y:
            return x
    return False

def run():
    game = ComputerBrain()
    game.createBoard()

    print("Computer is X and you are O");
    print("Who goes first? Computer (1) or You (2): ");
    choice = int(input())

    if choice == 1:
        _, bestMove1 = game.minimax("x")
        game.makeMove(bestMove1, "x")
        game.showBoard()
        # game.makeMove(randint(0, 9), "x")
        # game.showBoard()

    while (not game.complete()):

        canMove = True;
        badMove = False;
        while canMove:
            if (badMove):
                print("No More Spaces Available Here")
            print("Please enter your move: ")
            new_move = int(input()) - 1
            move = findPlace(game.getAvailableMoves(), new_move)
            if move == False:
                badMove = True;
                canMove = True;
            else:
                game.makeMove((move, new_move), "o")
                badMove = False;
                canMove = False;

        game.showBoard();

        if game.complete():
            break;

        _, bestMove1 = game.minimax("x")
        print("best move: ", bestMove1)
        game.makeMove(bestMove1, "x")
        game.showBoard()

    if game.getWinner() != None:
        print("\n Result: ", game.getWinner())
    else:
        print("\n DRAW")

    print("\n New Game? (y, n)")
    answer = str(input())
    if answer == "y":
        print("\n \n \n")
        run()
    else:
        return


run()

Answer 1

As @RicardoAbe stated in his comment, I incorrectly assigned -10 and +10 values to the initial minimax function rather than setting them to negative and positive infinity.