MinMax function crashes on depth = 3-CodePudding

I'm writing a chess AI by using a convolutional neural network to evaluate a specific board state, and then I'm using that evaluation to apply a min_max algorithm to get the AI's move. When I go passed a depth of 2 on my algorithm I get an error about comparing tuples to scalars.

def NN_evaluate(board):
    board3d = split_dims(board)
    board3d = np.expand_dims(board3d, 0)
    return model.predict(board3d)[0][0]


def minimax(board, depth, alpha, beta, maximizing_player):
    if depth == 0 or board.is_game_over(): 
        return NN_evaluate(board)
    moves = board.legal_moves
    
    if maximizing_player:
        max_eval = -np.Inf
        for move in moves:
            board.push(move)
            current_eval = minimax(board, depth-1, alpha, beta, False)
            board.pop()
            max_eval = max(max_eval, current_eval)
            best_move = move
            alpha = max(alpha, current_eval)
            if beta <= alpha:
                break
            return max_eval
    else: 
        min_eval = np.Inf
        for move in moves:
            board.push(move)
            current_eval = minimax(board, depth-1, alpha, beta, True)
            board.pop()
            min_eval = min(min_eval, current_eval)
            best_move = move
            beta = min(beta, current_eval) 
            if beta <= alpha:
                break 
        return min_eval
    
def get_ai_move(board, depth, maximizing_player):
    max_move = None
    max_eval = -np.inf  
    for move in board.legal_moves:
        board.push(move)
        current_eval = minimax(board, depth-1, -np.inf, np.inf, False)
        board.pop()
        if current_eval > max_eval:
            max_eval = current_eval
            max_move = move
    return max_move


board = chess.Board()

with chess.engine.SimpleEngine.popen_uci('C:\\Users\\coope\\Downloads\\Python\\Machine Learning\\Chess AI\\stockfish_15_win_x64_avx2\\stockfish_15_x64_avx2.exe') as engine:
    while True:
        move = get_ai_move(board, 3, True)
        board.push(move)
        print(f'\n{board}')
        if board.is_game_over():
            break

        move = engine.analyse(board, chess.engine.Limit(time=1), info=chess.engine.INFO_PV)['pv'][0]
        board.push(move)
        print(f'\n{board}')
        if board.is_game_over():
            break

The error goes as

TypeError                                 Traceback (most recent call last)
~\AppData\Local\Temp/ipykernel_1108/561116885.py in <module>
      3 with chess.engine.SimpleEngine.popen_uci('C:\\Users\\coope\\Downloads\\Python\\Machine Learning\\Chess AI\\stockfish_15_win_x64_avx2\\stockfish_15_x64_avx2.exe') as engine:
      4     while True:
----> 5         move = get_ai_move(board, 3, True)
      6         board.push(move)
      7         print(f'\n{board}')

~\AppData\Local\Temp/ipykernel_1108/3382320008.py in get_ai_move(board, depth, maximizing_player)
     40     for move in board.legal_moves:
     41         board.push(move)
---> 42         current_eval = minimax(board, depth-1, -np.inf, np.inf, False)
     43         board.pop()
     44         if current_eval > max_eval:

~\AppData\Local\Temp/ipykernel_1108/3382320008.py in minimax(board, depth, alpha, beta, maximizing_player)
     28             current_eval = minimax(board, depth-1, alpha, beta, True)
     29             board.pop()
---> 30             min_eval = min(min_eval, current_eval)
     31             best_move = move
     32             beta = min(beta, current_eval)

TypeError: '>' not supported between instances of 'float' and 'NoneType'

This seems to be an issue with my min(min_eval, current_eval) but I'm unsure how to fix it ifget_ai_move(board, 2, True) doesn't crash.

CodePudding user response：

Your

    return max_eval

is inside the for loop, and should be outside. Written like this there is a situation in which it does not execute, and thus the whole function returns None (default return value in python if you do not return anything in a function)