[Python-ideas] NAN handling in the statistics module

Chris Angelico rosuav at gmail.com
Sun Jan 6 23:29:08 EST 2019

On Mon, Jan 7, 2019 at 3:19 PM David Mertz <mertz at gnosis.cx> 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.


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