Thanks Alan. Well, it looks like I've worked it out as far as small example programs go, hope I can get it working in my actual codebase now.
On Wed, 2008-05-28 at 23:30 +0100, Alan Gauld wrote: > > It seemed to me that the most natural way to implement mixins in > > Python > > is to use multiple inheritance with new-style classes. But I > > certainly > > confused myself a lot trying to do it. > > Mixins are normally done using MI. I''m interested in your > experiences. > Did new style classes specifically add problems(I've only done mixins > in old style classes in Python, I've mainly used mixins in Lisp) The reason I started using new-style classes was that I had a diamond shape in my class inheritance hierarchy. All my mixin classes inherited from a single base class, and then all my widget classes multiple-inherited from selections of mixin classes. (I realise this is wrong now, but that's how I had it.) So to get the method resolution order working right with this diamond formation, I needed new-style classes. And with this diamond formation, to prevent the __init__ method of the base class being called many times when a widget class inherits many mixin classes, you have to use super() instead of explicit calls to all the parent __init__ methods. Then you run into a problem when the parameters taken the by __init__ methods of the mixins do not match each other or the paramaters taken by the __init__ method of the base class. If a multiple-inheriting widget class has in its class ancestry a number of different __init__ methods with different parameter lists, how can it call them all with a single use of super? So then you have to use *args and **kwargs in the __init__ method of _every_ class instead of specifying the parameters normally, and this has a disadvantage, that the checking of the parameter lists that Python would normally do is circumvented and you end up having to reimplement it yourself. By this time I was pretty put off by this approach, and was getting TypeErrors in my implementation, and decided not to push forward with it. Basically, I think that super in Python is a total mess, very tricky and unwieldy. To be avoided. And also, I think multiple inheritance is tricky and to be used minimally, avoiding diamond formations or worse. I understand now why Java does not allow multiple inheritance. But it all stemmed from my initial mistake, of having all the mixin classes inherit from a common base class. Take away that and you get rid of the diamond inheritance formation, you no longer need to use super, in fact, I probably don't need to use new-style classes at all. That's an entire days work summed up, trying to refactor my codebase to use mixins. _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
