python之使用魔术方法__getitem__和__len__

（1）像__getitem__这种由两个双下划线构成的方法，被称为魔术方法。

（2）魔术方法是为了给python解释器用的。当使用len(collection)时，实际上调用的就是collection.__len__方法。而在使用obj[key]的形式来访问元素时，实际上调用的是object.__getitem__(key)方法。

（3）魔术方法是属于类的方法，也就是说不需要实例化类就可以访问到该方法，同时，实例化的对象都可以访问到该方法。

（4）使用__getitem__和__len__方法，我们就可以实现一个对自定义数据类型的迭代和访问。

举个例子：

import collections
Card = collections.namedtuple("Card",["rank","suit"])
class FrenchDeck(object):
    ranks = [str(n) for n in range(2,11)] + list("JQKA")
    #黑桃 方块 红桃 梅花
    suits = "spades diamonds hearts clubs ".split()
    def __init__(self):
        self._cards = [Card(rank,suit) for rank in self.ranks
                       for suit in self.suits]
    def __getitem__(self, item):
        return self._cards[item]
    def __len__(self):
        return len(self._cards)

说明：

• Card = collections.namedtuple("Card",["rank","suit"])创建一个名字为Card的类，并且具有属性rank和suit，rank表示牌的数值，suit表示牌的花色。
• 一个FrenchDeck类，用于生成52张扑克牌，从2-A，红桃、方块、红桃、梅花。ranks、suits以及魔术方法都是类方法。

首先说明的是类属性和方法，直接用类名.属性或类名.方法访问即可。

print(FrenchDeck.ranks)
print(FrenchDeck.suits)
print(FrenchDeck.__len__(FrenchDeck()))

输出：

自定义的FrenchDeck类在重写了__getitem__和__len__方法之后，就可以对FrenchDeck实例化的对象进行类似于列表的操作。

1.得到对象的长度

deck = FrenchDeck()
print(len(deck))

输出：52

2.通过下标来获列表元素

print(deck[0])
print(deck[-1])

输出：

Card(rank='2', suit='spades') Card(rank='A', suit='clubs')

3.对列表进行遍历。当然也可以使用reversed方法进行翻转遍历

for d in reversed(deck):
    print(d)

输出：

Card(rank='A', suit='clubs') Card(rank='A', suit='hearts') Card(rank='A', suit='diamonds') Card(rank='A', suit='spades') Card(rank='K', suit='clubs') Card(rank='K', suit='hearts') Card(rank='K', suit='diamonds') Card(rank='K', suit='spades') Card(rank='Q', suit='clubs') Card(rank='Q', suit='hearts') Card(rank='Q', suit='diamonds') Card(rank='Q', suit='spades') Card(rank='J', suit='clubs') Card(rank='J', suit='hearts') Card(rank='J', suit='diamonds') Card(rank='J', suit='spades') Card(rank='10', suit='clubs') Card(rank='10', suit='hearts') Card(rank='10', suit='diamonds') Card(rank='10', suit='spades') Card(rank='9', suit='clubs') Card(rank='9', suit='hearts') Card(rank='9', suit='diamonds') Card(rank='9', suit='spades') Card(rank='8', suit='clubs') Card(rank='8', suit='hearts') Card(rank='8', suit='diamonds') Card(rank='8', suit='spades') Card(rank='7', suit='clubs') Card(rank='7', suit='hearts') Card(rank='7', suit='diamonds') Card(rank='7', suit='spades') Card(rank='6', suit='clubs') Card(rank='6', suit='hearts') Card(rank='6', suit='diamonds') Card(rank='6', suit='spades') Card(rank='5', suit='clubs') Card(rank='5', suit='hearts') Card(rank='5', suit='diamonds') Card(rank='5', suit='spades') Card(rank='4', suit='clubs') Card(rank='4', suit='hearts') Card(rank='4', suit='diamonds') Card(rank='4', suit='spades') Card(rank='3', suit='clubs') Card(rank='3', suit='hearts') Card(rank='3', suit='diamonds') Card(rank='3', suit='spades') Card(rank='2', suit='clubs') Card(rank='2', suit='hearts') Card(rank='2', suit='diamonds') Card(rank='2', suit='spades')

4.使用choice随机抽取一张牌

from random import choice
print(choice(deck))

5.利用in判断一张牌是否在对象列表中

print(Card("Q","clubs") in deck)

6.可以对牌组进行排序，按照花色桃心梅方以及数值大小

def spades_high(card):
    #首先得到该卡牌在数值中的位置
    rank_value = FrenchDeck.ranks.index(card.rank)
    #返回的是其位置*4+花色对应的级别
    #比如梅花2的大小为0，黑桃A的大小为12*4+3=51
    return rank_value*len(suit_values)+suit_values[card.suit]
#Card = Card("A","spades")
#print(spades_high(Card))
for card in sorted(deck,key=spades_high):
    print(card)

输出;

Card(rank='2', suit='diamonds') Card(rank='2', suit='clubs') Card(rank='2', suit='hearts') Card(rank='2', suit='spades') Card(rank='3', suit='diamonds') Card(rank='3', suit='clubs') Card(rank='3', suit='hearts') Card(rank='3', suit='spades') Card(rank='4', suit='diamonds') Card(rank='4', suit='clubs') Card(rank='4', suit='hearts') Card(rank='4', suit='spades') Card(rank='5', suit='diamonds') Card(rank='5', suit='clubs') Card(rank='5', suit='hearts') Card(rank='5', suit='spades') Card(rank='6', suit='diamonds') Card(rank='6', suit='clubs') Card(rank='6', suit='hearts') Card(rank='6', suit='spades') Card(rank='7', suit='diamonds') Card(rank='7', suit='clubs') Card(rank='7', suit='hearts') Card(rank='7', suit='spades') Card(rank='8', suit='diamonds') Card(rank='8', suit='clubs') Card(rank='8', suit='hearts') Card(rank='8', suit='spades') Card(rank='9', suit='diamonds') Card(rank='9', suit='clubs') Card(rank='9', suit='hearts') Card(rank='9', suit='spades') Card(rank='10', suit='diamonds') Card(rank='10', suit='clubs') Card(rank='10', suit='hearts') Card(rank='10', suit='spades') Card(rank='J', suit='diamonds') Card(rank='J', suit='clubs') Card(rank='J', suit='hearts') Card(rank='J', suit='spades') Card(rank='Q', suit='diamonds') Card(rank='Q', suit='clubs') Card(rank='Q', suit='hearts') Card(rank='Q', suit='spades') Card(rank='K', suit='diamonds') Card(rank='K', suit='clubs') Card(rank='K', suit='hearts') Card(rank='K', suit='spades') Card(rank='A', suit='diamonds') Card(rank='A', suit='clubs') Card(rank='A', suit='hearts') Card(rank='A', suit='spades')

说了这么多，就是为了说明通过实现__getitem__和__len__方法，FrenchDeck就和一个python自有的序列数据类型一样了。对合成的运用使得__len__和__getitem__的具体实现可以代理给self._card这个python列表。