父运行类方法

Question

我有这三个Python类：

class Card(object): 
    RANKS = ["2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K", "A"] 
    SUITS = ["c", "d", "h", "s"] 
    def __init__(self, rank, suit): 
     self.rank = rank 
     self.suit = suit 

    def __str__(self): 
     return self.rank + self.suit 

    def __lt__(self, other): 
     return self.value < other.value 

    def __gt__(self, other): 
     return self.value > other.value 

    @property 
    def value(self): 
     return RANKS.index(self.rank) + SUITS.index(self.suit)/4 


class Hand(object): 
    def __init__(self, cards = []): 
     self.cards = cards 
     self.tricks = 0 

    def __str__(self): 
     return " ".join([str(card) for card in self.cards]) 

    def add(self, card): 
     self.cards.append(card) 

    def remove(self, card): 
     self.cards.remove(card) 


class Deck(Hand): 
    def populate(self): 
     for rank in Card.RANKS: 
      for suit in Card.SUITS: 
       self.add(Card(rank, suit)) 

但是当我运行这段代码：

deck1 = Deck() 
deck1.populate() 
hand1 = Hand() 
print(hand1) 

卡打印的整个甲板。 Hand类似乎正在运行populate(self)。为什么？

Answer 1

你的问题是在这里：

def __init__(self, cards = []): 
     self.cards = cards 
     self.tricks = 0 

你看，像def __init__(self, cards=[]): Python的函数定义默认参数仅计算一次，当函数的定义是通过解释加载，所以他们的行为有点全球十岁上下。还要注意，该列表是一个可变对象，因此，根据定义，它可以更改它的元素。结果，当您拨打self.cards.append(card)时，它会附加到一次评估，每次都是相同的列表。 解决方法如下：

def __init__(self, cards=None): 
    if cards is None: 
     cards = [] 
    self.cards = cards 
    self.tricks = 0