当你使用len(a)获取a的长度，使用obj[key]获取一个key的值时的畅快和舒适，在于Python庞大的设计思想（Pythonic）。
而obj[key]背后其实是__getitem__方法，Python实现的会被框架本身调用的方法之一。像__len__,__getitem__这样的方法称为特殊方法或者双下方法。
下面用一个例子展示如何实现这两个特殊方法，以及见证特殊方法的强大：
此处用到collections模块中的namedtuple方法，返回一个具名元组子类。
# -*- coding: utf-8 -*-
# Nola import collections
from random import choice Card = collections.namedtuple('Card', ['rank', 'suit']) # 返回具有命名字段的元组的新子类 表示一张牌 suit_values = dict(spades=3, hearts=2, diamonds=1, clubs=0) # ♣->♦->♥->♠ 2-A 花色和大小从小到大 0(2梅花色)-51(A黑桃色) 花色大小映射 def spades_high(card):
"""
纸牌升序
:param card:
:return:
"""
rank_value = FrenchDeck.ranks.index(card.rank) # 获取纸牌大小index
return rank_value * len(suit_values) + suit_values[card.suit] # 纸牌大小index * 4色 + 花色权重 2clubs=0 2diamonds=1 class FrenchDeck(object):
# ['2', '3', '4', '5', '6', '7', '8', '9', '10', 'J', 'Q', 'K', 'A'] 13
ranks = [str(n) for n in range(2, 11)] + list('JQKA')
suits = 'spades diamonds clubs hearts'.split() # ['spades', 'diamonds', 'clubs', 'hearts'] 4 def __init__(self):
self._cards = [Card(rank, suit) for rank in self.ranks
for suit in self.suits] def __len__(self):
return len(self._cards) def __getitem__(self, position):
return self._cards[position] if __name__ == '__main__':
# namedtuple用法
Point = collections.namedtuple('Point', ['x', 'y'])
print(Point.__doc__) # docstring Point(x, y)
p = Point(11, 22)
print(p[0] + p[1]) # 使用index
x, y = p # 像元组一样解包
print(x, y)
print(p.x, p.y) # 使用字段名获取对应值
d = p._asdict() # 转为dict OrderedDict([('x', 11), ('y', 22)]) 11 22
print(d, d['x'], d['y'])
print(Point(**d)) # dict打包为新子类 Point(x=11, y=22)
print(p._replace(x=100)) # 类似str替换 Point(x=100, y=22)
print('@.@ '*20 + "\n"*2) # 得到一个纸牌对象
beer_card = Card('', 'diamonds')
print(beer_card) # Card(rank='7', suit='diamonds')
print('@.@ ' * 20 + "\n"*2) # len函数查看一叠牌有多少张
deck = FrenchDeck()
print(len(deck)) #
print('@.@ ' * 20 + "\n"*2) # 使用__getitem__方法从一叠牌中抽取特定的一张牌
print(deck[0]) # 第一张 Card(rank='2', suit='spandes')
print(deck[-1]) # 最后一张 Card(rank='A', suit='hearts')
print('@.@ ' * 20 + "\n" * 2) # 使用python内置函数random.choice随机抽取一张牌
print(choice(deck)) # Card(rank='9', suit='hearts')
print(choice(deck)) # Card(rank='3', suit='diamonds')
print('@.@ ' * 20 + "\n" * 2) # 抽取最上面3张
print(deck[:3])
# 只看牌面是A的牌
print(deck[-4:])
print(deck[48:])
print('@.@ ' * 20 + "\n" * 2) # 仅仅实现__getitem__方法，这一摞牌就编程可迭代了
for card in deck[:5]:
print(card)
print('@.@ ' * 20 + "\n" * 2) # 反迭代reverse list
for card in reversed(deck):
if card.rank == "K":
break
print(card)
print('@.@ ' * 20 + "\n" * 2) # in运算符可用在FrenchDeck类上，因为它是可迭代的
print(Card('Q', 'diamonds') in deck)
print(Card('S', 'diamonds') in deck)
print('@.@ ' * 20 + "\n" * 2) # spades_high函数对牌进行升序排序 标准库中sorted函数自增可根据指定key自增排序 reverse参数为True倒序
for card in sorted(deck[:8], key=spades_high, reverse=True):
print(card)
# Card(rank='3', suit='spades')
# Card(rank='3', suit='hearts')
# Card(rank='3', suit='diamonds')
# Card(rank='3', suit='clubs')
# Card(rank='2', suit='spades')
# Card(rank='2', suit='hearts')
# Card(rank='2', suit='diamonds')
# Card(rank='2', suit='clubs')
print('@.@ ' * 20 + "\n" * 2)