On Wed, Feb 20, 2013 at 2:38 PM, Terry Reedy <tjre...@udel.edu> wrote: > Liskov Substitution Principle (LSP): I met this over 15 years ago reading > debates among OOP enthusiasts about whether Rectangle should be a subclass > of Square or Square a subclass of Rectangle, and similarly, whether Ostrich > can be a legitimate subclass of Bird. > > The problem I see with the LSP for modeling either abstract or concrete > entities is that we in fact do define subclasses by subtraction or > limitation, as well as by augmentation, while the LSP only allows the > latter.
A plausible compromise is to demand LSP in terms of programming, but not necessarily functionality. So an Ostrich would have a fly() method that returns some kind of failure, in the same way that any instance of any flying-bird could have injury or exhaustion that prevents it from flying. It still makes sense to attempt to fly - an ostrich IS a bird - but it just won't succeed. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
