Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax]

From	Terry Reedy <tjreedy@udel.edu>
Subject	Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax]
Date	2011-12-14 15:57 -0500
References	(3 earlier) <qot8vmhn34u.fsf@ruuvi.it.helsinki.fi> <a5c77779-be8e-42ad-a652-01c594235683@y7g2000vbe.googlegroups.com> <4ee81592$0$11091$c3e8da3@news.astraweb.com> <qotipljcz19.fsf@ruuvi.it.helsinki.fi> <78d2ba1b-0ea2-49a8-88f4-d85186bb2317@f11g2000yql.googlegroups.com>
Newsgroups	comp.lang.python
Message-ID	<mailman.3656.1323896280.27778.python-list@python.org> (permalink)

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On 12/14/2011 5:09 AM, Eelco wrote:

> Arguably, the most elegant thing to do is to define integer division
> and remainder as a single operation;

It actually is, as quotient and remainder are calculated together. The 
microprocessors I know of expose this (as does Python). 'a divmod b' 
puts the quotient in one register and the remainder in another. If you 
ask for just one of the two values, both are calculated and one is 
grabbed while the other is returned.

> which is not only the logical
> thing to do mathematically, but might work really well
> programmatically too.
>
> The semantics of python dont really allow for this though. One could
> have:
>
> d, r = a // b

 >>> a,b = divmod(10,3)
 >>> a,b
(3, 1)

With CPython, int.__divmod__ lightly wraps and exposes the processor 
operation.

> But it wouldnt work that well in composite expressions; selecting the
> right tuple index would be messy and a more verbose form would be
> preferred.

That is why we have
 >>> a == 10 // 3
True
 >>> b == 10 % 3
True

In both cases, I believe CPython calls int.__divmod__ (or the lower 
level equivalent) to calculate both values, and one is returned while 
the other is ignored. It it the same when one does long division by hand.

> However, performance-wise its also clearly the best
> solution, as one often needs both output arguments and computing them
> simultaniously is most efficient.

As indicated above, there is really no choice but to calculate both at 
once. If one needs both a//b and a%b, one should explicitly call divmod 
once and save (name) both values, instead of calling it implicitly twice 
and tossing half the answer each time.

