流畅的python笔记：第一章，python数据模型

主要知识：

1，__len__是被解释器调用的，使用时只需len(obj)

2，__repr__得到对象的字符串表现形式

3，if obj，实际执行的是bool(obj)，python解释器调用obj.__bool__，如果没有__bool__，会调用__len__看是否为0

4，len(obj)，python解释器调用obj.__len__()

5，obj[key]，python解释器调用obj.__getitem__(key)

6，除了__init__，通常不需要写特殊方法

eg1，python风格的纸牌

import collections
import random

Card = collections.namedtuple('Card', ['rank', 'suit'])  # 定义了Card类，包含'rank'和'suit'属性

class FrenchDeck(object):
    ranks = [str(n) for n in range(2, 11)] + list('JQKA')
    suits = 'spades diamonds clubs hearts'.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]

suit_values = {'spades':3, 'hearts':2, 'diamonds':1, 'clubs':0}
def spades_high(card):
    rank_value = FrenchDeck.ranks.index(card.rank)
    return rank_value * len(suit_values) + suit_values[card.suit]

print(Card('7', 'diamond'))  # Card(rank='7', suit='diamond')
deck = FrenchDeck()
print(len(deck))  # 52，调用了__len__
print(deck[0])  # Card(rank='2', suit='spades'), 调用了__getitem__
print(deck[-1])  # Card(rank='A', suit='hearts')
print(random.choice(deck))  # 随机抽一张卡牌
print(deck[:3])  # 上面3张：[Card(rank='2', suit='spades'), Card(rank='3', suit='spades'), Card(rank='4', suit='spades')]
print(deck[12::13])  # 打印4张A，第13张，然后每隔步长13打印
for card in deck:
    print(card)  # 打印所有卡牌
for card in reversed(deck):
    print(card)  # 反向打印所有卡牌
for card in sorted(deck, key=spades_high):  # 按suit_values指定的花色判断进行排序
    print(card)

eg2，一个简单的二维向量

from math import hypot

class Vector(object):

    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    def __repr__(self):
        return 'Vector(%r, %r)'%(self.x, self.y)  # 应该要打印返回字符串,r改成s也行

    def __abs__(self):
        return hypot(self.x, self.y)   # hypot返回 sqrt(x*x + y*y),sqrt开根号

    def __bool__(self):
        return bool(abs(self))

    def __add__(self, other):
        x = self.x + other.x
        y = self.y + other.y
        return Vector(x, y)

    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)


v1 = Vector(2, 4)
v2 = Vector(3, 4)
print(v1)   # Vector(2, 4)