[Python-ideas] Consider (one day) adding an inheritance order class precedence mechanism

Neil Girdhar mistersheik at gmail.com
Fri Nov 17 18:03:13 EST 2017

On Fri, Nov 17, 2017 at 3:15 AM Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 17 November 2017 at 15:52, Steven D'Aprano <steve at 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).

This is really cool!  Like you said, this goes above and beyond what I was
proposing.   I guess my problem could be added as a motivation for
PEP-560's __mro_entries__.

(cc Ivan)

> Cheers,
> Nick.
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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