Why does __ne__ exist?

[Oren Ben-Kiki]

Good points. Well, this is pretty academic at this point - I don't think
anyone would seriously choose to obsolete __ne__, regardless of whether it
is absolutely necessary or not.

On Mon, Jan 8, 2018 at 4:51 PM, Thomas Jollans <tjol at tjol.eu> wrote:

> On 2018-01-08 15:25, Oren Ben-Kiki wrote:
> > I don't see a case in IEEE where (x == y) != !(x != y).
> > There _is_ a case where (x != x) is true (when x is NaN), but for such an
> > x, (x == x) will be false.
> >
> > I am hard pressed to think of a case where __ne__ is actually useful.
>
> See my earlier email and/or PEP 207. (tl;dr: non-bool return values)
>
> >
> > That said, while it is true you only need one of (__eq__, __ne__), you
> > could make the same claim about (__lt__, __ge__) and (__le__, __gt__).
> > That is, in principle you could get by with only (__eq__, __le__, and
> > __ge__) or, if you prefer, (__ne__, __lt__, __gt__), or any other
> > combination you prefer.
>
> PEP 207: "The above mechanism is such that classes can get away with not
> implementing either __lt__ and __le__ or __gt__ and __ge__."
>
>
> >
> > Or you could go where C++ is doing and say that _if_ one specifies a
> single
> > __cmp__ method, it should return one of LT, EQ, GT, and this will
> > automatically give rise to all the comparison operators.
>
> This used to be the case. (from version 2.1 to version 2.7, AFAICT)
>
>
> >
> > "Trade-offs... trafe-offs as far as the eye can see" ;-)
> >
> >
> > On Mon, Jan 8, 2018 at 4:01 PM, Thomas Nyberg <tomuxiong at gmx.com> wrote:
> >
> >> On 01/08/2018 12:36 PM, Thomas Jollans wrote:
> >>>
> >>> Interesting sentence from that PEP:
> >>>
> >>> "3. The == and != operators are not assumed to be each other's
> >>> complement (e.g. IEEE 754 floating point numbers do not satisfy this)."
> >>>
> >>> Does anybody here know how IEE 754 floating point numbers need __ne__?
> >>
> >> That's very interesting. I'd also like an answer to this. I can't wrap
> >> my head around why it would be true. I've just spent 15 minutes playing
> >> with the interpreter (i.e. checking operations on 0, -0, 7,
> >> float('nan'), float('inf'), etc.) and then also reading a bit about IEEE
> >> 754 online and I can't find any combination of examples where == and !=
> >> are not each others' complement.
> >>
> >> Cheers,
> >> Thomas
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