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

References (2 earlier) <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 2014-09-18 02:37 +1000
Subject Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))`
From Chris Angelico <rosuav@gmail.com>
Newsgroups comp.lang.python
Message-ID <mailman.14085.1410971877.18130.python-list@python.org> (permalink)

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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

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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

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