that would depend on the design you wanna achieve. If you're sure that you won't create different classes extending this one, and using different versions of R and I, using the class name is ok and makes it clear that you're using a class attribute. On the other side, if you're planning on doing several variations, in that case, yes you have to use self instead (but in that case, the real quesiton is: "shouldn't those arrays be instance properties rather than class properties, then?" )