Proposal: === and !=== operators

Roy Smith roy at panix.com
Wed Jul 9 14:27:28 CEST 2014


In article <53bce8a3$0$2746$c3e8da3$76491128 at news.astraweb.com>,
 Steven D'Aprano <steve at 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).



More information about the Python-list mailing list