Use starred expressions in subscripts

[PeterTillema]

BPO	45586
Nosy	@sweeneyde, @ajoino, @PeterTillema
Superseder	bpo-43812: Tuple unpacking in getitem

^{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 2021-10-24.02:45:48.052>
created_at = <Date 2021-10-23.08:58:24.257>
labels = ['interpreter-core', 'type-feature', '3.11']
title = 'Use starred expressions in subscripts'
updated_at = <Date 2021-10-26.23:05:06.171>
user = 'https://github.com/PeterTillema'

bugs.python.org fields:

activity = <Date 2021-10-26.23:05:06.171>
actor = 'Dennis Sweeney'
assignee = 'none'
closed = True
closed_date = <Date 2021-10-24.02:45:48.052>
closer = 'Dennis Sweeney'
components = ['Interpreter Core']
creation = <Date 2021-10-23.08:58:24.257>
creator = 'PeterTillema'
dependencies = []
files = []
hgrepos = []
issue_num = 45586
keywords = []
message_count = 7.0
messages = ['404860', '404908', '404909', '404915', '404954', '405029', '405062']
nosy_count = 3.0
nosy_names = ['Dennis Sweeney', 'ajoino', 'PeterTillema']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '43812'
type = 'enhancement'
url = 'https://bugs.python.org/issue45586'
versions = ['Python 3.11']

[PeterTillema]

It would be nice if you could starred expressions in list indices, for example this piece of code:

import numpy as np

a = np.array(0)
print(a[1, *[2, 3], 4])

[ajoino]

I don't understand, do you mean that lists should work like in your example? Or that your example code doesn't run?

If you mean the first issue, that is ok I guess but I've never used indexing like that outside of numpy, pandas and the like.

If you mean the second issue, it doesn't run because you are indexing a 0-dim array with 4 indices, if you instead try with a 1-dim array:

>>> import numpy as np
>>> a = np.array([0]) # 1-dim array instead of 0-dim
>>> print(a[[0, *[0, 0], 0]])
[0, 0, 0, 0]

You get the expected output.

[ajoino]

Oh yeah, the reason lists don't allow the starred expression has nothing to do with the starred expression itself, it's syntactically correct and in your case a[1, *[2, 3], 4] is equivalent to a[1, 2, 3, 4]. The "problem" is that lists do not allow indexing by tuples.

Perhaps the title of this issue should be changed to reflect that: "Allow lists to be indexed by tuples"

[sweeneyde]

Jacob wrote:
a[[0, *[0, 0], 0]]
which passes the *list* [0, 0, 0, 0] into __getitem__. In numpy, passing lists into __getitem__ does things like array([1, 10, 100])[[2, 1]] --> array([100, 10]).

But I think Peter is requesting that the following work:
a[0, *[0, 0], 0]
(this is currently a SyntaxError)
and the proposal is that it would pass the *tuple* (0, 0, 0, 0) into __getitem__, indexing into each dimension of the array.

A workaround is to use a[(0, *[0, 0], 0)] -- just one extra pair of parentheses.

This is a duplicate of https://bugs.python.org/issue43812 , where it was pointed out that this is currently being proposed as part of https://www.python.org/dev/peps/pep-0646/#implications , which it appears is in the queue of things for the steering council to rule on. I'll close this as a duplicate for now, but feel free to re-open if you feel something is missing from that PEP.