Python列表元素分配对象属性异常：发牌后所有玩家手牌相同

问题场景 #

随机向多名玩家分发52张编号卡牌，以下是编写的Python代码及运行结果：

分发卡牌的代码 #

import random

def deal(players):
    deck = [x for x in range(1,53)]
    random.shuffle(deck)
    internalObject = { 'cards': [], 'score': 0}
    starting = dict.fromkeys(players, internalObject)
    deckHasCards = True
    while (deckHasCards):
        for player in players:
            if deck == []:
                deckHasCards = False
                break
            card = deck.pop()
            starting[player]['cards'].append(card)
            card = 0

    return starting

def main():
    hands = deal(['Jack', 'Jill'])
    print(hands)

运行输出 #

{
'Jack': {'cards': [35, 31, 20, 15, 18, 26, 51, 19, 37, 21, 10, 44, 4, 9, 14, 52, 49, 29, 43, 5, 41, 47, 40, 7, 3, 25, 11, 22, 23, 48, 2, 32, 13, 30, 6, 16, 38, 34, 45, 36, 39, 28, 50, 24, 46, 42, 27, 1, 33, 17, 8, 12], 'score': 0}, 
'Jill': {'cards': [35, 31, 20, 15, 18, 26, 51, 19, 37, 21, 10, 44, 4, 9, 14, 52, 49, 29, 43, 5, 41, 47, 40, 7, 3, 25, 11, 22, 23, 48, 2, 32, 13, 30, 6, 16, 38, 34, 39, 28, 50, 24, 46, 42, 27, 1, 33, 17, 8, 12], 'score': 0}
}

问题 #

所有玩家最终手牌相同，请问代码存在什么问题？为何Python会给所有玩家分配同一张牌而非当前循环选中的玩家？

问题分析与修复 #

核心问题 #

问题出在starting = dict.fromkeys(players, internalObject)这一行：

• Python的dict.fromkeys()方法会把同一个对象的引用赋值给字典里的所有键。这里的internalObject是一个字典，所有玩家对应的value都是指向这个字典的指针，并非每个玩家拥有独立的字典实例。
• 所以当你执行starting[player]['cards'].append(card)时，不管循环到哪个玩家，都是在修改同一个cards列表，最终所有玩家的手牌自然完全相同。

修复代码 #

把创建玩家字典的方式改成字典推导式，为每个玩家生成独立的字典对象：

import random

def deal(players):
    deck = [x for x in range(1,53)]
    random.shuffle(deck)
    # 用字典推导式为每个玩家创建独立的卡牌字典
    starting = {player: {'cards': [], 'score': 0} for player in players}
    deckHasCards = True
    while deckHasCards:
        for player in players:
            if not deck:
                deckHasCards = False
                break
            card = deck.pop()
            starting[player]['cards'].append(card)
    return starting

def main():
    hands = deal(['Jack', 'Jill'])
    print(hands)

这样每个玩家都会拥有自己独立的cards列表，分发卡牌时各自累加，就不会出现手牌相同的问题了。

内容的提问来源于stack exchange，提问作者M Towseef Hossain