inconsistent behavior of __getslice__

[douady]

BPO	560064
Nosy	@gvanrossum, @brettcannon, @rhettinger
Files	str.diff: Patch to stringobject.c

^{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 = 'https://github.com/brettcannon'
closed_at = <Date 2003-05-20.02:43:14.000>
created_at = <Date 2002-05-24.11:59:51.000>
labels = ['interpreter-core']
title = 'inconsistent behavior of __getslice__'
updated_at = <Date 2003-05-20.02:43:14.000>
user = 'https://bugs.python.org/douady'

bugs.python.org fields:

activity = <Date 2003-05-20.02:43:14.000>
actor = 'brett.cannon'
assignee = 'brett.cannon'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2002-05-24.11:59:51.000>
creator = 'douady'
dependencies = []
files = ['492']
hgrepos = []
issue_num = 560064
keywords = []
message_count = 6.0
messages = ['10917', '10918', '10919', '10920', '10921', '10922']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'brett.cannon', 'rhettinger', 'douady']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue560064'
versions = ['Python 2.3']

[douady]

The following trace shows what I mean:
>>> help(str.__getslice__)
Help on wrapper_descriptor:

__getslice__(...)
    x.__getslice__(i, j) <==> x[i:j]
>>> 'ab'[-1]
'b'
>>> 'ab'.__getitem__(-1)
'b'
>>> 'ab'[-2:-1]
'a'
>>> 'ab'.__getslice__(-2,-1)
''

that is, __getslice__ does not add the length of the
sequence to its arguments when they are negative as
indexing with [:] does.
This contradicts the docstring which says __getslice__
is equivalent to indexing.
The same behavior applies to lists.

Note that __getitem__ does it right.

This bug may be linked to bug 473985 ('str, __getitem__
and slices') but should be much easier to fix.

[rhettinger]

Logged In: YES
user_id=80475

Tim, I attached a tiny patch which fixes the behavior for
strings. I've also tried out the same change in listobject.c
and unicodeobject.c. Is this the right this to do or are
there more subtleties to this than meet the eye?

[gvanrossum]

Logged In: YES
user_id=6380

This should NOT be fixed this way, Raymond!

The docs are (subtly) wrong. For historical reasons,
__getslice__ expects the Python VM to do the adjustment of
negative indices. If you did another adjustment inside getslice,
certain indices would start behaving differently (e.g. s[:-5]
where len(s)==3).

Maybe the docstring should be fixed.

[brettcannon]

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user_id=357491

If the docstring fix is still the best way to fix this I am willing to make the
change.

[gvanrossum]

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user_id=6380

Yes, please fix the docstring.

[brettcannon]

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Fixed as revision 2.232 on HEAD (after getting it right; bah) and as
2.126.4.35 on release22-maint.