Question

The move in recent versions of Python to passing a key function to sort() from the previous cmp function is making it trickier for me to perform complex sorts on certain objects.

For example, I want to sort a set of objects from newest to oldest, with a set of string tie-breaker fields. So I want the dates in reverse order but the strings in their natural order. With a comparison function I can just reverse the comparison for the date field compared to the string fields. But with a key function I need to find some way to invert/reverse either the dates or the strings.

It’s easy (although ugly) to do with numbers – just subtract them from something – but do I have to find a similar hack for dates (subtract them from another date and compare the timedeltas?) and strings (…I have no idea how I’d reverse their order in a locale-independent way).

I know of the existence of functools.cmp_to_key() but it is described as being “primarily used as a transition tool for programs being converted to Python 3 where comparison functions are no longer supported”. This implies that I should be able to do what I want with the key method – but how?

Answer 1

The slow-but-elegant way to do this is to create a value wrapper that has reversed ordering:

from functools import total_ordering
@total_ordering
class ReversedOrder:
    def __init__(self, value):
        self.value = value
    def __eq__(self, other):
        return other.value == self.value
    def __lt__(self, other):
        return other.value < self.value

If you don’t have functools.total_ordering, you’d have to implement all 6 comparisons, e.g.:

import operator
class ReversedOrder:
    def __init__(self, value):
        self.value = value
for x in ['__lt__', '__le__', '__eq__', '__ne__', '__ge__', '__gt__']:
    op = getattr(operator, x)
    setattr(ReversedOrder, x, lambda self, other, op=op: op(other.value, self.value))