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1. 序列getitem

如果沒有 __iter__ 和 __contains__ 方法，
Python 會調用 __getitem__ 方法，
設法讓迭代和 in 運算符可用

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
# 如果沒有 __iter__ 和 __contains__ 方法，
# Python 會調用 __getitem__ 方法，
# 設法讓迭代和 in 運算符可用

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))
# 牌被隨機打亂了
# 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')

或者在類外打補丁 FrenchDeck.__setitem__ = 函數(shù)

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

3. 抽象基類

class Test:
 def __len__(self):
  return 24
from collections import abc
print(isinstance(Test(), abc.Sized)) # True

只要實現(xiàn)了 __len__() 方法，就可以被 abc.Sized 識別為子類

4. 不要直接子類化內置類型

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'}

直接子類化內置類型（如 dict、list 或 str）容易出錯，因為內置類型的方法通常會忽略用戶覆蓋的方法。

不要子類化內置類型，用戶自己定義的類應該繼承 collections 模塊中的類，例如 UserDict、UserList 和 UserString，這些類做了特殊設計，因此易于擴展

5. 繼承順序

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}