“自我”如何正确更新原始变量？N皇后问题中的递归/回溯（Python）

这是我解决8皇后问题的python程序。除了打印解决方案的电路板的最后一步之外，所有步骤都可以正常工作。我使用递归/回溯功能在木板上填入皇后，直到找到解决方案为止。拥有解决方案的电路板对象是self，这是对的引用b1，因此我假设将b1初始化的原始电路板更新为包含最终解决的电路板，并使用来打印解决方案printBoard。但是，b1由于某种未知的原因，当我打印它时，它没有被更新并且持有一块失败的电路板。

编辑：增加了placeQueen在solve

EMPTY = 0
QUEEN = 1
RESTRICTED = 2

class Board:

    # initializes a 8x8 array
    def __init__ (self):
        self.board = [[EMPTY for x in range(8)] for y in range(8)]

    # pretty prints board
    def printBoard(self):
        for row in self.board:
            print(row)

    # places a queen on a board
    def placeQueen(self, x, y):
        # restricts row
        self.board[y] = [RESTRICTED for i in range(8)]

        # restricts column
        for row in self.board:
            row[x] = RESTRICTED

        # places queen
        self.board[y][x] = QUEEN

        self.fillDiagonal(x, y, 0, 0, -1, -1)   # restricts top left diagonal
        self.fillDiagonal(x, y, 7, 0, 1, -1)    # restructs top right diagonal
        self.fillDiagonal(x, y, 0, 7, -1, 1)    # restricts bottom left diagonal
        self.fillDiagonal(x, y, 7, 7, 1, 1)     # restricts bottom right diagonal

    # restricts a diagonal in a specified direction
    def fillDiagonal(self, x, y, xlim, ylim, xadd, yadd):
        if x != xlim and y != ylim:
            self.board[y + yadd][x + xadd] = RESTRICTED
            self.fillDiagonal(x + xadd, y + yadd, xlim, ylim, xadd, yadd)

    # recursively places queens such that no queen shares a row or
    # column with another queen, or in other words, no queen sits on a
    # restricted square. Should solve by backtracking until solution is found.
    def solve(self, col):

        if col == -1:
            return True

        for i in range(8):
            if self.board[i][col] == EMPTY:
                temp = self.copy()

                self.placeQueen(col, i)
                if self.solve(col - 1):
                    return True

                temp.board[i][col] = RESTRICTED
                self = temp.copy()

        return False

    # deep copies a board onto another board
    def copy(self):
        copy = Board()

        for i in range(8):
            for j in range (8):
                copy.board[j][i] = self.board[j][i]

        return copy


b1 = Board()
b1.solve(7)
b1.printBoard()

我知道我的实际求解器正在工作，因为当我添加printBoard如下代码时：

if col == -1:
    self.printBoard()
    return True

在该solve方法中，打印出已解决的板。简而言之，为什么self董事会实例未更新b1？