On Mon, 22 Sep 2008 10:11:58 +0200, Bruno Desthuilliers wrote: > Steven D'Aprano a écrit : >> I have a class which is not intended to be instantiated. Instead of >> using the class to creating an instance and then operate on it, I use >> the class directly, with classmethods. Essentially, the class is used >> as a function that keeps state from one call to the next. >> >> The problem is that I don't know what to call such a thing! "Abstract >> class" isn't right, because that implies that you should subclass the >> class and then instantiate the subclasses. >> >> What do you call such a class? >> >> > <nitpick> > Err... A possible design smell ?-) > </nitpick> > > > More seriously: this looks quite like a singleton, which in Python is > usually implemented way more simply using a module and plain functions.
I really don't know why everyone thinks I want a Singleton. I want to uncouple objects, not increase the coupling. Consider a factory function: def factory(x): # a toy example alist = [x] def foo(): return alist return foo Now suppose we "instantiate" the factory (for lack of a better term): >>> f1 = factory(0) >>> f2 = factory(0) Even though f1 and f2 have the same behaviour, they are obviously not the same object. And although both return a list [0], it is not the same list: >>> f1() == f2() == [0] True >>> f1() is f2() False They have a (very little) amount of state, which is *not* shared: >>> L = f1() >>> L.append(1) >>> f1() [0, 1] >>> f2() [0] But there's only a limited amount of state that functions carry around. I can give them more state like this: >>> f1.attr = 'x' but it isn't good enough if the function needs to refer to it's own state, because functions can only refer to themselves by name and the factory can't know what name the function will be bound to. As far as I know, the only objects that know how to refer to themselves no matter what name they have are classes and instances. And instances share at least some state, by virtue of having the same class. (Pedants will argue that classes also share state, by virtue of having the same metaclass. Maybe so, but that's at a deep enough level that I don't care.) I'm now leaning towards just having factory() instantiate the class and return the instance, instead of having to do metaclass chicanery. Because the class is built anew each time by the factory, two such instances aren't actually sharing the same class. I think that will reduce confusion all round (including mine!). Hopefully now that I've explained what I want in more detail, it won't seem so bizarre. Factory functions do it all the time. Is there a name for this pattern? Thanks to everyone who commented, your comments helped me reason out a better alternative to what I first suggested. -- Steven -- http://mail.python.org/mailman/listinfo/python-list