On Fri, Nov 17, 2017 at 10:14 AM, Nick Coghlan <ncogh...@gmail.com> wrote:
> On 17 November 2017 at 15:52, Steven D'Aprano <st...@pearwood.info> wrote: > > There's a lot of benefit to having a relatively simple, understandable > > algorithm for determining the MRO, as opposed to some sort of adaptive > > rule that will try to reorder classes according to potentially clashing > > constraints. If that means that some classes cannot go together in > > multiple inheritence because their MRO would be inconsistent, I think > > that's a price worth paying for not having to debug inheritence bugs > > caused by weirdly reordered MROs. > > 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). > > I can imagine someone wanting that, but it still doesn't allow customizing the *whole* MRO, AFAICT. Regarding the inheritance trees of B and R that is. Or at least trying to somehow achieve that without introducing a metaclass would probably become a terrible hack (if not just impossible). ––Koos -- + Koos Zevenhoven + http://twitter.com/k7hoven +
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