Python 3 preparation: Use "rich comparison" - std function cmp() is gone, method __cmp__ is ignored

[wluebbe]

Since Py2.1 there are the rich comparison special methods (__eq__, __ne__, __lt__, __le__, __gt__, __ge__) to implement comparison operators (==, !=, <, <=, >, >= respectively) for custom classes.

This is more flexible (but somewhat more tedious) than defining the special method __cmp__.

In Py3 the special method __cmp__ is ignored. The standard function cmp() (which is mostly used to implement __cmp__ methods) is gone.

One may define a replacement function like

def cmp(a, b):
    return (a > b) - (a < b)

But this does not improved performance. And it does not seem forward looking ...

Unfortunately __cmp__ and cmp() are used a LOT in Sage. And the migration to rich comparison can not be done purely mechanical :-(

Since Py2.7 there is a class decorator ​functools.total_ordering to help to create the full set of special method __lt__, __le__, __gt__, __ge__ when given one of those.
See Lennart Regebro's ​Chapter: Use rich comparison operators for a more detailed discussion.

This ticket is tracked as a dependency of meta-ticket ticket:15980.