In richcmp, fall back to reversed operation if coercion fails

[jdemeyer]

Change the implementation of x.__richcmp__(y, op) to try y.__richcmp__(x, revop) if y not a Sage Element and coercion fails. In Python terms: if x.__ge__(y) fails, try y.__le__(x). This is precisely what Python does when a comparison returns NotImplemented.

If this reversed operation is not implemented (either the type does not define comparisons or the comparison returns NotImplemented), go on as usual. Because of this, we cannot just return NotImplemented in the coercion model, we need to implement the reversing ourselves.

In #20767, the analogous thing was done for arithmetic.

Component: coercion

Author: Jeroen Demeyer

Branch/Commit: b3ea04f

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/21163

[jdemeyer]

Branch: u/jdemeyer/in_richcmp__fall_back_to_reversed_operation_if_coercion_fails

[jdemeyer]

Commit: b3ea04f

[jdemeyer]

New commits:

b3ea04f

Try reversed operation in richcmp if coercion fails

[fchapoton]

comment:8

ok, looks good to me. Nice formula for the revop involution.

[fchapoton]

Reviewer: Frédéric Chapoton

[jdemeyer]

comment:9

Replying to @fchapoton:

ok, looks good to me. Nice formula for the revop involution.

Thanks. That was a nice exercise :-)

[vbraun]

Changed branch from u/jdemeyer/in_richcmp__fall_back_to_reversed_operation_if_coercion_fails to b3ea04f