[Python知识库]python 接口 、继承、重载运算符

learn from 《流畅的python》

1. 序列__getitem__

如果没有 __iter__ 和 __contains__ 方法，
Python 会调用 __getitem__ 方法，
设法让 迭代 和 in 运算符可用

class Foo:
    def __getitem__(self, pos):
        return range(0, 30, 10)[pos]


f = Foo()
print(f[1])  # 10
for i in f:
    print(i)  # 0, 10, 20
# 如果没有 __iter__ 和 __contains__ 方法，
# Python 会调用 __getitem__ 方法，
# 设法让迭代和 in 运算符可用

2. __setitem__

import collections

Card = collections.namedtuple('Card', ['rank', 'suit'])


class FrenchDeck:
    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]
	
	def __str__(self):
        return ",".join(str(i) for i in self._cards)

from random import shuffle

deck = FrenchDeck()
print(str(deck))
# Card(rank='2', suit='spades'),Card(rank='3', suit='spades'),Card(rank='4', suit='spades'),Card(rank='5', suit='spades'),Card(rank='6', suit='spades'),Card(rank='7', suit='spades'),Card(rank='8', suit='spades'),Card(rank='9', suit='spades'),Card(rank='10', suit='spades'),Card(rank='J', suit='spades'),Card(rank='Q', suit='spades'),Card(rank='K', suit='spades'),Card(rank='A', suit='spades'),Card(rank='2', suit='diamonds'),Card(rank='3', suit='diamonds'),Card(rank='4', suit='diamonds'),Card(rank='5', suit='diamonds'),Card(rank='6', suit='diamonds'),Card(rank='7', suit='diamonds'),Card(rank='8', suit='diamonds'),Card(rank='9', suit='diamonds'),Card(rank='10', suit='diamonds'),Card(rank='J', suit='diamonds'),Card(rank='Q', suit='diamonds'),Card(rank='K', suit='diamonds'),Card(rank='A', suit='diamonds'),Card(rank='2', suit='clubs'),Card(rank='3', suit='clubs'),Card(rank='4', suit='clubs'),Card(rank='5', suit='clubs'),Card(rank='6', suit='clubs'),Card(rank='7', suit='clubs'),Card(rank='8', suit='clubs'),Card(rank='9', suit='clubs'),Card(rank='10', suit='clubs'),Card(rank='J', suit='clubs'),Card(rank='Q', suit='clubs'),Card(rank='K', suit='clubs'),Card(rank='A', suit='clubs'),Card(rank='2', suit='hearts'),Card(rank='3', suit='hearts'),Card(rank='4', suit='hearts'),Card(rank='5', suit='hearts'),Card(rank='6', suit='hearts'),Card(rank='7', suit='hearts'),Card(rank='8', suit='hearts'),Card(rank='9', suit='hearts'),Card(rank='10', suit='hearts'),Card(rank='J', suit='hearts'),Card(rank='Q', suit='hearts'),Card(rank='K', suit='hearts'),Card(rank='A', suit='hearts')
shuffle(deck)  # TypeError: 'FrenchDeck' object does not support item assignment

• 加入__setitem__

    def __setitem__(self, key, value):
        self._cards[key] = value

shuffle(deck) 
print(str(deck))
# 牌被随机打乱了
# Card(rank='6', suit='hearts'),Card(rank='2', suit='spades'),Card(rank='3', suit='hearts'),Card(rank='A', suit='clubs'),Card(rank='7', suit='clubs'),Card(rank='Q', suit='diamonds'),Card(rank='5', suit='hearts'),Card(rank='6', suit='spades'),Card(rank='5', suit='diamonds'),Card(rank='8', suit='diamonds'),Card(rank='K', suit='hearts'),Card(rank='Q', suit='hearts'),Card(rank='8', suit='hearts'),Card(rank='3', suit='diamonds'),Card(rank='Q', suit='clubs'),Card(rank='A', suit='diamonds'),Card(rank='9', suit='hearts'),Card(rank='7', suit='diamonds'),Card(rank='A', suit='spades'),Card(rank='J', suit='hearts'),Card(rank='3', suit='clubs'),Card(rank='4', suit='hearts'),Card(rank='Q', suit='spades'),Card(rank='10', suit='diamonds'),Card(rank='10', suit='hearts'),Card(rank='K', suit='diamonds'),Card(rank='2', suit='diamonds'),Card(rank='4', suit='spades'),Card(rank='3', suit='spades'),Card(rank='7', suit='hearts'),Card(rank='9', suit='clubs'),Card(rank='J', suit='clubs'),Card(rank='J', suit='diamonds'),Card(rank='10', suit='spades'),Card(rank='4', suit='diamonds'),Card(rank='7', suit='spades'),Card(rank='9', suit='diamonds'),Card(rank='2', suit='hearts'),Card(rank='6', suit='clubs'),Card(rank='K', suit='clubs'),Card(rank='4', suit='clubs'),Card(rank='A', suit='hearts'),Card(rank='K', suit='spades'),Card(rank='2', suit='clubs'),Card(rank='J', suit='spades'),Card(rank='8', suit='spades'),Card(rank='5', suit='spades'),Card(rank='9', suit='spades'),Card(rank='10', suit='clubs'),Card(rank='8', suit='clubs'),Card(rank='6', suit='diamonds'),Card(rank='5', suit='clubs')

或者 在类外打补丁 FrenchDeck.__setitem__ = 函数

def set_card(deck, position, card):
    deck._cards[position] = card


FrenchDeck.__setitem__ = set_card

3. 抽象基类

class Test:
    def __len__(self):
        return 24


from collections import abc

print(isinstance(Test(), abc.Sized)) # True

只要实现了 __len__() 方法，就可以被 abc.Sized 识别为子类

4. 不要直接子类化内置类型

class AnswerDict(dict):
    def __getitem__(self, key):
        return 24


ad = AnswerDict(a="good")
print(ad['a'])  # 24
d = {}
d.update(ad)
print(d['a'])  # good
print(ad)  # {'a': 'good'}
print(d)  # {'a': 'good'}

直接子类化内置类型（如 dict、list 或 str）容易出错， 因为 内置类型的方法 通常会 忽略用户覆盖的方法。
不要子类化内置 类型，用户自己定义的类 应该继承 collections 模块 中的类，例如 UserDict、UserList 和 UserString，这些类做了特殊设计，因 此易于扩展

import collections


class AnswerDict2(collections.UserDict):
    def __getitem__(self, key):
        return 24
ad = AnswerDict2(a="good")
print(ad['a'])  # 24
d = {}
d.update(ad)
print(d['a'])  # 24
print(ad)  # {'a': 'good'}
print(d)  # {'a': 24}

5. 继承顺序

• 多重继承的同名方法调用，根据类的 __mro__ 属性顺次在类中查找
• 推荐使用 super() 函数

6. 重载运算符

不可变类型，，一定不能实现就地特殊方法
__iadd__等增量赋值特殊方法 必须返回 self