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

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From	Terry Reedy <tjreedy@udel.edu>
Subject	Re: Why `divmod(float('inf'), 1) == (float('nan'), float('nan'))`
Date	Tue, 16 Sep 2014 21:12:48 -0400
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On 9/16/2014 5:40 PM, cool-RR wrote:
> While debugging my code I found that the bug was because I assumed
> that something like `divmod(float('inf'), 1)` would be equal to
> `(float('inf'), float('nan'))`,  while Python returns `(float('nan'),
> float('nan'))`. Why does Python make the division result be NaN in
> this case? `float('inf') / 1` is still `float('inf')`.

For integers, divmod(x, y) is defined as (x // y, x % y) == ((x - x%y) 
// y, x % y) == ((x - x%y) / y, x % y).

For floats, Python is documented as using the third expression.

"Help on built-in function divmod in module builtins:
divmod(...)
     divmod(x, y) -> (div, mod)
     Return the tuple ((x-x%y)/y, x%y).  Invariant: div*y + mod == x."

It does not really matter, however, as infinity cannot be 'floored' as 
required for //

 >>> math.floor(float('inf'))
Traceback (most recent call last):
   File "<pyshell#21>", line 1, in <module>
     math.floor(float('inf'))
OverflowError: cannot convert float infinity to integer

and hence

 >>> float('inf') // 1.0
nan

-- 
Terry Jan Reedy

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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