[Python-ideas] checking for identity before comparing built-in objects
tjreedy at udel.edu
Mon Oct 8 22:51:14 CEST 2012
On 10/8/2012 12:19 PM, Guido van Rossum wrote:
> I am not aware of an update to the standard. Being 20 years old does
> not make it outdated.
Similarly, being hundreds or thousands of years old does not make the
equality standard, which includes reflexivity of equality, outdated. The
IEEE standard violated that older standard.
illustrates some of the problems than come with that violation. But
given the compromise made to maintain sane behavior of Python's
collection classes, I see little reason to change nan in isolation.
I wonder if it would be helpful to make a NaN subclass of floats with
its own arithmetic and comparison methods. This would clearly mark a nan
as Not a Normal float. Since subclasses rule (at least some) binary
operations*, this might also simplify normal float code. But perhaps
this was considered and rejected before adding math.isnan in 2.6. (And
ditto for infinities.)
* in that class_ob op subclass_ob is delegated to subclass.__op__, but I
am not sure if this applies only to arithmetic, comparisons, or both.
Terry Jan Reedy
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