[Numpy-discussion] Integers to negative integer powers, time for a decision.

[Charles R Harris]

On Sat, Oct 8, 2016 at 9:12 AM, Nathaniel Smith <njs at pobox.com> wrote:

> On Sat, Oct 8, 2016 at 6:59 AM, Charles R Harris
> <charlesr.harris at gmail.com> wrote:
> >
> >
> > On Sat, Oct 8, 2016 at 4:40 AM, Nathaniel Smith <njs at pobox.com> wrote:
> >>
> >> On Fri, Oct 7, 2016 at 6:12 PM, Charles R Harris
> >> <charlesr.harris at gmail.com> wrote:
> >> > Hi All,
> >> >
> >> > The time for NumPy 1.12.0 approaches and I like to have a final
> decision
> >> > on
> >> > the treatment of integers to negative integer powers with the `**`
> >> > operator.
> >> > The two alternatives looked to be
> >> >
> >> > Raise an error for arrays and numpy scalars, including 1 and -1 to
> >> > negative
> >> > powers.
> >> >
> >> > Pluses
> >> >
> >> > Backward compatible
> >> > Allows common powers to be integer, e.g., arange(3)**2
> >> > Consistent with inplace operators
> >> > Fixes current wrong behavior.
> >> > Preserves type
> >> >
> >> >
> >> > Minuses
> >> >
> >> > Integer overflow
> >> > Computational inconvenience
> >> > Inconsistent with Python integers
> >> >
> >> >
> >> > Always return a float
> >> >
> >> > Pluses
> >> >
> >> > Computational convenience
> >> >
> >> >
> >> > Minuses
> >> >
> >> > Loss of type
> >> > Possible backward incompatibilities
> >> > Not applicable to inplace operators
> >>
> >> I guess I could be wrong, but I think the backwards incompatibilities
> >> are going to be *way* too severe to make option 2 possible in
> >> practice.
> >>
> >
> > Backwards compatibility is also a major concern for me.  Here are my
> current
> > thoughts
> >
> > Add an fpow ufunc that always converts to float, it would not accept
> object
> > arrays.
>
> Maybe call it `fpower` or even `float_power`, for consistency with `power`?
>
> > Raise errors in current power ufunc (**), for ints to negative ints.
> >
> > The power ufunc will change in the following ways
> >
> > +1, -1 to negative ints will error, currently they work
> > n > 1 ints to negative ints will error, currently warn and return zero
> > 0 to negative ints will error, they currently return the minimum integer
> >
> > The `**` operator currently calls the power ufunc, leave that as is for
> > backward almost compatibility. The remaining question is numpy scalars,
> > which we can make either compatible with Python, or with NumPy arrays.
> I'm
> > leaning towards NumPy array compatibility mostly on account of type
> > preservation and the close relationship between zero dimensionaly arrays
> and
> > scalars.
>
> Sounds good to me. I agree that we should prioritize within-numpy
> consistency over consistency with Python.
>
> > The fpow function could be backported to NumPy 1.11 if that would be
> helpful
> > going forward.
>
> I'm not a big fan of this kind of backport. Violating the
> "bug-fixes-only" rule makes it hard for people to understand our
> release versions. And it creates the situation where people can write
> code that they think requires numpy 1.11 (because it works with their
> numpy 1.11!), but then breaks on other people's computers (because
> those users have 1.11.(x-1)). And if there's some reason why people
> aren't willing to upgrade to 1.12 for new features, then probably
> better to spend energy addressing those instead of on putting together
> 1.11-and-a-half releases.
>

The power ufunc is updated in  https://github.com/numpy/numpy/pull/8127.
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