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Groups > comp.lang.python > #74249

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

From Roy Smith <roy@panix.com>
Newsgroups comp.lang.python
Subject Re: Proposal: === and !=== operators
Date 2014-07-09 08:27 -0400
Organization PANIX Public Access Internet and UNIX, NYC
Message-ID <roy-DFC5B5.08272809072014@news.panix.com> (permalink)
References <53bce8a3$0$2746$c3e8da3$76491128@news.astraweb.com>

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In article <53bce8a3$0$2746$c3e8da3$76491128@news.astraweb.com>,
 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.
> 
> 
> I propose: 
> 
> * The == operator be redefined to *always* assume reflexivity, that
>   is, it first compares the two arguments using `is` before calling
>   the __eq__ methods.
> 
> * That's a backwards-incompatible change, so you need to enable it
>   using "from __future__ import equals" in Python 3.5, and then to
>   become the default behaviour in 3.6.
> 
> * To support non-reflexive types, allow === and !=== operators, which
>   are like == and != except they don't call `is` first.
> 
> * The new === and !== operators call __eeq__ and __ene__ (extended
>   equal and extended not equal) methods; if they don't exist, they
>   fall back on __eq__ and __ne__.
> 
> * To support multi-valued logics, === and !== are not required to
>   return True or False, they can return anything you like and it is
>   up to the caller to ensure that they are sensible.
> 
> * Returning NotImplemented from __eeq__ and __ene__ has the same 
>   meaning as for __eq__ and __ne__.
> 
> * For the avoidance of doubt, `if...elif...else` are not expected to
>   be aware of multi-valued logics. No other changes to the language 
>   are expected.
> 
> 
> Thoughts? Comments?

-1.  This seems like it will just add additional complexity and 
confusion, for very little gain.  We would have *three* ways to compare 
for equality (==, ===, and is).

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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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