On 16 Nov, 16:35, Carl Banks <[EMAIL PROTECTED]> wrote: > On Thu, 15 Nov 2007 21:25:16 -0800, Dennis Lee Bieber wrote: > > On Thu, 15 Nov 2007 16:57:57 -0800 (PST), Carl Banks > > <[EMAIL PROTECTED]> declaimed the following in comp.lang.python: > > >> A source of confusion with "is a" is that it doesn't necessarily imply > >> a good inheritance relationship (aka Liskov substitutability). Consider > >> a Rectangle class that has methods set_width() and set_height(). > >> Should Square class inherit from Rectangle? A Square is a Rectangle, > >> but it's not suitable as a subclass of Rectangle, because the width and > >> height can't be set independently. You can't substitute a Square for a > >> Rectangle and get reasonable behavior. > > > Well, you would if you override the two set_* methods to set both > > height and width to the same value <G> > > But that breaks expectations: a user doesn't expect set_width() to affect > the height.
I can't speak for everyone but I certainly expect setting the width of a Square to change it's height. In fact, that would probably be the reason I used a Square rather than a Rectangle in the first place. -- http://mail.python.org/mailman/listinfo/python-list
