Comparable classes in Python 3

Question

What is the standard way of making a class comparable in Python 3? (For example, by id.)

Answer 1

For a full set of comparison functions I have used the following mixin, which you could put in say for example a mixin.py in your module.

class ComparableMixin(object):
    def _compare(self, other, method):
        try:
            return method(self._cmpkey(), other._cmpkey())
        except (AttributeError, TypeError):
            # _cmpkey not implemented, or return different type,
            # so I can't compare with "other".
            return NotImplemented

    def __lt__(self, other):
        return self._compare(other, lambda s, o: s < o)

    def __le__(self, other):
        return self._compare(other, lambda s, o: s <= o)

    def __eq__(self, other):
        return self._compare(other, lambda s, o: s == o)

    def __ge__(self, other):
        return self._compare(other, lambda s, o: s >= o)

    def __gt__(self, other):
        return self._compare(other, lambda s, o: s > o)

    def __ne__(self, other):
        return self._compare(other, lambda s, o: s != o)

To use the mixin above you need to implement a _cmpkey() method that returns a key of objects that can be compared, similar to the key() function used when sorting. The implementation could look like this:

>>> from .mixin import ComparableMixin

>>> class Orderable(ComparableMixin):
...
...     def __init__(self, firstname, lastname):
...         self.first = firstname
...         self.last = lastname
...
...     def _cmpkey(self):
...         return (self.last, self.first)
...
...     def __repr__(self):
...         return "%s %s" % (self.first, self.last)
...
>>> sorted([Orderable('Donald', 'Duck'), 
...         Orderable('Paul', 'Anka')])
[Paul Anka, Donald Duck]

The reason I use this instead of the total_ordering recipe is this bug. It's fixed in Python 3.4, but often you need to support older Python versions as well.