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

Nick Coghlan ncoghlan at gmail.com
Fri Nov 17 03:14:39 EST 2017

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


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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