[Numpy-discussion] Deprecate boolean math operators?

Sebastian Berg sebastian at sipsolutions.net
Fri Dec 6 15:50:42 EST 2013


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. 

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

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