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 conv…

Accepted Answer

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))
```

How to write sort key functions for descending values?

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