[Python-ideas] Consider (one day) adding an inheritance order class precedence mechanism
mistersheik at gmail.com
Sat Nov 18 15:56:43 EST 2017
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 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
Would you mind explaining why it's necessary for C3 to complain?
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.
> Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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