父类的Python子类调用方法

Question

I'm currently self-learning Python, and I came to the lesson about inheritance, so this is my first experience with subclasses. I'm learning from the book "Think Python", and the example is about creating classes that represent playing cards, decks, hands etc. The following example is copied from this book (with myself added method or two, as an exercise):

import random
from operator import itemgetter, attrgetter

class Card:
"""represents a standard playing card."""
    def __init__(self, suit=0, rank=2):
        self.suit = suit
        self.rank = rank

    suit_names = ['Clubs', 'Diamonds', 'Hearts', 'Spades']
    rank_names = [None, 'Ace', '2', '3', '4', '5', '6', '7', '8', '9', '10', 'Jack', 'Queen', 'King']

    def __str__(self):
        return '%s of %s' % (Card.rank_names[self.rank], Card.suit_names[self.suit])

    def __cmp__(self, other):
        t1 = self.suit, self.rank
        t2 = other.suit, other.rank
        return cmp(t1, t2)

class Deck:
"""represents a standard deck of cards"""
    def __init__(self):
        self.cards = []
        for suit in range(4):
            for rank in range(1, 14):
                card = Card(suit, rank)
                self.cards.append(card)

    def __str__(self):
        res = []
        for card in self.cards:
            res.append(str(card))
        return '\n'.join(res)

    def pop_card(self):
        return self.cards.pop()

    def add_card(self):
        self.cards.append(card)

    def shuffle(self):
        random.shuffle(self.cards)

    def sort(self):
        self.cards.sort(key=attrgetter('suit', 'rank'))

class Hand(Deck):
"""represents a hand of playing cards"""
    def __init__(self, label=''):
        self.cards = []
        self.label = label

    def move_cards(self, hand, num):
        for i in range(num):
            hand.add_card(self.pop_card())

My problem is with the Hand child class methods. If I try to invoke the move_cards method from the Hand child class, for example with a = Deck() , b = Hand() , trying to invoke move_cards(b,2) , or a.move_cards(b,2) gives me the same AttributeError: 'Deck' object has no attribute 'move_cards' .

Why is this happening or what I am doing wrong? Thanks.

Answer 1

move_cards is only defined for the subclass Hand , not the parent class Deck .

I think the order of inheritance is confusing in the example - Hand doesn't really feel like a subclass of Deck , rather they should both be subclasses of a common card collection ( Stack say). For example (not tested):

class Stack:
"""represents a collection of cards"""
    def __init__(self):
        self.cards = []

    def __str__(self):
        res = []
        for card in self.cards:
            res.append(str(card))
        return '\n'.join(res)

    def pop_card(self):
        return self.cards.pop()

    def add_card(self):
        self.cards.append(card)

    def shuffle(self):
        random.shuffle(self.cards)

    def sort(self):
        self.cards.sort(key=attrgetter('suit', 'rank'))

    def move_cards(self, dest, num):
        for i in range(num):
            dest.add_card(self.pop_card())

class Deck(Stack):
"""represents a deck of playing cards"""
    def __init__(self):
        Stack.__init__(self)
        for suit in range(4):
            for rank in range(1, 14):
                card = Card(suit, rank)
                self.cards.append(card)

class Hand(Stack):
"""represents a hand of playing cards"""
    def __init__(self, label=''):
        Stack.__init__(self)
        self.label = label

Notice how the overrided constructors for the subclasses Hand and Deck call the parent constructor explicitly with Stack.__init__(self) . This initialises the self.cards member in the same way for each.