[Python-Dev] Deprecating float.is_integer()

[David Mertz]

I've been using and teaching python for close to 20 years and I never
noticed that x.is_integer() exists until this thread. I would say the "one
obvious way" is less than obvious.

On the other hand, `x == int(x)` is genuinely obvious... and it immediately
suggests the probably better `math.isclose(x, int(x))` that is what you
usually mean.

On Wed, Mar 21, 2018, 2:08 PM Mark Dickinson <dickinsm at gmail.com> wrote:

> I'd prefer to see `float.is_integer` stay. There _are_ occasions when one
> wants to check that a floating-point number is integral, and on those
> occasions, using `x.is_integer()` is the one obvious way to do it. I don't
> think the fact that it can be misused should be grounds for deprecation.
>
> As far as real uses: I didn't find uses of `is_integer` in our code base
> here at Enthought, but I did find plenty of places where it _could_
> reasonably have been used, and where something less readable like `x % 1 ==
> 0` was being used instead. For evidence that it's generally useful: it's
> already been noted that the decimal module uses it internally. The mpmath
> package defines its own "isint" function and uses it in several places: see
> https://github.com/fredrik-johansson/mpmath/blob/2858b1000ffdd8596defb50381dcb83de2bcccc6/mpmath/ctx_mp_python.py#L764.
> MPFR also has an mpfr_integer_p predicate:
> http://www.mpfr.org/mpfr-current/mpfr.html#index-mpfr_005finteger_005fp.
>
> A concrete use-case: suppose you wanted to implement the beta function (
> https://en.wikipedia.org/wiki/Beta_function) for real arguments in
> Python. You'll likely need special handling for the poles, which occur only
> for some negative integer arguments, so you'll need an is_integer test for
> those. For small positive integer arguments, you may well want the accuracy
> advantage that arises from computing the beta function in terms of
> factorials (giving a correctly-rounded result) instead of via the log of
> the gamma function. So again, you'll want an is_integer test to identify
> those cases. (Oddly enough, I found myself looking at this recently as a
> result of the thread about quartile definitions: there are links between
> the beta function, the beta distribution, and order statistics, and the
> (k-1/3)/(n+1/3) expression used in the recommended quartile definition
> comes from an approximation to the median of a beta distribution with
> integral parameters.)
>
> Or, you could look at the SciPy implementation of the beta function, which
> does indeed do the C equivalent of is_integer in many places:
> https://github.com/scipy/scipy/blob/11509c4a98edded6c59423ac44ca1b7f28fba1fd/scipy/special/cephes/beta.c#L67
>
> In sum: it's an occasionally useful operation; there's no other obvious,
> readable spelling of the operation that does the right thing in all cases,
> and it's _already_ in Python! In general, I'd think that deprecation of an
> existing construct should not be done lightly, and should only be done when
> there's an obvious and significant benefit to that deprecation. I don't see
> that benefit here.
>
> --
> Mark
>
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