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
- 總結
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
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}
- 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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