[Python-Dev] PyObject_RichCompareBool identity shortcut

[Rob Cliffe]

I am not a specialist in this area (although I call myself a 
mathematician).  But they say that sometimes the outsider sees most of 
the game, or more likely that sometimes the idiot's point of view is useful.

To me the idea of non-reflexive equality (an object not being equal to 
itself) is abhorrent.  Nothing is more likely to put off new Python 
users if they happen to run into it.  And I bet even very experienced 
programmers will be tripped up by it a good proportion of the time they 
hit it.
Basically it's deferring to a wart, of dubious value, in floating point 
calculations and/or the IEEE754 standard, and allowing it to become a 
monstrous carbuncle disfiguring the whole language.
I think implementations of equal/not-equal which are make equality 
non-reflexive (and thus break "identity implies equality") should be 
considered broken.


On 27/04/2011 15:53, Guido van Rossum wrote:
> Maybe we should just call off the odd NaN comparison behavior?
Right on, Guido.  (A pity that a lot of people don't seem to be listening.)


On 27/04/2011 17:05, Isaac Morland wrote:
> Python could also provide IEEE-754 equality as a function (perhaps in 
> "math"), something like:
>
> def ieee_equal (a, b):
>     return a == b and not isnan (a) and not isnan (b)
>
Quite.  If atypical behaviour is required in specialised areas, it can 
be coded for.  (Same goes for specialised functions for comparing lists, 
dictionaries etc. in non-standard ways.  Forced explicit is better than 
well-hidden implicit.)
> Of course, the definition of math.isnan cannot then be by checking its 
> argument by comparison with itself
Damn right - a really dirty trick if ever I saw one (not even proof 
against the introduction of new objects which also have the same 
perverse non-reflexive equality).
> - it would have to check the appropriate bits of the float representation.
So it should.


On 28/04/2011 11:11, Nick Coghlan wrote:
> After all, why discard centuries of mathematical experience based on a 
> decision that the IEEE754 committe can't clearly recall the rationale 
> for, and didn't clearly document?
Sorry Nick, I have quoted you out of context - you WEREN'T arguing for 
the same point of view.  But you express it much better than I could.


It occurred to me that the very length of this thread [so far!] 
perfectly illustrates how controversial non-reflexive "equality"  is.  
(BTW I have read, if not understood, every post to this thread and will 
continue to read them all.)
And then I came across:
On 28/04/2011 09:43, Alexander Belopolsky wrote:
> If nothing else, annual reoccurrence of long threads on this topic is 
> a reason enough to reconsider which standard to follow.
Aha, this is is a regular, is it?  'Nuff said!

Best wishes
Rob Cliffe