自定义Python切片，请告知

问题描述

我有一个将列表对象子类化的类.现在，我需要处理切片.从我在管间读取的所有内容来看，都必须使用__getitem__方法来完成.至少在我使用的Python 2.7+中.我已经做到了(见下文)，但是当我传递切片时，不会调用__getitem__方法.相反，将完成切片并返回列表.我想要一个myList的新实例返回.

I have a class that subclasses the list object. Now I need to handle slicing. From everything I read on the intertubes this has to be done using the __getitem__ method. At least in Python 2.7+ which is what I'm using. I have done this (see below), but the __getitem__ method isn't called when I pass in a slice. Instead, a slice is done and a list is returned. I would like a new instance of myList returned.

请帮助我发现问题所在.

Please help me discover what is wrong.

谢谢！

class myList(list):

    def __init__(self, items):

        super(myList, self).__init__(items)
        self.name = 'myList'


    def __getitem__(self, index):

        print("__getitem__")
        if isinstance(index, slice):
            print("slice")
            return self.__class__(
                self[x] for x in range(*index.indices(len(self)))
                )
        else: return super(myList, self).__getitem__(index)

if __name__ == "__main__":
    print("\nI'm tesing out custom slicing.\n")

    N = 10
    L = myList(range(N))

    L3 = L[3]
    L02 = L[:2]

推荐答案

请参见:

自版本2.0起不推荐使用:支持 将对象切片为参数 __getitem__()方法. (但是，当前CPython中的内置类型 仍实施__getslice__(). 因此，您必须在 实现时的派生类 切片.

Deprecated since version 2.0: Support slice objects as parameters to the __getitem__() method. (However, built-in types in CPython currently still implement __getslice__(). Therefore, you have to override it in derived classes when implementing slicing.

因此，因为您继承了list的子类，所以即使不推荐使用它也必须覆盖__getslice__.

So, because you subclass list you have to overwrite __getslice__, even though it's deprecated.

我认为您通常应该避免对内建函数进行子类化，因为有太多奇怪的细节.如果您只想要一个行为类似于列表的类，则可以使用 ABC 可以帮助您解决此问题:

I think you should generally avoid subclassing builtins, there are too many weird details. If you just want a class that behaves like a list, there is a ABC to help with that:

from collections import Sequence

class MyList(Sequence):
    def __init__(self, *items):
        self.data = list(items)

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

    def __getitem__(self, slice):
        return self.data[slice]

s = MyList(1,2,3)
# lots of free methods
print s[1:2], len(s), bool(s), s.count(3), s.index(2), iter(s)

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