Question

I read something about slice in Python 3. Then I wrote a program, tried to implement __getitem__(self, slice(s)). Code goes below:

class NewList:
    def __init__(self, lst):
        print('new list')
        self._list = lst
    def __getitem__(self, x):
        if type(x) is slice:
            return [ self._list[n] for n in range(x.start, x.stop, x.step) ]  #error?
        else:
            return self._list[x]
    ...

nl1 = NewList([1,2,3,4,5])
nl1[1:3]  #error occurs

Then I found out x.step is None, which made range raise an exception.
So, how should I implement the __getitem__ method?

Answer 1

You need to use the slice.indices method. Given the length of your sequence, it returns a tuple of start, stop, step:

>>> s = slice(2, 5, None)
>>> s.indices(10)
(2, 5, 1)

>>> [x for x in range(*s.indices(10))]
[2, 3, 4]

>>> s.indices(3)
(2, 3, 1)

>>> s.indices(0)
(0, 0, 1)