Duncan Booth a écrit : > bruno at modulix wrote: > > >>class Base(object): >> def __init__(self, arg1): >> self.attr1 = arg1 >> self.dothis() >> >> def dothis(self): >> return self.attr1 >> >>class Derived(Base): >> def __init__(self, arg1, arg2=0): >> self.attr2 = arg2 >> Base.__init__(self, arg1) >> >> def dothis(self): >> return self.attr1 + self.attr2 >> >>(snip) >> >> >>>Perhaps if the base __init__ calls an overridden >>>method, but at that point it sounds to me like something wants >>>refactoring. >> >>Why so ? This is a well-known pattern (template method). I don't see >>what's wrong with it. > > > Apart from the fact that you can delete the method 'dothis' from both > classes with no effect on the code?
Mmmm... Oh, I see. Agreed, this is not a very good example. >>class Base(object): >> def __init__(self, arg1): >> self.attr1 = arg1 >> self.attr42 = self.dothis() Is that better ?-) Ok, let's be serious now... > Actually, this is quite an interesting example becaue it wouldn't work in > C++: if you tried the same trick the call to dothis from the base > constructor (even assuming it is virtual) would actually call Base.dothis. > I think you have demonstrated that you can do some things in Python which > you simply cannot do in static languages like C++: assigning to derived > attributes before calling a base initialiser, and calling a virtual method > from a base initialiser. I'm not arguing about that. What I don't have > though is an example of code where doing this would actually be a good > thing. I don't have any concrete example at hand, but I can tell you I've done such things often enough. > What I think I'm trying to get at is that I believe that most situations > where someone actually tries to do something in the base initialiser which > requires calling a virtual method I'm afraid I fail to see what's so special about 'virtual' methods - and FWIW, since all methods in Python are virtual, if you don't want to call virtual methods in the initializer, you won't get very far !-) > are probably also cases where the > initialiser is doing too much: i.e. separating the > construction/initialisation from actually doing something is usually a good > idea. What if some of the things one have to do at initialization is also used elsewhere ? You would not duplicate code, would you ? -- http://mail.python.org/mailman/listinfo/python-list
