On Jun 1, 2:38 am, Piet van Oostrum <p...@cs.uu.nl> wrote: > >>>>> LittleGrasshopper <seattleha...@yahoo.com> (L) wrote: > >L> On May 31, 3:59 pm, Carl Banks <pavlovevide...@gmail.com> wrote: > >>> On May 31, 3:52 pm, LittleGrasshopper <seattleha...@yahoo.com> wrote: > > >>> > This is some simple code which I got from Guido's paper on the > >>> > unification of classes and types, which Arnaud suggested to improve my > >>> > knowledge of metaclasses: > > >>> > class M1(type): > >>> > pass > >>> > class M2(M1): > >>> > pass > >>> > class M3(M2): > >>> > pass > >>> > class C1: > >>> > __metaclass__ = M1 > >>> > class C2(C1): > >>> > __metaclass__ = M2 > >>> > class C3(C1, C2): > >>> > __metaclass__ = M3 > > >>> > It is failing when processing C3: > >>> > Traceback (most recent call last): > >>> > File "metaerror.py", line 18, in <module> > >>> > class C3(C1, C2): > >>> > TypeError: Error when calling the metaclass bases > >>> > Cannot create a consistent method resolution > >>> > order (MRO) for bases C2, C1 > > [snip] > > >L> I guess the resulting MROs do not satisfy monotonicity, I > >L> just have to find out how to derive the MRO. I had the notion that it > >L> was constructed by listing the current class, followed by the MROs of > >L> each base in order, and then retaining the rightmost instance of each > >L> class in the MRO list, but I think that might be an > >L> oversimplification, since in this case that would be: > >L> (C1, object) > >L> (C2, C1, object) > >L> (C3, C2, C1, object) > >L> And I don't see any issues. But I'll read the paper to figure out what > >L> the problem is, thanks. > > Here is exactly the problem. Merging the two MRO's as you state would > give C3, C2, C1, object, i.e. C2 before C1. > > But your class definition: > > class C3(C1, C2): > says that C1 should be before C2. Conflict!! > Change it to class C3(C2, C1): > > You see it has nothing to do with the metaclasses. The following code > gives the same error: > > class C1(object): > pass > class C2(C1): > pass > class C3(C1, C2): > pass > > -- > Piet van Oostrum <p...@cs.uu.nl> > URL:http://pietvanoostrum.com[PGP 8DAE142BE17999C4] > Private email: p...@vanoostrum.org
Thank you, Piet. You are correct. I actually went through the whole C3 MRO paper from Michele and I realized the issue (local precedence is not maintained.) And, like you said, the way to fix it is to change the order of the bases in C3. -- http://mail.python.org/mailman/listinfo/python-list
