On Sun, 16 Jun 2019 at 12:54, Franklin? Lee
<leewangzhong+pyt...@gmail.com> wrote:
>
> On Sat, Jun 15, 2019 at 10:26 PM Guido van Rossum <gu...@python.org> wrote:
> > I don't actually know how viable this proposal is, but given that it's
> > being debated at some length, I'd like to put in my opinion that *if*
> > we're going to define an operator that's (roughly) synonymous with
> > issubclass(), it should be '<:', which is used in other languages (e.g.
> > Scala) and notational systems (https://en.wikipedia.org/wiki/Subtyping).
> > Overloading '<=' would be easier to implement, but would also cause enough
> > confusion that I think we should avoid it at all cost.
>
> Note that making new operators means class containment relationships
> can't be used as normal comparisons, such as for `sort`, especially of
> nested comparables such as tuples of classes. I'm not sure how to
> define a sort key that works even for a simple list of classes, other
> than to create a wrapper which implements class comparisons.
The subclass relationship between classes defines a partial rather
than total order so it can't be used with a normal sorting algorithm.
In general sorting a poset requires all-to-all O(N**2) comparison
(worst case: only one pair of objects is comparable).
You can sort comparable classes with something like:
key = cmp_to_key(lambda a, b: issubclass(b, a) - issubclass(a, b))
--
Oscar
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