python接口,继承,重载运算符详解

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

5. 继承顺序

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}

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

总结

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目录

1. 序列__getitem__

2. __setitem__