[Jim J Jewett]
CVS has previously restricted the shortcut to those two, but I agree it should be all or nothing.
[Tim Peters]
The rich comparison PEP explicitly promised that, for example, x <= y is not necessarily the same as "x < y or x == y":
...it's also not kosher to infer x != y is False from that x == y is True.
With consistent orderings, "x <= y" is strictly weaker than "x < y or x == y"; they differ only when a pair of objects don't know *which* of "<" and "==" is true. If we do not assume consistency, then "is" does not even imply "==", let alone "not !="
Any shortcuts based on object identity are optimizations, and dubious ones
Agreed.
because they violate promises made in that PEP.
PEP 207 http://www.python.org/peps/pep-0207.html said that you can't (generally) reason from one rich compare to another. It said nothing about reasoning from "is" to any of the comparisons directly. So this optimization may violate the spirit of __eq__, but it doesn't violate the letter of the PEP.
There's a strong pragmatic ("go fast") case for starting to infer "x == y" and "not x != y" from "x is y", but that doesn't extend to inferring others from "x is y"
For normal objects, consistency holds. When consistency does not hold, even Py_EQ and Py_NE are wrong. Doing (only) the other operations correctly won't fix the whole program; it will just make the bugs less obvious (and therefore harder to catch). -jJ