On Sat, Nov 18, 2017 at 9:34 AM Nick Coghlan <ncogh...@gmail.com> wrote:
> On 18 November 2017 at 09:03, Neil Girdhar <mistersh...@gmail.com> wrote: > > On Fri, Nov 17, 2017 at 3:15 AM Nick Coghlan <ncogh...@gmail.com> wrote: > >> I'll note that an interesting side effect of > >> https://www.python.org/dev/peps/pep-0560/#mro-entries will be to allow > >> folks to write: > >> > >> class MyCustomMRO: > >> def __init__(self, *bases): > >> self._resolved_bases = my_mro_resolver(bases) > >> def __mro_entries(self, orig_bases): > >> if len(orig_bases) > 1 or orig_bases[0] is not self: > >> raise TypeError("CustomMRO instance must be sole base > >> class") > >> return self._resolved_bases > >> > >> > >> class Z(MyCustomMRO(B, R)): > >> ... > >> > >> The custom MRO algorithm may then allow for use case specific hints to > >> handle situations that the default C3 resolver will reject as > >> inconsistent or ambiguous. (I'll also note that such a custom resolver > >> would be able to manufacture and inject synthetic metaclasses if > >> that's what someone decided they really wanted to do, by also > >> synthesising a custom base class to stick at the head of the list of > >> bases). > > > > This is really cool! > > Unfortunately, as Koos noted, it doesn't actually work as simply as I > presented it: even if you spell out a full MRO in the most-derived > class, the C3 resolution algorithm will complain that it's > inconsistent with the order in one of the two conflicting base > classes. > 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. > > So to truly get a custom method resolution order, you're likely going > to end up needing a custom metaclass involved. > > Cheers, > Nick. > > -- > Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia >
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