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

Nick Coghlan ncoghlan at gmail.com
Sat Nov 18 09:34:10 EST 2017

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

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

More information about the Python-ideas mailing list