OOP为不同的实例返回相同的值[重复]

Question

您好，我正在为黑杰克创建一个小测试脚本，在使用 OOP 时，我发现它为两个不同的实例返回相同的值

import random

color=['Hearts', 'Diamonds', 'Spades', 'Clubs']
shape=['Jack', 'Queen', 'King']
deck={}
for x in color:
    for i in range(1,11):
        commpination=str(i)+" "+x
        deck[commpination] = i
    for y in shape:
        commpination=y+" "+x
        deck[commpination] = 10
class Player:
    def __init__(self,name='Dealer',money=0,bet=0,player_cards=[],player_value=0):
        self.name=name
        self.money=money
        self.bet=bet
        self.player_cards=player_cards
        self.player_value=player_value

    def hit(self):
        global deck
        #here will choose random card from the cards and add it to the player deck, 
        #will return Random key and put it in the new_card list
        new_card=random.choice(list(deck.keys()))
        #remove the choosen card from the deck
        deck.pop(new_card)
        self.player_cards.append(new_card)

ahmed=Player('Ahmed',200,20)    # create new instance called Ahmed
sara=Player()                   # create new instance called Sara
ahmed.name                      # outputs 'Ahmed'
mohamed.name                    # outputs 'Dealer'
ahmed.hit()                     # run the method on ahmed
ahmed.player_cards              # returns '5 Hearts'
mohamed.player_cards            # returns '5 Hearts'

我的问题是，为什么 mohamed.player_cards 在没有首先运行 mohamed.hit() 的情况下返回 '5 Hearts' 并且我的脚本中的所有内容，每当我在第一个实例上运行方法时，我发现第二个实例也被修改了？

Answer 1

>>> sara.player_cards is ahmed.player_cards
True

[] 默认值在定义时计算一次，因此两个实例引用同一个列表。

使用 None 作为默认值。

class Player:
    def __init__(self,name='Dealer',money=0,bet=0,player_cards=None,player_value=0):
        self.name=name
        self.money=money
        self.bet=bet
        self.player_cards=player_cards if player_cards is not None else []
        self.player_value=player_value

通常您可以通过测试任何“虚假”值而不是明确地测试 None（参见 Is there shorthand for returning a default value if None in Python?）

        self.player_cards=player_cards or []