在Python中，如果我們想實現(xiàn)創(chuàng)建類似于序列和映射的類（可以迭代以及通過[下標(biāo)]返回元素），可以通過重寫魔法方法__getitem__、__setitem__、__delitem__、__len__方法去模擬。

魔術(shù)方法的作用：

__getitem__(self,key):返回鍵對應(yīng)的值。
__setitem__(self,key,value)：設(shè)置給定鍵的值
__delitem__(self,key):刪除給定鍵對應(yīng)的元素。
__len__():返回元素的數(shù)量

【注釋】只要實現(xiàn)了__getitem__和 __len__方法，就會被認(rèn)為是序列。

• 可以用__len__():函數(shù)來查看對象長度
• __getitem__(self,key): 可以對對象進(jìn)行[]操作，如切片，索引，iterd等高級操作。
• 如果在類中定義了__getitem__()方法，那么他的實例對象（假設(shè)為P）就可以這樣P[key]取值。當(dāng)實例對象做P[key]運算時，就會調(diào)用類中的__getitem__()方法。

這些魔術(shù)方法的原理就是：當(dāng)我們對類的屬性item進(jìn)行下標(biāo)的操作時，首先會被__getitem__()、__setitem__()、__delitem__()攔截，從而執(zhí)行我們在方法中設(shè)定的操作，如賦值，修改內(nèi)容，刪除內(nèi)容等等。
這個方法應(yīng)該以與鍵相關(guān)聯(lián)的方式存儲值，以便之后能夠使用__setitem__來獲取。當(dāng)然，這個對象可變時才需要實現(xiàn)這個方法。

舉個栗子：
定義一副撲克牌（不包括大小王），對牌進(jìn)行洗牌，然后發(fā)牌。

mport collections
Card = collections.namedtuple('Card',['rank','suit'])
#也可以使用一個類來定義Card
# class Card:
#     def __init__(self,rank,suit):
#         self.rank = rank
#         self.suit = suit


class Puke:
    ranks = [str(n) for n in range(2,11) ] + list('JQKA')
    suits = "黑桃 方塊 梅花 紅心".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, item):
        return self._cards[item]

    def __setitem__(self, key, value):
        print(key,value)
        self._cards[key] = value

pk = Puke()

# print(pk._cards)
# for card in pk:
    print(card)
print(pk[2:6])
print(pk[12::13])
pk[1:3] = [Card(rank='A',suit='紅桃')] * 3
print(pk[1:3])

Output：

[Card(rank='2', suit='黑桃'), Card(rank='3', suit='黑桃'), Card(rank='4', suit='黑桃'), Card(rank='5', suit='黑桃'), Card(rank='6', suit='黑桃'), Card(rank='7', suit='黑桃'), Card(rank='8', suit='黑桃'), Card(rank='9', suit='黑桃'), Card(rank='10', suit='黑桃'), Card(rank='J', suit='黑桃'), Card(rank='Q', suit='黑桃'), Card(rank='K', suit='黑桃'), Card(rank='A', suit='黑桃'), Card(rank='2', suit='方塊'), Card(rank='3', suit='方塊'), Card(rank='4', suit='方塊'), Card(rank='5', suit='方塊'), Card(rank='6', suit='方塊'), Card(rank='7', suit='方塊'), Card(rank='8', suit='方塊'), Card(rank='9', suit='方塊'), Card(rank='10', suit='方塊'), Card(rank='J', suit='方塊'), Card(rank='Q', suit='方塊'), Card(rank='K', suit='方塊'), Card(rank='A', suit='方塊'), Card(rank='2', suit='梅花'), Card(rank='3', suit='梅花'), Card(rank='4', suit='梅花'), Card(rank='5', suit='梅花'), Card(rank='6', suit='梅花'), Card(rank='7', suit='梅花'), Card(rank='8', suit='梅花'), Card(rank='9', suit='梅花'), Card(rank='10', suit='梅花'), Card(rank='J', suit='梅花'), Card(rank='Q', suit='梅花'), Card(rank='K', suit='梅花'), Card(rank='A', suit='梅花'), Card(rank='2', suit='紅心'), Card(rank='3', suit='紅心'), Card(rank='4', suit='紅心'), Card(rank='5', suit='紅心'), Card(rank='6', suit='紅心'), Card(rank='7', suit='紅心'), Card(rank='8', suit='紅心'), Card(rank='9', suit='紅心'), Card(rank='10', suit='紅心'), Card(rank='J', suit='紅心'), Card(rank='Q', suit='紅心'), Card(rank='K', suit='紅心'), Card(rank='A', suit='紅心')]
[Card(rank='4', suit='黑桃'), Card(rank='5', suit='黑桃'), Card(rank='6', suit='黑桃'), Card(rank='7', suit='黑桃')]
[Card(rank='A', suit='黑桃'), Card(rank='A', suit='方塊'), Card(rank='A', suit='梅花'), Card(rank='A', suit='紅心')]
slice(1, 3, None) [Card(rank='A', suit='紅桃'), Card(rank='A', suit='紅桃'), Card(rank='A', suit='紅桃')]
[Card(rank='A', suit='紅桃'), Card(rank='A', suit='紅桃')]

【注意】:我們會發(fā)現(xiàn)output中，輸出了：slice(1, 3, None)，下面給出解釋。

切片原理

語法：

class slice(stop)
class slice(start, stop[, step])

參數(shù)說明：

• start -- 起始位置
• stop -- 結(jié)束位置
• step -- 間距

slice() 函數(shù)實現(xiàn)切片對象，主要用在切片操作函數(shù)里的參數(shù)傳遞。

slice用于規(guī)定序列的選取規(guī)則

舉兩個栗子來看看：

step = slice(0,5,2)
components = [11, 22, 66, 88, 99, 00, 123]
print(components[step])

Output:

[11, 66, 99]

切片原理

class MySeq:
    def __getitem__(self, item):
        return item

s = MySeq()
print(s[1])
print(s[1:4])
print(s[1:4:2])
print(s[1:4:2,7:9])

output

1
slice(1, 4, None)
slice(1, 4, 2)
(slice(1, 4, 2), slice(7, 9, None))

(程序員必會的 hhhhh.....)
看看slice在python3.7中是怎么描述的:

help(slice)

Help on class slice in module builtins:

class slice(object)
 |  slice(stop)
 |  slice(start, stop[, step])
 |  
 |  Create a slice object.  This is used for extended slicing (e.g. a[0:10:2]).
 |  
 |  Methods defined here:

 |  有點長。。。忽略  ********

 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  start
 |  
 |  step
 |  
 |  stop
 |  
 |  ----------------------------------------------------------------------
 |  Data and other attributes defined here:
 |  
 |  __hash__ = None