《流畅的python 第一章 python数据模型》

2021-07-06 20:50:27

第一章 python数据模型

摘要：本章主要讲一些特殊方法（前后带双下划线写法的方法，如__len__，__getitem__），为什么有特殊方法，特殊方法的应用。

特殊方法

python是一种面向对象语言，那么为什么会有len(x)这种写法，而不是x.len()呢

Python 解释器遇到len()这种特殊句法时，会使用特殊方法去激活一些基本的对象操作，这些特殊方法的名字以两个下划线开头，以两个下划线结尾。如__len__，__getitem__。比如obj[key]背后就是__getitem__方法，为了能求得obj[key]的值，解释器实际上调用的是obj.__getitem__(key)。

通过合理使用这些方法，能让自己的对象实现和支持以下语言结构，并与之交互：

迭代
集合类
属性访问
运算符重载
函数和方法的调用
对象的创建和销毁
字符串表示形式和格式化
管理上下文（with模块）

魔术方法（magic method）是特殊方法的昵称，又叫双下方法（dunder method）。

一摞python风格的纸牌

用一个非常简单的例子来展示如何实现__getitem__、__len__这两个特殊方法。

import collections
Card=collections.namedtuple('Card',['rank','suit'])
#使用collections.namedtuple创建了一个元组类型的子类，子类类名为Card，子类中有两个key，key值分别为'rank'、'suit'
class FrenchDeck:
    ranks=[str(n) for n in range(2,11)]+list('JQKA')#ranks=['2', '3', '4', '5', '6', '7', '8', '9', '10', 'J', 'Q', 'K', 'A']
    suits='spades diamonds clubs hearts'.split()#suits=['spades', 'diamonds', 'clubs', 'hearts']
    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, item):
        return self._cards[item]

利用namedtuple，我们可以很轻松地得到一个纸牌对象：

bear_card=Card(rank='7',suit='spades')
print(bear_card)

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
Card(rank='7', suit='spades')

Process finished with exit code 0

然后，我们来关注FrenchDeck这个类，将它实例化后，我们可以用len()函数来查看这叠纸牌共有多少张。

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

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
52

Process finished with exit code 0

也可以使用index下标来访问特定的纸牌：

print(deck[5])

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
Card(rank='3', suit='diamonds')

Process finished with exit code 0

如果在FrenchDeck类中注释掉__len__方法，就无法使用len(deck)，同理，如果在FrenchDeck类中注释掉__getitem__方法，就无法使用deck[key]访问特定元素。

而且我们不需要再写一个方法用来随机抽取一张纸牌，只需要调用python内置的random.choice。

from random import choice
deck=FrenchDeck()
print(choice(deck))

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
Card(rank='A', suit='spades')

Process finished with exit code 0

从上述试验中，我们发现了通过实现特殊方法来利用python数据模型的两个好处：

用户实例化该类后，不必费心去记标准操作的格式名称，比如说获取元素总数，是size方法还是length方法
可以更加方便的利用python标准库，而不需要重新自己写代码

并且由于__getitem__方法把下标访问[]的操作交给了self._cards列表，deck自动支持了切片操作。

print(deck[:3])
print(deck[12::13])

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
[Card(rank='2', suit='spades'), Card(rank='2', suit='diamonds'), Card(rank='2', suit='clubs')]
[Card(rank='5', suit='spades'), Card(rank='8', suit='diamonds'), Card(rank='J', suit='clubs'), Card(rank='A', suit='hearts')]

Process finished with exit code 0

另外，仅仅实现了__getitem__方法，deck就变成可迭代的了：

for card in deck:
    if card.rank=='J':
        print(card)
    else:
        pass

上述代码找到所有’J’的纸牌

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
Card(rank='J', suit='spades')
Card(rank='J', suit='diamonds')
Card(rank='J', suit='clubs')
Card(rank='J', suit='hearts')

Process finished with exit code 0

反向迭代也可以：

for card in reversed(deck):
    if card.rank=='J':
        print(card)
    else:
        pass

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
Card(rank='J', suit='hearts')
Card(rank='J', suit='clubs')
Card(rank='J', suit='diamonds')
Card(rank='J', suit='spades')

Process finished with exit code 0

迭代通常是隐式的，比如说一个集合类型没有实现__contains__方法，那么in运算符就会按顺序做一次迭代搜索，因为它是可迭代的：

print(Card(rank='J', suit='hearts') in deck)
print(Card(rank='O', suit='hearts') in deck)

