Strange behavior with python slicing

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?

if you had correctly understood first two expressions `a[:: -1]` and `a[3:8]` *then* your expected result should come from `a[3: 8][:: -1]` expression, rather than `a[3: 8: -1]` — Grijesh Chauhan, Dec 03 '19 at 07:00
Also: [Why does extended slicing not reverse the list?](https://stackoverflow.com/q/56959836/7851470) — Georgy, Dec 03 '19 at 09:38

Carcigenicate · Accepted Answer · 2019-12-02T22:37:01.900

20

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.

edited Dec 02 '19 at 22:37

answered Dec 02 '19 at 22:21

Carcigenicate

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Jean-François Fabre · Answer 2 · 2019-12-02T22:27:55.410

7

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)

edited Dec 02 '19 at 22:27

answered Dec 02 '19 at 22:23

Jean-François Fabre

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Why would python set the start to "end of list" when doing a[:5:-1]?? Shouldn't it be "start of list"? – user3105173 Dec 02 '19 at 22:27
1

since the step is negative, default start & stop are reversed. Else `[::-1]` notation would not work at all. – Jean-François Fabre Dec 02 '19 at 22:28
1

@user3105173 because, slice notation is for *convenience*. When the steps are reversed, the implied starts-end change accordingly – juanpa.arrivillaga Dec 02 '19 at 23:16

score 4 · Answer 3 · edited Jun 20 '20 at 09:12

This behavior is explained in the documentation.

The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j. If i or j is greater than len(s), use len(s). If i is omitted or None, use 0. If j is omitted or None, use len(s). If i is greater than or equal to j, the slice is empty.

The slice of s from i to j with step k.... stopping when j is reached (but never including j). When k is positive, i and j are reduced to len(s) if they are greater. When k is negative, i and j are reduced to len(s) - 1 if they are greater. If i or j are omitted or None, they become “end” values (which end depends on the sign of k).

Strange behavior with python slicing

3 Answers3

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