[Python-ideas] PEP 531: Existence checking operators

[Steven D'Aprano]

On Sun, Oct 30, 2016 at 06:26:13AM -0700, David Mertz wrote:

> NaN's *usually* propagate.  The NaN domain isn't actually closed under IEEE
> 754.

[...]
> >>> min(1, nan)
> 1
> 
> The last one isn't really mandated by IEEE 754, and is weird when you
> consider `min(nan, 1)`.

Python's min() and max() don't treat NANs correctly according to IEEE 
754. The 2008 revision to the standard says that:

    min(x, NaN) = min(NaN, x) = x
    max(x, NaN) = max(NaN, x) = x

https://en.wikipedia.org/wiki/IEEE_754_revision#min_and_max

Professor Kahan, one of the IEEE 745 committee members, writes:

    For instance max{x, y} should deliver the same result as max{y, x} 
    but almost no implementations do that when x is NaN. There are good
    reasons to define max{NaN, 5} := max{5, NaN} := 5 though many would
    disagree.

Page 9, https://people.eecs.berkeley.edu/~wkahan/ieee754status/IEEE754.PDF


I believe that the standard does allow implementations to define a 
second pair of functions that implement "NAN poisoning", that is, they 
return NAN when given a NAN argument.



-- 
Steve