On Jun 5, 8:40 am, Gabriel Rossetti <[EMAIL PROTECTED]> wrote: > Hello everyone, > > I had read somewhere that it is preferred to use > self.__class__.attribute over ClassName.attribute to access class (aka > static) attributes. I had done this and it seamed to work, until I > subclassed a class using this technique and from there on things started > screwing up. I finally tracked it down to self.__class__.attribute! What > was happening is that the child classes each over-rode the class > attribute at their level, and the parent's was never set,
That's a misunderstanding of classes vs instances. If you have an instance of MyClass(Superclass), there is one instance but several classes. The instance is of MyClass; there is no instance of Superclass. 'self' has a .__class__ attribute because it's an instance, but MyClass and Superclass do not because they're already classes. Going further, self.a retrieves the instance attribute if it exists, or or MyClass.a if it doesn't, or Superclass.a if that doesn't. But assigning to self.a always assigns to an instance attribute. To assign a class attribute you must use .__class__ or TheClass. Likewise, to retrieve a class attribute that has been overridden by a subclass or instance, you must use .__class__ or TheClass. There's a very good reason to use self.__class__: it makes it possible to subclass your class. In Jason Orendorff's path.py, some path methods return new path objects. He uses 'path()' rather than self.__class__ to construct these. So if you subclass 'path' to extend it, these methods return path objects rather than your subclass objects. In my Unipath package which was written later, I use self.__class__ in these cases to allow users to extend my class. It's a little annoying that if you want to print a class's name in some unknown object, you have to use obj.__class__.__name__ if it's an instance, and obj.__name__ if it's a class. I sometimes wish classes had a .__class__ attribute that's the class itself, but I can see how that would cause its own confusion (and recursion). -- Mike Orr <[EMAIL PROTECTED]> -- http://mail.python.org/mailman/listinfo/python-list
