3.0/3.1: Bad bug in range() computation (or possible Integer problem)

[mfxmfx]

BPO	6334
Nosy	@rhettinger, @mdickinson, @benjaminp, @ezio-melotti
Files	issue6334.patch

^{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/mdickinson'
closed_at = <Date 2009-06-24.19:24:52.866>
created_at = <Date 2009-06-24.10:51:41.833>
labels = ['interpreter-core', 'type-bug', 'release-blocker']
title = '3.0/3.1: Bad bug in range() computation (or possible Integer problem)'
updated_at = <Date 2009-06-25.11:31:15.735>
user = 'https://bugs.python.org/mfxmfx'

bugs.python.org fields:

activity = <Date 2009-06-25.11:31:15.735>
actor = 'mfxmfx'
assignee = 'mark.dickinson'
closed = True
closed_date = <Date 2009-06-24.19:24:52.866>
closer = 'mark.dickinson'
components = ['Interpreter Core']
creation = <Date 2009-06-24.10:51:41.833>
creator = 'mfxmfx'
dependencies = []
files = ['14355']
hgrepos = []
issue_num = 6334
keywords = ['patch']
message_count = 8.0
messages = ['89660', '89661', '89665', '89667', '89669', '89670', '89674', '89702']
nosy_count = 5.0
nosy_names = ['rhettinger', 'mark.dickinson', 'benjamin.peterson', 'ezio.melotti', 'mfxmfx']
pr_nums = []
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue6334'
versions = ['Python 3.0']

[mfxmfx]

Please note that the correct answer is 25, and the last element is missing !

This bug does not show on 64-bit versions (but 46337**2 is near 2**31).

~Markus

C:\Python31>python
Python 3.1rc2 (r31rc2:73414, Jun 13 2009, 16:43:15) [MSC v.1500 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> len(list(range(46337**2, 46349**2, 46337)))
24

C:\Python30>python.exe
Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> len(list(range(46337**2, 46349**2, 46337)))
24

[ezio-melotti]

Simpler test case:
Py2.6:
>>> n = 46349**2
>>> n
2148229801L
>>> range(n-10, n, 3)
[2148229791L, 2148229794L, 2148229797L, 2148229800L]

Py3.0:
>>> n = 46349**2
>>> n
2148229801
>>> list(range(n-10, n, 3))
[2148229791, 2148229794, 2148229797]

[mdickinson]

The length calculation in range_iter in Objects/rangeobject.c is
incorrect, when using a longrangeiterobject. The length is computed
as: (stop - start)//step. It should be ceiling((stop-start)/step), or
1 + (stop - start - 1)//step, provided that start <= stop and step > 0.
It's not clear to me right now whether there may also be problems with
negative steps, and with cases where start < stop, etc.

I think this is serious enough to be considered a release blocker for 3.1;
I'm working on a patch, and will post it later today.

[mdickinson]

Here's a patch for py3k.

There are also a whole bunch of tests that are commented
out in BuiltinTest.test_range in Lib/test/test_builtin.py.
Some of those tests fail with the current py3k; with this
patch applied, they all pass except the one involving 'badzero'.

[benjaminp]

The patch looks good. Please apply.

[mdickinson]

Applied to py3k in r73547. Will backport to 3.0.

[mdickinson]

Backported to release30-maint branch in r73549.

Thanks for catching this, Markus!

[mfxmfx]

Many thanks for your quick fix! ~Markus