Clarify Python Tutorial Gap on Negative Step Slicing

[NK5W]

Bug report

Bug description:

Negative/reverse slicing (_variable[_one:_two:-1]) is easy to misinterpret using only the two‑argument “indices between characters” model from the tutorial.

Example:

_variable = [1, 2, 3, 4]
print(_variable[2:0:-1])

Expected from two‑arg mental model: [2, 1]
Actual: [3, 2] — because:

• Stop index is always excluded, even when step is negative.
• Traversal starts at start and moves in step increments until it passes stop, never including stop.

The tutorial’s diagram is correct for step=1 but does not cover step at all, so readers extending that model to negative steps will predict the wrong result.

Proposed doc improvement: keep current explanation for two‑argument slices, but add a short note or link to the full slicing definition in the language reference showing how negative steps work, with a concrete example like [2:0:-1] → [3, 2]. This would prevent the confusion without overloading beginners.

CPython versions tested on:

3.15

Operating systems tested on:

No response

Linked PRs

• gh-137960: Add note about extended slicing in tutorial #144605

[skirpichev]

two‑argument “indices between characters” model from the tutorial.

Where in the tutorial index stop is treated as it should be included?

Could you please argue which exactly parts of the tutorial should be changed and how exactly?

So far I see here:

>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'

Note how the start is always included, and the end always excluded.

I think it's the first time indexing was introduced in the tutorial. Are you sure you have started to read the tutorial from beginning?

[terryjreedy]

This is a partial duplicate of NK5W's #137359, a week ago. The focus there was whether the [i:j] description is correct, which it is. I ended #137359 (comment) with "We might add a reference to this section, but the intention of the tutorial is not confuse beginners with slice strides. I lean to leaving the text as is.". NK5W is here proposing that we 'lean' the other way.

If we add anything to 'this section', the best place might between the end of the [i:j] discussion and the start of the next, "Python strings cannot be changed ... immutable." I propose something like:

(Some sequences support extended slicing with a third, possibly negative parameter. See here, footnote 5.)

With or without the parentheses. I am still not sure we should but do not mind if we do.

[picnixz]

Since this is not going forward and since I personally think we should not burden more the already long docs, I'm going to close this one. I also don't think the suggested location would be a good fit in the overall paragraph.