Re: Proposal: === and !=== operators

References	<53bce8a3$0$2746$c3e8da3$76491128@news.astraweb.com>
From	Devin Jeanpierre <jeanpierreda@gmail.com>
Date	2014-07-09 05:01 -0700
Subject	Re: Proposal: === and !=== operators
Newsgroups	comp.lang.python
Message-ID	<mailman.11687.1404907345.18130.python-list@python.org> (permalink)

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On Wed, Jul 9, 2014 at 12:00 AM, Steven D'Aprano <steve@pearwood.info> wrote:
> At the moment, Python has two (in)equality operators, == and != which
> call __eq__ and __ne__ methods. Some problems with those:
>
>
> * Many people expect == to always be reflexive (that is, x == x for
>   every x) but classes which customise __eq__ may not be.
>
> * The == operator requires __eq__ to return True or False
>   (or NotImplemented) and raises TypeError if it doesn't, which
>   makes it impossible to use == with (say) three-valued or fuzzy
>   logic.

Counter-proposal: The second use case doesn't matter that much, and
can be added to == without breaking backwards compatibility
meaningfully. The first case is covered by all sorts of things, but
I'd suggest that the most meaningful way in which nan "is the same as"
nan is that they both behave the same, as objects.

The Python "is" operator returns True when two objects are identical.
Is it a great leap to guarantee, in Python, that equivalent immutable
objects are always identical? Then float('nan') is float('nan') should
always return True. We already implicitly allow that a Python
implementation may choose to do this (e.g. small integers are cached),
but it isn't so hard to guarantee it. (You don't even have to cache
anything. PyPy makes things is-identical even when they are behind the
scenes different objects in memory, IIRC.)

See also http://wiki.ecmascript.org/doku.php?id=harmony:egal , which
is a similar proposal for ES6.

-- Devin

Thread

Proposal: === and !=== operators Steven D'Aprano <steve@pearwood.info> - 2014-07-09 07:00 +0000
  Re: Proposal: === and !=== operators Chris Angelico <rosuav@gmail.com> - 2014-07-09 17:21 +1000
    Re: Proposal: === and !=== operators Steven D'Aprano <steve@pearwood.info> - 2014-07-09 09:17 +0000
      Re: Proposal: === and !=== operators Rustom Mody <rustompmody@gmail.com> - 2014-07-09 09:20 -0700
      Re: Proposal: === and !=== operators Ian Kelly <ian.g.kelly@gmail.com> - 2014-07-09 11:50 -0600
      Re: Proposal: === and !=== operators Cameron Simpson <cs@zip.com.au> - 2014-07-10 09:16 +1000
      Re: Proposal: === and !=== operators Johannes Bauer <dfnsonfsduifb@gmx.de> - 2014-07-12 13:54 +0200
        Re: Proposal: === and !=== operators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-12 16:35 +0000
          Re: Proposal: === and !=== operators Chris Angelico <rosuav@gmail.com> - 2014-07-13 02:54 +1000
            Re: Proposal: === and !=== operators Roy Smith <roy@panix.com> - 2014-07-12 16:39 -0400
          Re: Proposal: === and !=== operators Johannes Bauer <dfnsonfsduifb@gmx.de> - 2014-07-12 20:14 +0200
            Re: Proposal: === and !=== operators Chris Angelico <rosuav@gmail.com> - 2014-07-13 09:01 +1000
              Re: Proposal: === and !=== operators Roy Smith <roy@panix.com> - 2014-07-12 19:06 -0400
                Re: Proposal: === and !=== operators Chris Angelico <rosuav@gmail.com> - 2014-07-13 09:15 +1000
            Re: Proposal: === and !=== operators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-13 04:48 +0000
  Re: Proposal: === and !=== operators Cameron Simpson <cs@zip.com.au> - 2014-07-09 18:17 +1000
    Re: Proposal: === and !=== operators Steven D'Aprano <steve@pearwood.info> - 2014-07-09 09:02 +0000
      Re: Proposal: === and !=== operators Chris Angelico <rosuav@gmail.com> - 2014-07-09 19:23 +1000
  Re: Proposal: === and !=== operators Devin Jeanpierre <jeanpierreda@gmail.com> - 2014-07-09 05:01 -0700
  Re: Proposal: === and !=== operators Roy Smith <roy@panix.com> - 2014-07-09 08:27 -0400
    Re: Proposal: === and !=== operators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-09 12:48 +0000
      Re: Proposal: === and !=== operators Tim Chase <python.list@tim.thechases.com> - 2014-07-09 13:05 -0500
        Re: Proposal: === and !=== operators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-10 01:10 +0000
      Re: Proposal: === and !=== operators Ian Kelly <ian.g.kelly@gmail.com> - 2014-07-09 12:31 -0600
      Re: Proposal: === and !=== operators Roy Smith <roy@panix.com> - 2014-07-09 16:47 -0400
  Re: Proposal: === and !=== operators Ethan Furman <ethan@stoneleaf.us> - 2014-07-09 05:43 -0700
  Re: Proposal: === and !=== operators Robert Kern <robert.kern@gmail.com> - 2014-07-09 16:27 +0100
  Re: Proposal: === and !=== operators Alex Burke <alexjeffburke@gmail.com> - 2014-07-10 18:33 +0200
  Re: Proposal: === and !=== operators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-12 03:30 +0000
    Re: Proposal: === and !=== operators Alan Bawden <alan@scooby-doo.csail.mit.edu> - 2014-07-12 01:07 -0400
      Re: Proposal: === and !=== operators Torsten Bronger <bronger@physik.rwth-aachen.de> - 2014-07-12 08:05 +0200
        Re: Proposal: === and !=== operators Torsten Bronger <bronger@physik.rwth-aachen.de> - 2014-07-12 08:14 +0200
      Re: Proposal: === and !=== operators Chris Angelico <rosuav@gmail.com> - 2014-07-12 16:06 +1000
      Re: Proposal: === and !=== operators Ethan Furman <ethan@stoneleaf.us> - 2014-07-11 23:11 -0700
      Re: Proposal: === and !=== operators Chris Angelico <rosuav@gmail.com> - 2014-07-12 16:39 +1000
        Re: Proposal: === and !=== operators Marko Rauhamaa <marko@pacujo.net> - 2014-07-12 10:06 +0300
      Re: Proposal: === and !=== operators Ethan Furman <ethan@stoneleaf.us> - 2014-07-11 23:53 -0700
      Re: Proposal: === and !=== operators Chris Angelico <rosuav@gmail.com> - 2014-07-12 17:25 +1000
      Re: Proposal: === and !=== operators Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-12 08:33 +0000

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