On Sat, Nov 18, 2017 at 9:29 PM Nick Coghlan <ncogh...@gmail.com> wrote:
> On 19 November 2017 at 06:56, Neil Girdhar <mistersh...@gmail.com> wrote: > > Would you mind explaining why it's necessary for C3 to complain? > > > > In: > > > > S < C > > B < S, E > > R < E, C > > Z < B, R > > > > If Z is told to have MRO: > > > > (Z, B, S, R, E, C) > > > > then there are no conflicts with any base classes. > > I don't actually know what C3 allows in principle, I only know that > CPython's resolver still complains in practice: > > >>> class C: pass > ... > >>> class S(C): pass > ... > >>> class E: pass > ... > >>> class B(S, E): pass > ... > >>> class R(E, C): pass > ... > >>> class Z(B, S, R, E, C): pass > ... > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: Cannot create a consistent method resolution order > (MRO) for bases C, E > > I think the problem is that the resolver isn't looking at the declared > bases of "B", and "R", it's looking at their full MRO: > > >>> B.__mro__ > (<class '__main__.B'>, <class '__main__.S'>, <class '__main__.C'>, > <class '__main__.E'>, <class 'object'>) > >>> R.__mro__ > (<class '__main__.R'>, <class '__main__.E'>, <class '__main__.C'>, > <class 'object'>) > > Changing the heuristics used to generate B's MRO such that "C" and "E" > appeared in the opposite order wouldn't really help, since that would > just flip the problematic case to be the "R(C, E)" declaration. > Sorry, I wrote back too quickly. I meant also to change B's requested MRO to be: (B, S, E, C) It works with that change. > > Cheers, > Nick. > > -- > Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia >
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