On Mon, Jan 7, 2019 at 3:19 PM David Mertz email@example.com wrote:
OK, let me be more precise. Obviously if the implementation in a class is:
class Foo: def __lt__(self, other): return random.random() < 0.5
Then we aren't going to rely on much.
- If comparison of any two items in a list (under __lt__) is deterministic, is the resulting sort order deterministic? (Pretty sure this is a yes)
If you guarantee that exactly one of "x < y" and "y < x" is true for any given pair of values from the list, and further guarantee that if x < y and y < z then x < z, you have a total order. Without those two guarantees, you could have deterministic comparisons (eg "nan < 5" is always false, but so is "5 < nan"), but there's no way to truly put the elements "in order". Defining __lt__ as "rock < paper", "paper < scissors", "scissors < rock" means that you can't guarantee the sort order, nor determinism.
Are those guarantees safe for your purposes? If so, sort() is, AIUI, guaranteed to behave sanely.