[Numpy-discussion] Deprecate boolean math operators?

[josef.pktd at gmail.com]

On Fri, Dec 6, 2013 at 3:50 PM, Sebastian Berg
<sebastian at sipsolutions.net> wrote:
> On Fri, 2013-12-06 at 15:30 -0500, josef.pktd at gmail.com wrote:
>> On Fri, Dec 6, 2013 at 2:59 PM, Nathaniel Smith <njs at pobox.com> wrote:
>> > On Fri, Dec 6, 2013 at 11:55 AM, Alexander Belopolsky <ndarray at mac.com> wrote:
>> >>
>> >>
>> >>
>> >> On Fri, Dec 6, 2013 at 1:46 PM, Alan G Isaac <alan.isaac at gmail.com> wrote:
>> >>>
>> >>> On 12/6/2013 1:35 PM, josef.pktd at gmail.com wrote:
>> >>> > unary versus binary minus
>> >>>
>> >>> Oh right; I consider binary `-` broken for
>> >>> Boolean arrays. (Sorry Alexander; I did not
>> >>> see your entire issue.)
>> >>>
>> >>>
>> >>> > I'd rather write ~ than unary - if that's what it is.
>> >>>
>> >>> I agree.  So I have no objection to elimination
>> >>> of the `-`.
>> >>
>> >>
>> >> It looks like we are close to reaching a consensus on the following points:
>> >>
>> >> 1. * is well-defined on boolean arrays and may be used in preference of & in
>> >> code that is designed to handle 1s and 0s of any dtype in addition to
>> >> booleans.
>> >>
>> >> 2. + is defined consistently with * and the only issue is the absence of
>> >> additive inverse.  This is not a problem as long as presence of - does not
>> >> suggest otherwise.
>> >>
>> >> 3. binary and unary minus should be deprecated because its use in
>> >> expressions where variables can be either boolean or numeric would lead to
>> >> subtle bugs.  For example -x*y would produce different results from -(x*y)
>> >> depending on whether x is boolean or not.  In all situations, ^ is
>> >> preferable to binary - and ~ is preferable to unary -.
>> >>
>> >> 4. changing boolean arithmetics to auto-promotion to int is precluded by a
>> >> significant use-case of boolean matrices.
>> >
>> > +1
>>
>> +0.5
>> (I would still prefer a different binary minus, but it would be
>> inconsistent with a logical unary minus that negates.)
>>
>
> The question is if the current xor behaviour can make sense? It doesn't
> seem to make much sense mathematically? Which only leaves that `abs(x -
> y)` is actually what a (python) programmer might expect.
> I think I would like to deprecate at least the unary one. The ~ kind of
> behaviour just doesn't fit as far as I can see.

I haven't seen any real use cases for xor yet.
My impression is that both plus and minus are just overflow accidents
and not intentional. plus works in a useful way, minus as xor might be
used once per century.

I would deprecate both unary and binary minus.

(And when nobody is looking in two versions from now, I would add a
binary minus that overflows to the clipped version, so I get a set
subtraction. :)

>
>> 5. `/` is useless
>> 6 `**` follows from 1.

>>> m1 ** m2
array([1, 0, 1, 1], dtype=int8)
>>> m1 ** 2
array([False, False,  True,  True], dtype=bool)
>>> m1 ** 3
array([0, 0, 1, 1])

but I'm using python with an old numpy right now
>>> np.__version__
'1.6.1'

>
> Both of these are currently not defined, they will just cause upcast to
> int8. I suppose it would be possible to deprecate that upcast though
> (same goes for most all other ufuncs/operators in principle).

We would have to start the discussion again for all other
operators/ufuncs to see if they are useful in some cases.
For most treating as int will make sense, I guess.

Josef

>
>>
>> Josef
>>
>>
>> >
>> > --
>> > Nathaniel J. Smith
>> > Postdoctoral researcher - Informatics - University of Edinburgh
>> > http://vorpus.org
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