我在Python中收到TypeError。如何解决“ NoneType”对象不可迭代错误？

Question

我正在创建二十一点模拟程序。我有两个班级，分别是手牌和策略。在策略课程中，我有一个玩功能，应该玩一手牌。我得到TypeError'NoneType'对象是不可迭代的。我的代码如下。我究竟做错了什么？我该如何解决？

import random
import sys
import csv
from collections import namedtuple
from collections import defaultdict

Stand = namedtuple('Stand', 'stand total')

class Hand:
"""Class that encapsulates a blackjack hand."""

    def __init__(self, cards=None):
        super(Hand, self).__init__()
        self.cards = cards
        self.total, self.soft_ace_count = self.score()

    def __str__(self):
        return str(self.cards)

    def get_card():
        return random.randint(1, 13)

    def add_card(self):
        self.cards.append(get_card())
        self.score()


    def is_blackjack(self):
        if self.total == 21 and self.soft_ace_count == 1 and 10 in self.cards or 11 in self.cards or 12 in self.cards or 13 in self.cards:
        if 10 in self.cards or 11 in self.cards or 12 in self.cards or 13 in self.cards:
            return True
        return False


    def is_bust(self):
        return self.total > 21

    def score(self):
        self.total = 0
        self.soft_ace_count = 0

        for card in self.cards:
            if card == 1:
                self.total+=11
                self.soft_ace_count+=1
            elif card == 11 or card == 12 or card == 13:
                self.total+=10
            else:
                self.total+=card

        for x in self.cards:
            if self.total > 21 and self.soft_ace_count == 1:
                if x == 1:
                    self.total-=10
                    self.soft_ace_count-=1
        return (self.total, self.soft_ace_count)

class Strategy:
"""docstring for Strategy."""

    def __init__(self, stand_on_value, stand_on_soft):
        super(Strategy, self).__init__()
        self.stand_on_value = stand_on_value
        self.stand_on_soft = stand_on_soft

    def stand(self, hand):
        total, soft_ace_count = hand.score()
        if total < self.stand_on_value:
            return False
        if total > self.stand_on_value:
            return True
        if soft_ace_count > 0 and not self.stand_on_soft:
            return False
        else:
            return True


    def play(self):
        h = Hand(self.cards)
        while not self.stand(h):
            h.self.cards.append(get_card())
            self.stand(h.self.cards)
        return h.self.cards