Class-override of a sort-key method?

[Chris Rebert]

On Mon, Dec 20, 2010 at 3:23 PM,  <pythonlist.calin79 at spamgourmet.com> wrote:
> Hi all - it would seem that these days, all the cool kids use the sort
> function's 'key' kwarg in order to sort a list of custom objects quickly.

Really? They don't bother to define __cmp__ or similar? Sounds lazy
and poorly structured.

> Unfortunately, as opposed to using 'cmp', where you can implent __cmp__ to
> get 'automatic sorting' in a similar fashion, there doesn't seem to be a
> direct analogue for a class-overridable method for providing a sort key.
>  (ie, something like '__sortkey__' or '__key__').

Just simply delegate to key comparison in your __cmp__ (or similar) method(s).

> Is there one, and I'm just missing it? If not, are there any plans to add
> one? (I did a quick search of the PEP list, and the only hits for 'sort' I
> saw had to do with sorting dictionaries by value).

If you know at class-definiton-time how you want instances to be
sorted, then just define __cmp__ (or the rich comparison methods)
appropriately, possibly even delegating to a comparison of keys (as
the class defines them).

For example:

from functools import total_ordering

@total_ordering
class Person(object):
    def __init__(self, first_name, last_name):
        self.first_name = first_name
        self.last_name = last_name

    @property
    def _key(self):
        """Identifying key for a Person, by which they are sorted"""
        return (self.last_name, self.first_name)

    def __eq__(self, other):
        return isinstance(other, Person) and self._key == other._key

    def __lt__(self, other):
        return self._key < other._key

If you want to abstract even this away, then just write a class
decorator; there's no need to add yet another (rather complicated due
to all the interactions with the existing comparison methods) special
method.

Cheers,
Chris
--
http://blog.rebertia.com