Question

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.

Please help me discover what is wrong.

Thanks!

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]

Answer 1

See this note:

object.__getslice__(self, i, j)

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.

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

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)