extended slicing not sufficiently covered in docs

[donlorenzo]

BPO	7460
Nosy	@birkenfeld, @terryjreedy

^{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 = None
created_at = <Date 2009-12-08.17:20:56.139>
labels = ['type-feature', '3.8', '3.9', '3.10', 'docs']
title = 'extended slicing not sufficiently covered in docs'
updated_at = <Date 2020-11-21.16:51:08.801>
user = 'https://bugs.python.org/donlorenzo'

bugs.python.org fields:

activity = <Date 2020-11-21.16:51:08.801>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2009-12-08.17:20:56.139>
creator = 'donlorenzo'
dependencies = []
files = []
hgrepos = []
issue_num = 7460
keywords = []
message_count = 3.0
messages = ['96141', '100501', '100503']
nosy_count = 3.0
nosy_names = ['georg.brandl', 'terry.reedy', 'donlorenzo']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue7460'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[donlorenzo]

I find the documentation regarding extended slicing to be lacking.
To be honest I couldn't find any documentation about it.
The only - and quite good - source of information I found was [1].
Maybe I didn't search enough. If so I would appreciate a link to the
relevant section.

In fact I didn't really know about extended slicing (except for the
start, stop, step notation) until I implemented a C extension and got a
TypeError in Python 3 because I was only supporting simple slicing. And
this is despite the fact that I don't consider myself a python n00b.

As I am still learning about this protocol at the time of writing. I
might have some things backwards and I don't feel qualified to improve
the docs myself. Instead I would like to point out where I would have
expected some information about this topic:

To begin improvement a hint around "PySequence_GetSlice" that this is a
more or less deprecated API and "tp_as_mapping->mp_subscript" should be
used instead would be useful.
In addition "PyObject_GetItem/tp_as_mapping->mp_subscript" could say
that a slice object is passed when extended slicing is used.

sincerely yours
//Lorenz

PS: I marked this Python 3.x even though the documentation of 2.x lacks
as well because I only got the TypeError in 3.x whereas in 2.x
everything works fine with simple slicing.

[1] http://mail.python.org/pipermail/python-3000/2007-August/009851.html

[terryjreedy]

This may be duplicative of bpo-1446619,
but I have not looked at that enough.
I agree that revision is needed and will try to develop patch suggestions.

[donlorenzo]

When I wrote this bug report I was mainly thinking about the C-API documentation which doesn't mention extended slicing at all.
Since the C-API isn't mentioned in the other bug I don't think it's a duplicate but they could maybe be merged if you see any advantage in that.

And thanks for looking into it.