文章目錄

• • 1. 序列`__getitem__`
  • 2. `__setitem__`
  • 3. 抽象基類
  • 4. 不要直接子類化內置類型
  • 5. 繼承順序
  • 6. 重載運算符

learn from 《流暢的python》

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 collectionsCard = 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 shuffledeck = 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

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

3. 抽象基類

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

只要實現了 __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 24ad = 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

import collectionsclass 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. 繼承順序

• 多重繼承的同名方法調用，根據類的 __mro__ 屬性順次在類中查找
• 推薦使用 super() 函數

6. 重載運算符

不可變類型，，一定不能實現就地特殊方法
__iadd__等增量賦值特殊方法 必須返回 self

總結

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