magic methods 6

Protocols : it is similar with the port in other program language. In Python, it functions as a guide.

Container : Alignment[list, tuple, string] and mapping[dictionary].

The container you customize is sorted as two kinds. One is the unchangeable container, in which you only need to define the __len__() and __getitem__(). The other is the changeable one, in which you should define the __setitem__() and __delitem__() based on the former container.

Trial:

1. Write an unchangeable user-defined list, the number of everyone element being visited should be recorded.

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class CountList:
    def __init__(self, *args):
        self.values = [x for x in args]     #列表推导式
        self.count = {}.fromkeys(range(len(self.values)), 0)    #利用下标，初始0

    def __len__(self):
        return len(self.values)

    def __getitem__(self, key):
        self.count[key] += 1
        return self.values[key]

Result:

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>>> c1 = CountList(1,3,5,7,9)
>>> c2 = CountList(2,4,6,8,10)
>>> c1[1]
3
>>> c2[1]
4
>>> c1[1]+c2[1]
7
>>> c1.count
{0: 0, 1: 2, 2: 0, 3: 0, 4: 0}

• How to write an unchangeable list?