#17890: Remove _(rich)cmp_c_impl
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Reporter: jdemeyer | Owner:
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-6.7
Component: cython | Resolution:
Keywords: | Merged in:
Authors: Jeroen Demeyer | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/jdemeyer/ticket/17890 | 04570b3dfd194fea8c2437c8717c3dfb8704b4af
Dependencies: | Stopgaps:
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Comment (by vdelecroix):
Replying to [comment:56 jdemeyer]:
> Replying to [comment:55 vdelecroix]:
> > You wrote in the doc of `_richcmp`
> > {{{
> > If a class wants to implement rich comparison without
coercion,
> > then ``_richcmp`` should be defined (as well as
``__richcmp__``
> > as usual).
> > }}}
> > Why not define directly `__richcmp__` in that case?
>
> Because rich comparison after coercion calls `_richcmp`, not
`__richcmp__`. The only reason this works for `Integer` is that `ZZ` has
no sub-rings: it is never the common parent of two different parents.
Right, but `_richcmp` will call `_richcmp_`/`_cmp_`. In Integers (and
Rationals after #18304) there is also this `_cmp_` implemented and if
everything else failed in `__richcmp__` there is precisely
`Element._richcmp(self, other, op)`.
So even if you implement a subset of `ZZ`/`QQ` as a new parent (let say
the additive group `3 ZZ` or the ring `ZZ[1/p]`) then it would be fine
with comparisons with integers/rationals as soon as you define a coercion
`3 ZZ -> ZZ`/`ZZ[1/p] -> QQ`. So my mistake, the situation for `ZZ` and
`QQ` is a bit different: `Integer/Rational` do not avoid coercion in rich
comparisons, they just try to avoid it for most common cases.
Do you have an example of a `Parent` where we want to avoid coercions?
--
Ticket URL: <http://trac.sagemath.org/ticket/17890#comment:60>
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