python的特殊方法

import collections
Card = collections.namedtuple('Card', ['rank', 'suit'])
class FrenDeck:
    ranks = [str(n) for n in range(2,11)] + list('JQKA')
    suits = 'a b c d'.split()
    def __init__(self):
        self._cards = [Card(rank,suit) for suit in self.suits
                                       for rank in self.ranks]
    def __len__(self):
        return len(self._cards)
    def __getitem__(self, position):
        return self._cards[position]
beer_card = Card('7', 'da')
beer_card
Out[3]: Card(rank='7', suit='da')

card = FrenDeck()

len(card)
Out[6]: 52

    def __len__(self):
        return 10

len(card)
Out[12]: 10