[EMAIL PROTECTED] wrote: > On Thu, 11 Aug 2005 01:19:19 +0100 > phil hunt wrote: > > > According to Wikipedia, the Liskov substitution principle is: > > > > Let q(x) be a property provable about objects x of type T. Then > > q(y) should be true for objects y of type S where S is a subtype of T > > > > To me, this is nonsense. Under this definition any subtype must > > behave the same as its parent type, becausde if it doesn't there > > will be some q(y) that are different to q(x). > > > > But if it behaves the same, what's the point of having a subtype? > > It does not behave the same, it has the same properties.
Doesn't it? "What is wanted here is something like the following substitution property: 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." Barbara Liskov, "Data Abstraction and Hierarchy" SIGPLAN Notices, 23,5 (May, 1988). IOW if an arbitrary program P works with objects of type T and you replace each object of type T by an object of type S and each P works still the same way one can imply that S is a subtype of T. It is a funny aspect of this definition that it doesn't work for reflective programming languages. If you have access to properties AS properties ( on the meta-level ) it is easy to break LSP. Preserving LSP becomes a challenge or a requirement. Example: The statement if type(obj) == T: ... breaks LSP. One might say that that T has no subtypes at all. But since T is arbitrary chosen no type in Python has a subtype in a strict sense. On the other hand this statement preserves LSP: if isinstance(obj,T): ... Therefore one can write programs that work as if LSP was valid and subtypes indeed exist. Kay -- http://mail.python.org/mailman/listinfo/python-list
