On Mon, Dec 05, 2016 at 08:04:11AM +0100, Jeroen Demeyer wrote:
> On 2016-12-04 19:39, Marc Mezzarobba wrote:
> >Hi,
> >
> >Does Element.__richcmp__() (via CoercionModel_cache_maps.richcmp())
> >really need to fall back on comparing by type/id when no common parent
> >is found?
>
> No. As far as I know, It does so for historical reasons only. I am in favour
> of making elements without a common parent uncomparable.
Note that there are not only "mathemathical" orderings, but also
"algorithmic" ones. It is useful (and i bet assumed in various Sage
algorithms) to be able to totally order any pair of Python objects, e.g.
when you want to traverse a graph, or even identify edges:
sage: Graph({2:['a']}).edges(labels=False)
[(2, 'a')]
sage: Graph({'a':[2]}).edges(labels=False)
[(2, 'a')]
sage: 2 < 'a'
True
I guess we can try to use equality test and `id()` for such a purpose, but
it might force us to change a lot of code.
Ciao,
Thierry
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