Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))`

Path	csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.xs4all.nl!newsfeed4a.news.xs4all.nl!xs4all!news.tele.dk!news.tele.dk!small.news.tele.dk!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path	<rosuav@gmail.com>
X-Original-To	python-list@python.org
Delivered-To	python-list@mail.python.org
X-Spam-Status	OK 0.009
X-Spam-Evidence	'H': 0.98; 'S': 0.00; 'true,': 0.05; 'wednesday,': 0.07; 'subject:Why': 0.09; 'cc:addr:python-list': 0.11; 'developers,': 0.11; 'chris,': 0.16; 'division.': 0.16; 'from:addr:rosuav': 0.16; 'from:name:chris angelico': 0.16; 'wrote:': 0.18; 'thu,': 0.19; 'cc:addr:python.org': 0.22; 'documented': 0.24; 'cc:2**0': 0.24; "i've": 0.25; 'values': 0.27; 'header:In-Reply-To:1': 0.27; 'chris': 0.29; 'am,': 0.29; 'message-id:@mail.gmail.com': 0.30; "i'm": 0.30; 'mod': 0.31; 'sep': 0.31; 'maybe': 0.34; 'agree': 0.35; 'but': 0.35; 'received:google.com': 0.35; 'there': 0.35; 'like,': 0.36; 'returning': 0.36; "didn't": 0.36; 'possible': 0.36; 'should': 0.36; 'too': 0.37; 'whatever': 0.38; 'expect': 0.39; 'more': 0.64; 'talking': 0.65; 'to:none': 0.92
DKIM-Signature	v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:cc :content-type; bh=n/c7mzZiHEOcpzH/2DqYrCUAgnm3yXp+hgHpKI6ZyXA=; b=WQK/ycavhMYtUyyOqDwSwRTfNVZON52vCih9aeJc1tqbOkW3jvLGfNQFCle6nTKWdk +3bvdk+Le3w3t8+CHsoOw7TO15LcjzwO3Kt85iHgeFx0Aaex6bha2ARFKP5YkQJymwZM TNBSBK54k0XQyVxc7J1LvnHyTpuqcSXnaM4QpJ8uHMCXmIG5vdsA4X0eSSS0CelvZ1qa jUO2pYPzTTjFYQVVCZGC95eMA6L/GwW2WYUB3jcwrbSPceEBe60Z8Twu1QB4961lztQg ybhgq+nB94PByrhTICXIShnx48y+56XInJflPv9Ky0/AmtD/PcLxCJ86a9nlRswYOqFY 5Ivw==
MIME-Version	1.0
X-Received	by 10.51.17.2 with SMTP id ga2mr5782068igd.2.1410971870745; Wed, 17 Sep 2014 09:37:50 -0700 (PDT)
In-Reply-To	<adaa42e9-3b9a-40e0-a03d-e092c41ccaec@googlegroups.com>
References	<06397cb3-8baf-4af1-8152-ff7c3f6ceb62@googlegroups.com> <mailman.14066.1410916403.18130.python-list@python.org> <fd859066-7068-4a43-a9f0-0e9cc6949cf6@googlegroups.com> <mailman.14076.1410966617.18130.python-list@python.org> <151e45ce-444d-441c-b59b-67e2aee3882b@googlegroups.com> <mailman.14077.1410967765.18130.python-list@python.org> <adaa42e9-3b9a-40e0-a03d-e092c41ccaec@googlegroups.com>
Date	Thu, 18 Sep 2014 02:37:50 +1000
Subject	Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))`
From	Chris Angelico <rosuav@gmail.com>
Cc	"python-list@python.org" <python-list@python.org>
Content-Type	text/plain; charset=UTF-8
X-BeenThere	python-list@python.org
X-Mailman-Version	2.1.15
Precedence	list
List-Id	General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe	<https://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive	<http://mail.python.org/pipermail/python-list/>
List-Post	<mailto:python-list@python.org>
List-Help	<mailto:python-list-request@python.org?subject=help>
List-Subscribe	<https://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups	comp.lang.python
Message-ID	<mailman.14085.1410971877.18130.python-list@python.org> (permalink)
Lines	20
NNTP-Posting-Host	2001:888:2000:d::a6
X-Trace	1410971877 news.xs4all.nl 2974 [2001:888:2000:d::a6]:56323
X-Complaints-To	abuse@xs4all.nl
Xref	csiph.com comp.lang.python:77979

