最近编写了一个井字棋AI，命名为“洛书幻方”，用Python编的，结果有TypeError，求解决方法

代码：

def joiner(x: list, p: any):
    x.append(p)

# --------------------------------------------------------------------

def win(s: list):
    n: int = len(s)
    if n < 3:
        return False
    s = sorted(s)
    for i in range(n - 2):
        lft = i + 1
        rgt = n - 1
        while lft < rgt:
            total = s[i] + s[lft] + s[rgt]
            if total == 15:
                return True
            elif total < 15:
                lft = lft + 1
            else:
                rgt = rgt - 1
    return False


WIN = 10
LOSE = -10
TIE = 0
INF = 1000

# 洛书幻方
MAGIC_SQUARE = [4, 9, 2,
                3, 5, 7,
                8, 1, 6]


def _eval(x, o):
    if win(x):
        return WIN
    if win(o):
        return LOSE
    return TIE


def finished(x, o):
    return len(x) + len(o) >= 9


def minmax(x, o, depth, maximize):
    score = _eval(x, o)
    if score == WIN:
        return score - depth
    if score == LOSE:
        return score + depth
    if finished(x, o):
        return 0  # Draw
    best = -INF if maximize else INF
    for i in MAGIC_SQUARE:
        if (i not in x) and (
                i not in o):
            if maximize:
                best = max(
                    best,
                    minmax(
                        i + x,
                        o,
                        depth + 1,
                        not maximize))
            else:
                best = min(
                    best,
                    minmax(x,
                           i + o,
                           depth + 1,
                           not maximize))


def findbest(x, o, maximize):
    best = -INF if maximize else INF
    move = 0
    for i in MAGIC_SQUARE:
        if (i not in x) and (
                i not in o):
            if maximize:
                val = minmax(
                    joiner(x, i), o,
                    0,
                    not maximize)
                if val > best:
                    best = val
                    move = i
            else:
                val = minmax(
                    x, joiner(o, i),
                    0,
                    not maximize)
                if val < best:
                    best = val
                    move = i
    return move


def board(x, o):
    for row in range(3):
        print(" -----------")
        for col in range(3):
            p = MAGIC_SQUARE[
                row * 3 + col]
            if p in x:
                print(" |X", end="")
            elif p in o:
                print(" |O", end="")
            else:
                print(" | ", end="")
        print(" | ")
    print(" -----------")


# Let's go!
def play():
    print("A1 is 4, A2 is 9, A3 is 2, \n"
          "B1 is 3, B2 is 5, B3 is 7, \n"
          "C1 is 8, C2 is 1, C3 is 6. \n")
    x = []
    o = []
    while not (win(x) or win(
            o) or finished(x, o)):
        board(x, o)
        while True:
            i = int(input("Please input your piece: "))
            if (
                    i not in MAGIC_SQUARE) or (
                    MAGIC_SQUARE[i - 1] in x) or (
                    MAGIC_SQUARE[i - 1] in o):
                print("Invalid move!!!!!!!!")
            else:
                x.append(MAGIC_SQUARE[i - 1])
                break
        o.append(findbest(x, o, False))
    board(x, o)


if __name__ == '__main__':
    play()

报错信息：

TypeError: object of type 'NoneType' has no len()