How to write sort key functions for descending values?

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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).

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