On Sat, 5 Nov 2022 at 02:21, Greg Ewing <greg.ew...@canterbury.ac.nz> wrote: > > > r...@zedat.fu-berlin.de (Stefan Ram) writes [that Barbara Liskov said]: > > > >> |If for each object o1 of type S there is an object o2 of > >> |type T such that for all programs P defined in terms of T, > >> |the behavior of P is unchanged when o1 is substituted for o2 > >> |then S is a subtype of T. > > That seems overly restrictive, because it wouldn't allow S to > override a method of T and make it do something different -- > which we do all the time in practice. > > >> Class contracts must hold for subclasses. > > That sounds like a much better way of saying it! >
Yeah, I would agree with that latter definition. The trouble is that few programs - and fewer programmers - really define which parts are contracts, so it's much easier to say "a subclass behaves just like the superclass(es) do(es)" - but it would be more accurate to qualify that with "in the ways that the superclass(es) define as standard behaviour". ChrisA -- https://mail.python.org/mailman/listinfo/python-list
