Add start and stop parameters to the array.index()

[nikispahiev]

BPO	31956
Nosy	@rhettinger, @serhiy-storchaka, @Phaqui, @csabella, @corona10, @ZackerySpytz, @nanjekyejoannah, @kamilturek
PRs	bpo-31956: Add start and stop parameters to array.index() #4435 bpo-31956: Add start and stop parameters to array.index() #25059
Files	WIP-issue-31956

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-04-09.01:26:32.579>
created_at = <Date 2017-11-06.11:13:47.630>
labels = ['extension-modules', 'type-feature', '3.10']
title = 'Add start and stop parameters to the array.index()'
updated_at = <Date 2021-04-09.01:26:32.577>
user = 'https://bugs.python.org/nikispahiev'

bugs.python.org fields:

activity = <Date 2021-04-09.01:26:32.577>
actor = 'corona10'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-09.01:26:32.579>
closer = 'corona10'
components = ['Extension Modules']
creation = <Date 2017-11-06.11:13:47.630>
creator = 'niki.spahiev'
dependencies = []
files = ['47261']
hgrepos = []
issue_num = 31956
keywords = ['patch']
message_count = 14.0
messages = ['305629', '305643', '305650', '305758', '306133', '306143', '306150', '306178', '330746', '350405', '350619', '352499', '369732', '390070']
nosy_count = 10.0
nosy_names = ['rhettinger', 'serhiy.storchaka', 'Phaqui', 'cheryl.sabella', 'corona10', 'ZackerySpytz', 'niki.spahiev', 'nanjekyejoannah', 'Ryan G.', 'kamilturek']
pr_nums = ['4435', '25059']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31956'
versions = ['Python 3.10']

[nikispahiev]

Sequence protocol specifies 2 optional argument for index method:
seq.index(value, [start, [stop]])

array.index(value) needs start and stop arguments too.

[rhettinger]

+1

[vstinner]

Николай Спахиев: Are you only asking for the feature, or are you interested to work on a concrete patch?

[nikispahiev]

I plan to work on a patch.

[phaqui]

I decided to work on this, and I would like some review, as this would be my second contribution to cpython. Also, a general question:

As I defined the start and end arguments Py_ssize_t, bigger indexes (more negative or more positive) than what can fit in Py_ssize_t will trigger an overflow error. This should be OK, though, as other functions with index arguments has them as Py_ssize_t - and getarrayitem() itself accepts a Py_ssize_t. Or?

[serhiy-storchaka]

Just take list.index as an example.

[vstinner]

Anders Lorentsen: Py_ssize_t is the correct type for an index in the C language. It's not technically possible to create an array object larger than PY_SSIZE_T_MAX items :-)

Serhiy> Just take list.index as an example.

I concur with Serhiy :-)

[phaqui]

Writing my tests, I originally looked at Lib/test/seq_tests.py. One test case uses indexes that are (+-)4*sys.maxsize. This does not fit in Py_ssize_t, and so these tests cause my array implementation to raise an overflow exception.

A solution is of course to have the function take general objects instead, and then truncate them down to a number that can fit in Py_ssize_t if it's too negative or positive).

But I concur. It seems more reasonable to stay consistent with the rest of the module, too.

I'll look over the test code to make sure I test for every given scenario (or as many as I can think of), and prepare a PR for this, then :)

[RyanG]

This functionality is useful to me. Is this issue still alive? If not, how can I help?

[nanjekyejoannah]

There is a pending PR here : #4435 by phaqui. @Phaqui do you want to finish your PR ?

[phaqui]

As far as I can recall, the patch is generally speaking good to go. A number of discussions arose on various details, however. In any event, I'll take a look at it during the next few days.

[phaqui]

I have actually managed to lost my local branch of this fix, though I assume I can just start another one, manually copy over the changes, somehow mark this current PR as cancelled, aborted, or in my option the best: "replaced/superseeded by: [new PR]". In any case, there were discussions that seem to be unresolved, allow me to summarize:

• Document that index() raises ValueError when *value* is not found?

vstinner: We don't do this, remove this addition.
serhiy: Other index() methods does this.
---> My patch current does this. Who has final saying here?

• 'start' and 'stop' arguments are not keyword arguments, and also not shown in the signature as '.. start=0 ..' for this very reason (it may make them look as keyword arguments). Also, this lines up with list.index() for consistency. Wishes about changing this for all index()-methods has been expressed, but it seems to be consensus on doing this in unison for all index()-methods at once, in a bigger change... So, what is currently in the PR is good enough for now, or?

• Wording in documentation: Clarify that "the returned index is still relative to the start of the array, not the searched sub sequence" or not?

• Comment in the code about checking the length of the array on each iteration? There were comments about it being "confusing" - and while I agree, the other index()-code for lists, does not comment on this. Again I followed the path of most consistency, but I did receive comments about this. Yes to descriptive comments, or not?

----

Generally speaking: In the end, all I really did was mimic how list.index() is both written and documented, and that's when discussions about issues related to that started occurring, and so I now remember that I halted my PR, waiting for these issues to be resolved.

[csabella]

I found this SO about reclaiming an orphaned pull request:
https://stackoverflow.com/a/53383772

But you may still need to open a new PR for it.

[corona10]

New changeset afd1265 by Zackery Spytz in branch 'master':
bpo-31956: Add start and stop parameters to array.index() (GH-25059)
afd1265