Question

I have multiple subclasses of a superclass that store something in a instance_of_a_class.value and I override __cmp__() to provide reasonable ==, <, > etc. comparisons.

However, I have multiple places in my code where I do

min(list_of_instances_of_class, key=lambda _: _.value) or max(list_of_instances_of_class, key=lambda _: _.value) and an occasional sorted(...

Is there a function to override in the class so that I don’t have to specify the key function for each call to the said functions or do I need to subclass list and override the max, min and sorted methods?

Answer 1

Just implement __lt__:

class Obj(object):
    def __init__(self, value):
        self.value = value
    def __lt__(self, other):
        return self.value < other.value
    def __repr__(self):
        return 'Obj(%r)' % self.value

obj_list = [Obj(2), Obj(1), Obj(4), Obj(3)]

print max(obj_list)
print min(obj_list)
print sorted(obj_list)

__cmp__ is deprecated, and all of the functions that you mentioned use only __lt__ not the other comparisons.

If for some reason you really can’t have them compare this way, you can do something like:

from operator import attrgetter
from functools import partial

valget = attrgetter('value')

maxval = partial(max, key=valget)
minval = partial(max, key=valget)
sortedval = partial(sorted, key=valget)
sortval = partial(list.sort, key=valget)

Where you call them just as maxval(obj_list) instead of max(obj_list) etc., and sortval(obj_list) to sort in-place instead of obj_list.sort()