> No, that is an argument for multiple-inheritance, mixin classes etc. You > know when constructing the object what behaviour you want it to have. It > isn't an argument for changing the behaviour of an existing object > dynamically.
Partially true. I don't want to change the behavior of an exiting object. I know what behavior I want to have at the time of creating. This can be achieved by creating classes dynamically by parameterizing the base class, which I thought is ugly. > > for an example where it is useful to extend the behaviour of objects have a > look at Zope3 interfaces and adaptors: an object can be defined to > implement one or more interfaces, and you can associate additional > interfaces with objects at runtime. Sometimes an object will implement an > interface directly, but often the implementation is taken out into a > separate 'adaptor' class. Whenever you want to use a specified interface on > an object you simply use the interface class as a constructor on the > object, and that might find the appropriate adapter or it might just return > the original object if the interface is provided directly. > > e.g. seehttp://plone.org/documentation/tutorial/five-zope3-walkthrough Interesting! > Note that this is very different than the forwarding mechanism which > started this thread: when you use an adaptor you get something which > implements just the specified interface, you don't have to worry about > conflicting names for methods or attributes. Also you can have a common > interface associated with objects which have completely different adapters > implementing that interface. It is not a simple one-way forwarding mechanism. It is two-way. An example: class SimpleDB(Extend): def withID(self, id): # do query def withIDs(self, ids): return [self.withID(id) for id in ids] class CachedDB(Extend): def withID(self, id): if id not in self.cache: self.cache[id] = self.super_withID(id) return self.cache[id] db = CachedDB(SimpleDB()) When I call db.withIDs, it should make use of the cache, which I think is not possible with a simple forwarding mechanism. -- http://mail.python.org/mailman/listinfo/python-list
