python接口,继承,重载运算符详解(方法子类)

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}