[Numpy-discussion] (no subject)

[Petr Viktorin]

On Thu, Sep 18, 2014 at 7:14 PM, Jonathan Helmus <jjhelmus at gmail.com> wrote:
> On 09/18/2014 12:01 PM, Chris Barker wrote:
>
> Well,
>
> First of all, numpy and the python math module have a number of differences
> when it comes to handling these kind of special cases -- and I think that:
>
> 1) numpy needs to do what makes the most sense for numpy and NOT mirror the
> math lib.

Sure.

> 2) the use-cases of the math lib and numpy are different, so they maybe
> _should_ have different handling of this kind of thing.

If you have a reason for the difference, I'd like to hear it.

> 3) I'm not sure that the core devs think these kinds of issues are "wrong"
> 'enough to break backward compatibility in subtle ways.

I'd be perfectly fine with it being documented and tested (in CPython)
as either a design mistake or design choice.

> But it's a fun topic in any case, and maybe numpy's behavior could be
> improved.
>>
>> My vote is that NumPy is correct here. I see no reason why
>> >>> float('inf') / 1
>> and
>> >>> float('inf') // 1
>>
>> should return different results.
>
>
> Well, one argument is that "floor division" is supposed to return an integer
> value, and that inf is NOT an integer value. The integral part of infinity
> doesn't exist and thus is Not a Number.
>
>
> But nan is not an integer value either:
>
>>>> float('inf') // 1
> nan
>>>> int(float('inf') // 1)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ValueError: cannot convert float NaN to integer
>
> Perhaps float('inf') // 1 should raise a ValueError directly since there is
> no proper way perform the floor division on infinity.

inf not even a *real* number; a lot of operations don't make
mathematical sense on it. But most are defined anyway, and quite
sanely.