Show key headers only | View raw

On Thu, Sep 18, 2014 at 2:33 AM, cool-RR <ram.rachum@gmail.com> wrote:
> On Wednesday, September 17, 2014 6:30:04 PM UTC+3, Chris Angelico wrote:
>> On Thu, Sep 18, 2014 at 1:16 AM, cool-RR <ram.rachum@gmail.com> wrote:
>> > I didn't ask for the modulo, I agree it should remain NaN. I'm talking about the floor division.
>>
>> Invariant: div*y + mod == x.
>> If mod is NaN, there is no possible value for div that will make the
>> invariant true, ergo it too has to be NaN.
>> ChrisA
>
> Chris, why is this invariant `div*y + mod == x` so important? Maybe it's more important to return a mathematically reasonable result for the the floor-division result than to maintain this invariant?

It's important because it's documented :) You can expect whatever you
like, but if something's a documented invariant, that's more important
than your expectations.

Now, why was divmod documented as returning values adhering to that
invariant? You'd have to ask the developers, I've no idea.

ChrisA

Thread

Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` cool-RR <ram.rachum@gmail.com> - 2014-09-16 14:40 -0700
  Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Terry Reedy <tjreedy@udel.edu> - 2014-09-16 21:12 -0400
    Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` cool-RR <ram.rachum@gmail.com> - 2014-09-17 07:55 -0700
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Chris Angelico <rosuav@gmail.com> - 2014-09-18 01:10 +1000
        Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` cool-RR <ram.rachum@gmail.com> - 2014-09-17 08:16 -0700
          Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Chris Angelico <rosuav@gmail.com> - 2014-09-18 01:29 +1000
            Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` cool-RR <ram.rachum@gmail.com> - 2014-09-17 09:33 -0700
              Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Chris Angelico <rosuav@gmail.com> - 2014-09-18 02:37 +1000
              Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-18 13:11 +1000
                Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` cool-RR <ram.rachum@gmail.com> - 2014-09-18 01:43 -0700
          Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Ian Kelly <ian.g.kelly@gmail.com> - 2014-09-17 09:34 -0600
          Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Chris Angelico <rosuav@gmail.com> - 2014-09-18 01:40 +1000
            Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-18 12:50 +1000
          Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Ian Kelly <ian.g.kelly@gmail.com> - 2014-09-17 10:12 -0600
          Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Ian Kelly <ian.g.kelly@gmail.com> - 2014-09-17 10:30 -0600
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Ian Kelly <ian.g.kelly@gmail.com> - 2014-09-17 12:03 -0600
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Chris Angelico <rosuav@gmail.com> - 2014-09-18 07:37 +1000
        Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-18 13:29 +1000
          Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Chris Angelico <rosuav@gmail.com> - 2014-09-18 13:47 +1000
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Terry Reedy <tjreedy@udel.edu> - 2014-09-17 19:23 -0400
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Ian Kelly <ian.g.kelly@gmail.com> - 2014-09-18 00:51 -0600
  Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Ian Kelly <ian.g.kelly@gmail.com> - 2014-09-17 09:35 -0600
  Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` wxjmfauth@gmail.com - 2014-09-17 23:22 -0700
    Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Marko Rauhamaa <marko@pacujo.net> - 2014-09-18 09:45 +0300
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` wxjmfauth@gmail.com - 2014-09-18 00:13 -0700
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-09-18 19:25 +1000
        Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` wxjmfauth@gmail.com - 2014-09-18 05:57 -0700
        Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Grant Edwards <invalid@invalid.invalid> - 2014-09-18 14:33 +0000
    Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` chris.barker@noaa.gov - 2014-09-18 09:35 -0700
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` Ian Kelly <ian.g.kelly@gmail.com> - 2014-09-18 11:05 -0600
      Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))` wxjmfauth@gmail.com - 2014-09-18 23:48 -0700

csiph-web