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
True
False

Process finished with exit code 0

给纸牌排序：2最小，A最大，黑桃最大，红桃次之，方块再次，梅花最小。

suit_values=dict(spades=3,hearts=2,diamonds=1,clubs=0)#spades:黑桃 hearts:红桃 diamonds:方块 clubs:梅花
# spades A>hearts A>diamonds A>clubs A>
# spades K>hearts K>diamonds K>clubs K>
# spades Q>hearts Q>diamonds Q>clubs Q>
# spades J>hearts J>diamonds J>clubs J>
# spades 10>hearts 10>diamonds 10>clubs 10>
# spades 9>hearts 9>diamonds 9>clubs 9>
# spades 8>hearts 8>diamonds 8>clubs 8>
# spades 7>hearts 7>diamonds 7>clubs 7>
# spades 6>hearts 6>diamonds 6>clubs 6>
# spades 5>hearts 5>diamonds 5>clubs 5>
# spades 4>hearts 4>diamonds 4>clubs 4>
# spades 3>hearts 3>diamonds 3>clubs 3>
#  1*4+3    1*4+2     1*4+1     1*4+0
# spades 2>hearts 2>diamonds 2>clubs 2
#  0*4+3    0*4+2     0*4+1     0*4+0
def spades_high(card):
    rank_value=FrenchDeck.ranks.index(card.rank)
    return rank_value*len(suit_values)+suit_values[card.suit]#排序规则的公式

for card in sorted(deck,key=spades_high):#先将card传入spades_high()函数进行计算，按最终返回值排序
    print(card,spades_high(card))

有了spades_high函数，就能对这摞纸牌进行升序排序了。

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
Card(rank='2', suit='clubs') 0
Card(rank='2', suit='diamonds') 1
Card(rank='2', suit='hearts') 2
Card(rank='2', suit='spades') 3
Card(rank='3', suit='clubs') 4
Card(rank='3', suit='diamonds') 5
Card(rank='3', suit='hearts') 6
Card(rank='3', suit='spades') 7
Card(rank='4', suit='clubs') 8
Card(rank='4', suit='diamonds') 9
Card(rank='4', suit='hearts') 10
Card(rank='4', suit='spades') 11
Card(rank='5', suit='clubs') 12
Card(rank='5', suit='diamonds') 13
Card(rank='5', suit='hearts') 14
Card(rank='5', suit='spades') 15
Card(rank='6', suit='clubs') 16
Card(rank='6', suit='diamonds') 17
Card(rank='6', suit='hearts') 18
Card(rank='6', suit='spades') 19
Card(rank='7', suit='clubs') 20
Card(rank='7', suit='diamonds') 21
Card(rank='7', suit='hearts') 22
Card(rank='7', suit='spades') 23
Card(rank='8', suit='clubs') 24
Card(rank='8', suit='diamonds') 25
Card(rank='8', suit='hearts') 26
Card(rank='8', suit='spades') 27
Card(rank='9', suit='clubs') 28
Card(rank='9', suit='diamonds') 29
Card(rank='9', suit='hearts') 30
Card(rank='9', suit='spades') 31
Card(rank='10', suit='clubs') 32
Card(rank='10', suit='diamonds') 33
Card(rank='10', suit='hearts') 34
Card(rank='10', suit='spades') 35
Card(rank='J', suit='clubs') 36
Card(rank='J', suit='diamonds') 37
Card(rank='J', suit='hearts') 38
Card(rank='J', suit='spades') 39
Card(rank='Q', suit='clubs') 40
Card(rank='Q', suit='diamonds') 41
Card(rank='Q', suit='hearts') 42
Card(rank='Q', suit='spades') 43
Card(rank='K', suit='clubs') 44
Card(rank='K', suit='diamonds') 45
Card(rank='K', suit='hearts') 46
Card(rank='K', suit='spades') 47
Card(rank='A', suit='clubs') 48
Card(rank='A', suit='diamonds') 49
Card(rank='A', suit='hearts') 50
Card(rank='A', suit='spades') 51

Process finished with exit code 0

通过实现__len__和__getitem__这两个特殊方法，FrenchDeck这个类就像python自有的序列数据类型一样，拥有迭代、切片等python核心语言特性。

练习：创建一个书本类

Book=collections.namedtuple('Book',['name','editor','id'])
# a_book=Book(name='救赎',editor='lily',id='20211101')
# print(a_book)
class MyBook:
    names='abandon baby cloud dead'.split()
    editors='lili susan yang alice'.split()
    ids=[x for x in range(2,6)]
    def __init__(self):
        self._book=[Book(name,editor,id) for name in self.names for editor in self.editors for id in self.ids] #这种写法对names,editors,ids进行了全排列组合

    def __len__(self):
        return len(self._book)

    def __getitem__(self, item):
        return self._book[item]

b=MyBook()
print(len(b))
print(b[2])

执行结果如下：

/Users/zy/PycharmProjects/learnpython/venv/bin/python /Users/zy/PycharmProjects/learnpython/collect.py
64
Book(name='abandon', editor='lili', id=4)

Process finished with exit code 0

码农公寓

特殊方法

一摞python风格的纸牌

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