On Wednesday, November 15, 2017 at 5:32:00 PM UTC-5, Koos Zevenhoven wrote: > > On Wed, Nov 15, 2017 at 11:49 PM, Neil Girdhar <miste...@gmail.com > <javascript:>> wrote: > >> Sometimes I get MRO failures when defining classes. For example, if >> >> R < E, C >> B < S, E >> S < C >> Z < B, R >> >> Then Z cannot be instantiated because C precedes E in B and E precedes C >> in R. The problem is that when the inheritance graph was >> topologically-sorted in B, the order was S, C, E. It could just as easily >> have been S, E, C. So, one solution is to add C explicitly to B's base >> class list, but this is annoying because B might not care about C (it's an >> implementation detail of S). It also means that if the hierarchy changes, >> a lot of these added extra base classes need to be fixed. >> >> I propose adding a magic member to classes: >> >> __precedes__ that is a list of classes. So, the fix for the above >> problem would be to define E as follows: >> >> class E: >> from whatever import C >> __precedes__ = [C] >> >> Then, when topologically-sorting (so-called linearizing) the ancestor >> classes, Python can try to ensure that E precedes C when defining B. >> >> > So it sounds like you are talking about the way that the siblings in the > inheritance tree (the bases of each class) get "flattened" into the mro in > a depth-first manner, and the relative order of siblings is not preserved. >
It is preserved, but there are insufficient constraints, which causes problems with future class definitions. > What would you think about not topologically sorting the inheritance tree > as such, but sorting a graph that has additional edges according to the > base lists of each class involved? In this case those edges would be E->C, > S->E, B->R. Is this what your talking about, or do I misinterpret the > problem? > That's already part of the topological sorting algorithm. You have to do that. I'm suggesting additional constraints. > > -- Koos > > > -- > + Koos Zevenhoven + http://twitter.com/k7hoven + >
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