Python之Sequence Protocol 序列协议

这里介绍Python的Sequence Protocol 序列协议

实践

对于我们自定义的对象，如果期望其可以支持序列的相关操作，只需对该类实现Sequence Protocol序列协议即可。这里我们提供了一个自定义类——一副扑克牌（不包含大小王）

class PlayingCard:
    """
    一副扑克牌
    """
    # 扑克牌花色
    suits = ["黑桃","红桃","梅花","方块"]
    # 扑克牌点数
    nums = ["A"] + [str(n) for n in range(2,11)] + ["J", "Q", "K"] 

    def __init__(self):
        self.cards = [suit+num for suit in PlayingCard.suits for num in PlayingCard.nums]
    
    def __len__(self):
        return len(self.cards)
    
    def __getitem__(self, index):
        return self.cards[index]
    
    def __setitem__(self, index, value):
        self.cards[index] = value

    def __delitem__(self, index):
        self.cards[index] = None

len方法

调用len()方法获取长度，Python会自动调用自定义对象的__len__()方法

print("-------------------len 方法 -----------------------")
playingCard = PlayingCard()
cards_num = len( playingCard )
print("一副扑克牌中的扑克牌总数:", cards_num)

索引访问

使用索引访问元素时，Python会自动调用自定义对象的__getitem__()方法

print("------------------- 索引访问 -----------------------")
playingCard = PlayingCard()
print("playingCard[0]: ", playingCard[0])
print("playingCard[14]: ", playingCard[14])
print("playingCard[-1]: ", playingCard[-1])

切片

切片时，Python会自动调用自定义对象的__getitem__()方法

print("------------------- 切片 -----------------------")
playingCard = PlayingCard()

print("playingCard[:4]: ", playingCard[:4])
print("playingCard[-4:-1]: ", playingCard[-4:-1])

迭代

for循环时我们虽然没有实现__iter__()方法，但是实现了__getitem__()方法。故Python会自动创建一个迭代器，尝试按从索引0开始按顺序获取元素

print("------------------- 循环迭代 -----------------------")
playingCard = PlayingCard()

count=1
for card in playingCard:
    print(f"第{count}张扑克牌: {card}")
    count = count+1
    if( count==4 ):
        break

索引修改

通过索引修改元素时，Python会自动调用自定义对象的__setitem__()方法

print("------------------- 索引修改 -----------------------")
playingCard = PlayingCard()

print("Before Modify: playingCard[:4]: ", playingCard[:4])
playingCard[0], playingCard[1], playingCard[2] = playingCard[1], playingCard[2], playingCard[0]
print("After Modify: playingCard[:4]: ", playingCard[:4])

del删除

使用del关键字删除元素时，Python会自动调用自定义对象的__delitem__()方法

print("------------------- del删除 -----------------------")
playingCard = PlayingCard()

print("Before Delete: playingCard[10:15]: ", playingCard[10:15])
del playingCard[11]
del playingCard[13]
print("After Delete: playingCard[10:15]: ", playingCard[10:15])

参考文献

1. Python编程·第3版：从入门到实践 Eric Matthes著
2. Python基础教程·第3版 Magnus Lie Hetland著
3. 流畅的Python·第1版 Luciano Ramalho著