On 17 November 2017 at 15:52, Steven D'Aprano <steve@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@gmail.com | Brisbane, Australia
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