[Python-ideas] What math.floor(inf) should return? Was: Make `float('inf') //1 == float('inf')`

[Alexander Belopolsky]

On Thu, Sep 18, 2014 at 6:57 PM, Case Van Horsen <casevh at gmail.com> wrote:
>
> > I don't think there is any dispute over what math.floor(inf) should
return.
> > POSIX, C99, IEEE and probably many other standards agree that inf
should be
> > returned as long as the resulting type can represent it.
>
> I dispute that there is no dispute over what math.floor(inf) should
return. ;-)
>

I am changing the subject so that we don't mix making new decisions with a
critique and defense of the decisions that were made in the past.

I wrote: "inf should be returned as long as the resulting type can
represent it".  This is the part that I still believe is not disputed.   No
one has suggested that math.floor(inf)  should return nan, for example.


> All the standards specify a result type can represent +-Inf and +-0. A
> standards compliant version should return +-Inf and +-0.  lrint() and
llrint()
> are defined to return long or long long, respectively. It would be fine if
> they raised an exception. The current math.floor() actually behaves more
> like llrint() than floor().
>

POSIX does not preclude raising an exception: "If the correct value would
cause overflow, a range error shall occur" (
http://pubs.opengroup.org/onlinepubs/009695399/functions/floor.html).

>
> I accept that having math.floor() return an integer (and raise an
exception
> for +-Inf) may be useful in many cases but it is different from the
standard.
> Other floating-point libraries still return a floating-point value.

The standards are influenced by the limitation inherent in many languages
where ints have finite range and cannot represent floor() of many finite
floating point values.  Python does not have this limitation.  (Granted -
PEP 3141 could do a better job explaining why floor, ceil, round, //, etc.
should return Integer rather than Real.)
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