自制Python残局国际象棋引擎遇AssertionError报错求助

问题场景 #

自主开发残局国际象棋引擎时，运行代码触发AssertionError，报错提示san() and lan() expect move to be legal or null, but got f6g6 in 6k1/8/6K1/2Q5/8/8/8/8 b - - 1 1。

原始代码 #

import chess
import time

def ddfs(board, depth, is_white_turn):
    if board.is_checkmate():
        return True, []
    if depth == 0 or board.is_game_over():
        return False, []
    legal_moves = list(board.legal_moves)
    for move in legal_moves:
        board.push(move)
        found_mate, mate_path = ddfs(board, depth - 1, not is_white_turn)
        board.pop()
        if found_mate:
            return True, [move] + mate_path
    return False, []

def iterative_deepening_dfs(board):
    depth = 0
    while True:
        found_mate, mate_sequence = ddfs(board.copy(), depth, board.turn == chess.WHITE)
        if found_mate:
            return depth, mate_sequence
        depth += 1

def print_move_sequence(board, move_sequence):
    print("Initial board state:")
    print(board, "\n")
    for move in move_sequence:
        if board.is_legal(move):
            board.push(move)
            print(f"Move {board.san(move)}:")
            print(board, "\n")
        else:
            print("An illegal move found in the sequence, which should not happen.")
            break

def main():
    initial_fen = "6k1/8/5K2/2Q5/8/8/8/8 w - - 0 1"
    board = chess.Board(initial_fen)

    start_time = time.time()
    mate_depth, mate_sequence = iterative_deepening_dfs(board)
    elapsed_time = time.time() - start_time

    if mate_depth is not None:
        print(f"Mate found in {mate_depth} move(s), time taken: {elapsed_time:.2f} seconds.")
        board.reset()
        board.set_fen(initial_fen)  # Reset the board to initial state
        print_move_sequence(board, mate_sequence)
    else:
        print("No mate found.")

if __name__ == "__main__":
    main()

报错信息 #

Mate found in 3 move(s), time taken: 0.00 seconds.

Initial board state:
     . . . . . . k .
     . . . . . . . .
     . . . . . K . .
     . . Q . . . . .
     . . . . . . . .
     . . . . . . . .
     . . . . . . . .
     . . . . . . . . 
    ---------------------------------------------------------------------------
    AssertionError                            Traceback (most recent call last)
    Cell In[19], line 55
         52         print("No mate found.")
         54 if __name__ == "__main__":
    ---> 55     main()
    
    Cell In[19], line 50, in main()
         48     board.reset()
         49     board.set_fen(initial_fen)  # Reset the board to initial state
    ---> 50     print_move_sequence(board, mate_sequence)
         51 else:
         52     print("No mate found.")
    
    Cell In[19], line 32, in print_move_sequence(board, move_sequence)
         30 if board.is_legal(move):
         31     board.push(move)
    ---> 32     print(f"Move {board.san(move)}:")
         33     print(board, "\n")
         34 else:
    
    File ~\myenvGPU32\Lib\site-packages\chess\__init__.py:2866, in Board.san(self, move)
       2861 def san(self, move: Move) -> str:
       2862     """
       2863     Gets the standard algebraic notation of the given move in the context
       2864     of the current position.
       2865     """
    -> 2866     return self._algebraic(move)
    
    File ~\myenvGPU32\Lib\site-packages\chess\__init__.py:2879, in Board._algebraic(self, move, long)
       2878 def _algebraic(self, move: Move, *, long: bool = False) -> str:
    -> 2879     san = self._algebraic_and_push(move, long=long)
       2880     self.pop()
       2881     return san
    
    File ~\myenvGPU32\Lib\site-packages\chess\__init__.py:2884, in Board._algebraic_and_push(self, move, long)
       2883 def _algebraic_and_push(self, move: Move, *, long: bool = False) -> str:
    -> 2884     san = self._algebraic_without_suffix(move, long=long)
       2886     # Look ahead for check or checkmate.
       2887     self.push(move)
    
    File ~\myenvGPU32\Lib\site-packages\chess\__init__.py:2920, in Board._algebraic_without_suffix(self, move, long)
       2917         return "O-O"
       2919 piece_type = self.piece_type_at(move.from_square)
    -> 2920 assert piece_type, f"san() and lan() expect move to be legal or null, but got {move} in {self.fen()}"
       2921 capture = self.is_capture(move)
       2923 if piece_type == PAWN:
    
    AssertionError: san() and lan() expect move to be legal or null, but got f6g6 in 6k1/8/6K1/2Q5/8/8/8/8 b - - 1 1

问题根源 #

在print_move_sequence函数中，执行board.push(move)后调用board.san(move)：

• push(move)会修改棋盘状态，将棋子从move.from_square移走，此时该位置为空
• san()方法需要基于走法执行前的棋盘状态生成代数记谱，此时无法找到起始位置的棋子，触发断言失败

修复方案 #

修改print_move_sequence函数，在push之前获取走法的代数记谱，避免修改原棋盘后再调用san()：

修改后的代码 #

import chess
import time

def ddfs(board, depth, is_white_turn):
    if board.is_checkmate():
        return True, []
    if depth == 0 or board.is_game_over():
        return False, []
    legal_moves = list(board.legal_moves)
    for move in legal_moves:
        board.push(move)
        found_mate, mate_path = ddfs(board, depth - 1, not is_white_turn)
        board.pop()
        if found_mate:
            return True, [move] + mate_path
    return False, []

def iterative_deepening_dfs(board):
    depth = 0
    while True:
        found_mate, mate_sequence = ddfs(board.copy(), depth, board.turn == chess.WHITE)
        if found_mate:
            return depth, mate_sequence
        depth += 1

def print_move_sequence(board, move_sequence):
    print("Initial board state:")
    print(board, "\n")
    for move in move_sequence:
        if board.is_legal(move):
            # 在push之前获取代数记谱
            move_san = board.san(move)
            board.push(move)
            print(f"Move {move_san}:")
            print(board, "\n")
        else:
            print("An illegal move found in the sequence, which should not happen.")
            break

def main():
    initial_fen = "6k1/8/5K2/2Q5/8/8/8/8 w - - 0 1"
    board = chess.Board(initial_fen)

    start_time = time.time()
    mate_depth, mate_sequence = iterative_deepening_dfs(board)
    elapsed_time = time.time() - start_time

    if mate_depth is not None:
        print(f"Mate found in {mate_depth} move(s), time taken: {elapsed_time:.2f} seconds.")
        board.reset()
        board.set_fen(initial_fen)  # Reset the board to initial state
        print_move_sequence(board, mate_sequence)
    else:
        print("No mate found.")

if __name__ == "__main__":
    main()

额外优化提示 #

1. 深度计数逻辑修正：当前iterative_deepening_dfs返回的depth是递归深度，但国际象棋中"mate in N"通常指进攻方走N步（包含防守方的应对），可根据需求调整深度计数逻辑
2. 剪枝优化：在ddfs中加入剪枝逻辑，跳过明显无法形成杀棋的走法，提升搜索效率
3. 终止条件完善：当前ddfs仅判断is_checkmate()和is_game_over()，可补充判断长将、和棋等情况

内容的提问来源于stack exchange，提问作者user22758952