#18305: Element comparison for Python classes
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Reporter: vdelecroix | Owner:
Type: enhancement | Status: new
Priority: major | Milestone: sage-6.7
Component: coercion | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: #17890, #18303 | Stopgaps:
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Comment (by vdelecroix):
Replying to [comment:1 jdemeyer]:
> I think in general, rich comparisons (both Cython's `__richcmp__` and
Python's `__eq__` and friends) should be discouraged in favour of `_cmp_`,
because the latter needs coercion only once. Only for more complicated
types where rich comparisons cannot be implemented in terms of `__cmp__`
(intervals for example) should rich comparisons be used.
>
> Do you agree?
Partly. I agree that if `_cmp_` is doable '''and''' cheap then it is
preferable. But
- consider lists (or words or sequences), you have a cheap test to check
whether they are different: comparing the length. So in that case, you
might want to use it before comparing items one by one for getting the
lexicographic ordering.
- there are elements where equality makes sense but not comparison.
`_richcmp_` can handle that but not `_cmp_`. In such case I would
implement `_richcmp_` and raise an error if the operator is not `Py_EQ` or
`Py_NE`.
And I am not sure that I understand why `_cmp_` needs less coercion than
`_richcmp_`.
--
Ticket URL: <http://trac.sagemath.org/ticket/18305#comment:2>
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