On Jun 1, 3:44 am, Piet van Oostrum <p...@cs.uu.nl> wrote: > >>>>> Piet van Oostrum <p...@cs.uu.nl> (I) wrote: > >I> But your class definition: > >I> class C3(C1, C2): > >I> says that C1 should be before C2. Conflict!! > >I> Change it to class C3(C2, C1): > > Of course the C1 is then superfluous. > > I wonder why you want this. What is the problem you want to solve? > > Apart from the metaclasses (that you still can use with `class C3(C2)') > I could think of the following use case: > > class C1(object): > def m1(self): > return 'C1.m1' > > class C2(C1): > # override m1 > def m1(self): > return 'C2.m1' > def m2(self): > return 'C2.m2'+self.m1() > > class C3(C1, C2): > def test(self): > print self.m1()+self.m2() > > i.e. in C3 we `ignore' the override of m1 in C2 but still want to make > use of the m2 from C2. > > The question that arises then is: the self.m1() inside m2, which m1 > should it use? For an instance of C3 it would use C1.m1, but for an > instance of C2 it would use C2.m2. > > However, every instance of C3 can also be considered an instance of C2, > (Liskov substitution principle), therefore there is a conflict. That is > exactly the conflict that the MRO signals. > > If you want that kind of behaviour it can be solved by using a mixin > class for the m2 method: > > class C1(object): > __metaclass__ = M1 > def m1(self): > return 'C1.m1' > class Cmix(object): > def m2(self): > return 'C2.m2'+self.m1() > class C2(C1, Cmix): > __metaclass__ = M2 > # override m1 > def m1(self): > return 'C2.m1' > class C3(C1, Cmix): > __metaclass__ = M3 > def test(self): > print self.m1()+self.m2() > > -- > Piet van Oostrum <p...@cs.uu.nl> > URL:http://pietvanoostrum.com[PGP 8DAE142BE17999C4] > Private email: p...@vanoostrum.org
Your discussion of this scenario, why it justifiably fails, and the alternative solution using a mixing, is all excellent. I am going to keep this for future reference. I am at a stage of my Python journey that some of these things seem rather mysterious, so I especially appreciate this very detailed discussion. To answer your question, I actually got this hierarchy directly from Guido's paper on class unification that came out with 2.2.3. -- http://mail.python.org/mailman/listinfo/python-list
