How to write sort key functions for descending values?

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 most generic way to do this is simply to sort separately by each key in turn. Python's sorting is always stable so it is safe to do this:

sort(data, key=tiebreakerkey) sort(data, key=datekey, reverse=True) 

will (assuming the relevant definitions for the key functions) give you the data sorted by descending date and ascending tiebreakers.

Note that doing it this way is slower than producing a single composite key function because you will end up doing two complete sorts, so if you can produce a composite key that will be better, but splitting it out into separate sorts gives a lot of flexibility: given a key function for each column you can make any combination of them and specify reverse for any individual column.

For a completely generic option:

keys = [ (datekey, True), (tiebreakerkey, False) ] for key, rev in reversed(keys):     sort(data, key=key, reverse=rev) 

and for completeness, though I really think it should be avoided where possible:

from functools import cmp_to_key sort(data, key=cmp_to_key(your_old_comparison_function)) 

The reason I think you should avoid this you go back to having n log n calls to the comparison function compared with n calls to the key function (or 2n calls when you do the sorts twice).

Answer 2

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))