[David Mertz <me...@gnosis.cx>]
> 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)
Yes, but not defined unless __lt__ also defines a total ordering.
> * If the pairwise comparisons are deterministic, is sorting idempotent?
Not necessarily. For example, the 2-element list here swaps its
elements every time `.sort()` is
invoked, because the second element always claims it's "less than" the
first element, regardless of which order they're in:
class RelentlesslyTiny:
def __init__(self, name):
self.name = name
def __repr__(self):
return self.name
def __lt__(self, other):
return self is not other
a = RelentlesslyTiny("A")
b = RelentlesslyTiny("B")
xs = [a, b]
print(xs)
xs.sort()
print("after sorting once", xs)
xs.sort()
print("after sorting twice", xs)
[A, B]
after sorting once [B, A]
after sorting twice [A, B]
> This statement is certainly false:
>
> * If two items are equal, and pairwise inequality is deterministic, exchanging
> the items does not affect the sorting of other items in the list.
What I said at the start ;-) The only thing .sort() always guarantees
regardless of how goofy __lt__ may be is that the result list will be
some permutation of the input list. This is so even if __lt__ raises
an uncaught exception, killing the sort mid-stream.
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