-- 
Terry Jan Reedy

Thread

Verbose and flexible args and kwargs syntax Eelco Hoogendoorn <hoogendoorn.eelco@gmail.com> - 2011-12-12 00:44 +0100
  Re: Verbose and flexible args and kwargs syntax Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-12 01:11 +0000
    Re: Verbose and flexible args and kwargs syntax Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-12-12 21:09 +1300
      Re: Verbose and flexible args and kwargs syntax Terry Reedy <tjreedy@udel.edu> - 2011-12-12 10:36 -0500
      Re: Verbose and flexible args and kwargs syntax Nick Dokos <nicholas.dokos@hp.com> - 2011-12-12 10:55 -0500
      Re: Verbose and flexible args and kwargs syntax Arnaud Delobelle <arnodel@gmail.com> - 2011-12-12 16:15 +0000
      Re: Verbose and flexible args and kwargs syntax Chris Angelico <rosuav@gmail.com> - 2011-12-13 03:16 +1100
      Re: Verbose and flexible args and kwargs syntax Chris Angelico <rosuav@gmail.com> - 2011-12-13 03:19 +1100
    Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-12 04:21 -0800
      Re: Verbose and flexible args and kwargs syntax Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-12 11:09 -0700
        Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-12 10:17 -0800
          Re: Verbose and flexible args and kwargs syntax Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-12 11:35 -0700
            Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-12 10:51 -0800
              Re: Verbose and flexible args and kwargs syntax Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-12 17:34 -0700
                Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 00:31 -0800
            Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-12 11:05 -0800
              Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-12 11:20 -0800
      Re: Verbose and flexible args and kwargs syntax alex23 <wuwei23@gmail.com> - 2011-12-12 15:54 -0800
      Re: Verbose and flexible args and kwargs syntax Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-13 02:43 +0000
        Re: Verbose and flexible args and kwargs syntax Chris Angelico <rosuav@gmail.com> - 2011-12-13 14:08 +1100
        Re: Verbose and flexible args and kwargs syntax Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-12 21:52 -0700
        Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 01:15 -0800
          Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 01:50 -0800
            Re: Verbose and flexible args and kwargs syntax Arnaud Delobelle <arnodel@gmail.com> - 2011-12-13 10:15 +0000
              Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 02:39 -0800
              Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 02:46 -0800
                Re: Verbose and flexible args and kwargs syntax Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-13 11:28 +0000
                Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 04:19 -0800
          Re: Verbose and flexible args and kwargs syntax Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-13 11:13 +0000
            Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 04:47 -0800
              Re: Verbose and flexible args and kwargs syntax Chris Angelico <rosuav@gmail.com> - 2011-12-14 00:14 +1100
                Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 05:35 -0800
    Re: Verbose and flexible args and kwargs syntax "OKB (not okblacke)" <brenNOSPAMbarn@NObrenSPAMbarn.net> - 2011-12-12 19:45 +0000
  Re: Verbose and flexible args and kwargs syntax Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-12 12:59 +0200
    Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-12 03:22 -0800
      Re: Verbose and flexible args and kwargs syntax Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-12 13:46 +0200
    Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-12 03:18 -0800
    Re: Verbose and flexible args and kwargs syntax Terry Reedy <tjreedy@udel.edu> - 2011-12-12 10:39 -0500
      Re: Verbose and flexible args and kwargs syntax Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-12 18:52 +0200
        Re: Verbose and flexible args and kwargs syntax Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-12 09:29 -0800
          Re: Verbose and flexible args and kwargs syntax Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-12 20:09 +0200
          % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-14 03:18 +0000
            Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-13 23:49 -0800
              Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Arnaud Delobelle <arnodel@gmail.com> - 2011-12-14 11:55 +0000
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-14 04:33 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Arnaud Delobelle <arnodel@gmail.com> - 2011-12-14 16:13 +0000
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-14 09:15 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] rusi <rustompmody@gmail.com> - 2011-12-14 19:43 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-15 01:44 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] rusi <rustompmody@gmail.com> - 2011-12-15 02:56 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-15 03:37 -0800
            Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-14 10:56 +0200
              Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-14 02:09 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-14 14:22 +0200
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-14 04:41 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-14 12:38 +0000
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-14 05:29 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Chris Angelico <rosuav@gmail.com> - 2011-12-15 00:39 +1100
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-14 08:45 -0700
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Terry Reedy <tjreedy@udel.edu> - 2011-12-14 15:57 -0500
              Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] rusi <rustompmody@gmail.com> - 2011-12-14 03:47 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Chris Angelico <rosuav@gmail.com> - 2011-12-14 22:53 +1100
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-14 15:09 +0200
              Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-14 12:32 +0000
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-14 15:21 +0200
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Robert Kern <robert.kern@gmail.com> - 2011-12-15 10:47 +0000
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-15 02:59 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] alex23 <wuwei23@gmail.com> - 2011-12-15 18:14 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] MRAB <python@mrabarnett.plus.com> - 2011-12-16 02:58 +0000
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-16 02:36 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Chris Angelico <rosuav@gmail.com> - 2011-12-16 16:01 +1100
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] alex23 <wuwei23@gmail.com> - 2011-12-15 21:30 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-16 02:25 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] rusi <rustompmody@gmail.com> - 2011-12-16 09:38 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-16 11:40 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and   kwargs syntax] Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-12-17 12:49 +1300
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Eelco <hoogendoorn.eelco@gmail.com> - 2011-12-16 16:00 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Roy Smith <roy@panix.com> - 2011-12-16 19:03 -0500
                Re: % is not an operator [was Re: Verbose and flexible args and   kwargs syntax] Grant Edwards <invalid@invalid.invalid> - 2011-12-17 20:02 +0000
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-17 00:54 +0000
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] David Robinow <drobinow@gmail.com> - 2011-12-16 21:11 -0500
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Chris Angelico <rosuav@gmail.com> - 2011-12-15 21:58 +1100
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] rusi <rustompmody@gmail.com> - 2011-12-15 03:04 -0800
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-15 14:48 +0200
                Re: % is not an operator [was Re: Verbose and flexible args and kwargs syntax] Terry Reedy <tjreedy@udel.edu> - 2011-12-15 18:15 -0500
            Re: % is not an operator Paul Rudin <paul.nospam@rudin.co.uk> - 2011-12-14 09:43 +0000
        Re: Verbose and flexible args and kwargs syntax Nick Dokos <nicholas.dokos@hp.com> - 2011-12-12 12:58 -0500
          Re: Verbose and flexible args and kwargs syntax Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-12-14 11:04 +0200
  Re: Verbose and flexible args and kwargs syntax gene heskett <gheskett@wdtv.com> - 2011-12-12 12:46 -0500
  Re: Verbose and flexible args and kwargs syntax Dave Angel <d@davea.name> - 2011-12-12 13:04 -0500
  Re: Verbose and flexible args and kwargs syntax Nick Dokos <nicholas.dokos@hp.com> - 2011-12-12 13:02 -0500

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