Strange behavior with python slicing [duplicate]

Question

Suppose we have this list:

>>> a = [x for x in range(10)]
>>> print(a)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Separately, both ways to slice work as expected:

>>> a[3:8]
[3, 4, 5, 6, 7]

>>> a[::-1]
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

But, when combined:

>>> a[3:8:-1]
[]

I would expect it to be [7, 6, 5 ,4, 3] or perhaps [6, 5, 4, 3, 2] (if reversing happened first). It is also interesting to consider what happens when either start or stop parameters are not passed:

>>> a[:5:-1]
[9, 8, 7, 6]

This is almost what I would expect, only its one item short. Tested this with numpy and it seems to behave in the same way.

Whats going on here?

Answer 1

With

a[3:8:-1]

The start and stop positions of the slice aren't adjusted based on the step. With a negative step, you're having it go backwards from 3, but there are no elements with indices in the range 3 to 8 counting back from 3, so you get an empty list.

You need to set the start and stop accordingly:

a[8:3:-1]

Which will count back from 8 to 4.

Answer 2

a[3:8:-1] instructs python to start from 3 and go to 8 by steps of -1

This creates an empty list: it's not possible to reach 8 from 3 by adding -1 (just like list(range(3,8,-1)) which gives an empty list too)

When you do a[:5:-1] then start is the default start, which python sets to "end of list" so it "works"

Same as when you do a[::-1] the start & stop are the default ones, and python understands that they're from end to start (else this notation wouldn't be useable)