list.index does not accept None as start or stop

[cfbolz]

BPO	13340
Nosy	@rhettinger, @amauryfa, @mdickinson, @cfbolz, @alex, @bitdancer, @asmeurer, @akheron, @berkerpeksag, @serhiy-storchaka, @iritkatriel

^{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 2020-09-16.07:15:26.001>
created_at = <Date 2011-11-04.09:23:03.449>
labels = ['interpreter-core', 'type-bug']
title = 'list.index does not accept None as start or stop'
updated_at = <Date 2020-09-16.07:15:25.999>
user = 'https://github.com/cfbolz'

bugs.python.org fields:

activity = <Date 2020-09-16.07:15:25.999>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-09-16.07:15:26.001>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2011-11-04.09:23:03.449>
creator = 'Carl.Friedrich.Bolz'
dependencies = []
files = []
hgrepos = []
issue_num = 13340
keywords = []
message_count = 19.0
messages = ['147000', '147108', '147111', '147113', '147114', '147116', '147117', '147120', '147121', '147151', '147153', '147162', '147163', '147171', '147172', '228937', '263582', '376958', '376977']
nosy_count = 12.0
nosy_names = ['rhettinger', 'amaury.forgeotdarc', 'mark.dickinson', 'Carl.Friedrich.Bolz', 'alex', 'r.david.murray', 'Aaron.Meurer', 'python-dev', 'petri.lehtinen', 'berker.peksag', 'serhiy.storchaka', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue13340'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']

[cfbolz]

The list.index method does not accept None as start and stop, which makes the error message quite confusing:

>>> [1, 2, 3].index(2, None, None)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: slice indices must be integers or None or have an __index__ method

I checked this in 3.2.2 and 2.7.2. Seems similar to bpo-12163.

[akheron]

The same issue exists for tuples:

>>> (1, 2, 3).index(2, None)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: slice indices must be integers or None or have an __index__ method

[python-dev]

New changeset 0f0eda4daac7 by Petri Lehtinen in branch '2.7':
Accept None as start and stop parameters for list.index() and tuple.index()
http://hg.python.org/cpython/rev/0f0eda4daac7

New changeset 5c1fcaf3cf1c by Petri Lehtinen in branch '3.2':
Accept None as start and stop parameters for list.index() and tuple.index()
http://hg.python.org/cpython/rev/5c1fcaf3cf1c

New changeset c33aa14f4edb by Petri Lehtinen in branch 'default':
Accept None as start and stop parameters for list.index() and tuple.index().
http://hg.python.org/cpython/rev/c33aa14f4edb

[akheron]

Committed a fix for both list and tuple. I considered this a bug fix rather than a new feature based on discussion in bpo-11828, especially msg133532, and applied the fix to 2.7 and 3.2, too.

[rhettinger]

The relevant code is in _PyEval_SliceIndex() in Python/ceval.c.

[rhettinger]

I think this "fix" was too hastily committed.

1. It was an API change.
2. It probably should have been done in _PyEval_SliceIndex().

Be careful. Don't rush to commit. Especially for backports.

[akheron]

2. It probably should have been done in _PyEval_SliceIndex().

I saw that other code used the same approach as I used in the fix.

The comment above _PyEval_SliceIndex() suggests it's used in other
contexts too. It seems that 2.7 uses it to implement the SLICE opcode,
while in 3.x it's only used to convert slice-like arguments.

What do you suggest? Doing it in _PyEval_SliceIndex() in 2.7 is
problematic, as we don't want x[None:2], right? :)

[rhettinger]

The API in 2.7 shouldn't be changed.
The error message can be fixed though.

Also, it is not clear that the API should change even in 3.3. The list.index() method is not required to accept None. It is not different than other APIs that use PyArg_ParseTuple() with an "i" field for a start, stop, or step argument.

[amauryfa]

str.index does accept None, though

[mdickinson]

What do you suggest? Doing it in _PyEval_SliceIndex() in 2.7 is
problematic, as we don't want x[None:2], right? :)

Eh? Don't we already have this?

Python 2.7.2 (default, Aug 22 2011, 13:53:27) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> range(5)[None:2]
[0, 1]

Or am I misunderstanding?

[akheron]

Or am I misunderstanding?

Ah, no, sorry. I wasn't aware of this.

Now the error message set by _PyEval_SliceIndex() makes sense. It doesn't itself accept None, but apply_slice() and assign_slice() handle the None case.

There's still the question whether{list,tuple}.index() should accept None or not.