[Numpy-discussion] nanargmax failure case (was: Re: [SciPy-Dev] 1.8.0rc1)

Nathaniel Smith njs at pobox.com
Tue Oct 1 10:12:26 EDT 2013

[switching subject to break out from the giant 1.8.0rc1 thread]

On Tue, Oct 1, 2013 at 2:52 PM, Charles R Harris
<charlesr.harris at gmail.com> wrote:
> On Tue, Oct 1, 2013 at 7:25 AM, Nathaniel Smith <njs at pobox.com> wrote:
>> On Tue, Oct 1, 2013 at 1:56 PM, Charles R Harris
>> <charlesr.harris at gmail.com> wrote:
>> > On Tue, Oct 1, 2013 at 4:43 AM, Nathaniel Smith <njs at pobox.com> wrote:
>> >>
>> >> On Mon, Sep 30, 2013 at 10:51 PM, Christoph Gohlke <cgohlke at uci.edu>
>> >> wrote:
>> >> > 2) Bottleneck 0.7.0
>> >> >
>> >> >
>> >> > https://github.com/kwgoodman/bottleneck/issues/71#issuecomment-25331701
>> >>
>> >> I can't tell if these are real bugs in numpy, or tests checking that
>> >> bottleneck is bug-for-bug compatible with old numpy and we just fixed
>> >> some bugs, or what. It's clearly something to do with the
>> >> nanarg{max,min} rewrite -- @charris, do you know what's going on here?
>> >>
>> >
>> > Yes ;) The previous behaviour of nanarg for all-nan axis was to cast nan
>> > to
>> > intp when the result was an array, and return nan when a scalar. The
>> > current
>> > behaviour is to return the most negative value of intp as an error
>> > marker in
>> > both cases and raise a warning. It is a change in behavior, but I think
>> > one
>> > that needs to be made.
>> Ah, okay! I kind of lost track of the nanfunc changes by the end there.
>> So for the bottleneck issue, it sounds like the problem is just that
>> bottleneck is still emulating the old numpy behaviour in this corner
>> case, which isn't really a problem. So we don't really need to worry
>> about that, both behaviours are correct, just maybe out of sync.
>> I'm a little dubious about this "make up some weird value that will
>> *probably* blow up if people try to use it without checking, and also
>> raise a warning" thing, wouldn't it make more sense to just raise an
>> error? That's what exceptions are for? I guess I should have said
>> something earlier though...
> I figure the blowup is safe, as we can't allocate arrays big enough that the
> minimum intp value would be a valid index. I considered raising an error,
> and if there is a consensus the behavior could be changed. Or we could add a
> keyword to determine the behavior.

Yeah, the intp value can't be a valid index, so that covers 95% of
cases, but I'm worried about that other 5%. It could still pass
silently as the endpoint of a slice, or participate in some sort of
integer arithmetic calculation, etc. I assume you also share this
worry to some extent or you wouldn't have put in the warning ;-).

I guess the bigger question is, why would we *not* use the standard
method for signaling an exceptional condition here, i.e., exceptions?
That way we're 100% guaranteed that if people aren't prepared to
handle it then they'll at least know something has gone wrong, and if
they are prepared to handle it then it's very easy and standard, just
use try/except. Right now I guess you have to check for the special
value, but also do something to silence warnings, but just for that
one line? Sounds kind of complicated...


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