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

[Neil Girdhar]

On Sat, Nov 18, 2017 at 9:34 AM Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 18 November 2017 at 09:03, Neil Girdhar <mistersheik at gmail.com> wrote:
> > On Fri, Nov 17, 2017 at 3:15 AM Nick Coghlan <ncoghlan at 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   |   ncoghlan at gmail.com   |   Brisbane, Australia
>
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