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

Neil Girdhar mistersheik at gmail.com
Mon Nov 20 21:42:47 EST 2017 
On Mon, Nov 20, 2017 at 9:34 PM Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 21 November 2017 at 11:09, Neil Girdhar <mistersheik at gmail.com> wrote:
> >
> > On Sat, Nov 18, 2017 at 9:29 PM Nick Coghlan <ncoghlan at gmail.com> wrote:
> >>
> >>     >>> class C: pass
> >>     ...
> >>     >>> class S(C): pass
> >>     ...
> >>     >>> class E: pass
> >>     ...
> >>     >>> class B(S, E): pass
> >>     ...
> >>     >>> class R(E, C): pass
> >>     ...
> >>     >>> class Z(B, S, R, E, C): pass
> >>     ...
> >>     Traceback (most recent call last):
> >>      File "<stdin>", line 1, in <module>
> >>     TypeError: Cannot create a consistent method resolution order
> >> (MRO) for bases C, E
>
> > Sorry, I wrote back too quickly.  I meant also to change B's requested
> MRO
> > to be:
> >
> > (B, S, E, C)
> >
> > It works with that change.
>
> Right, but once you do that, then the existing resolver is already
> able to handle things:
>
>     >>> class C: pass
>     ...
>     >>> class S(C): pass
>     ...
>     >>> class E: pass
>     ...
>     >>> class B(S, E, C): pass
>     ...
>     >>> class R(E, C): pass
>     ...
>     >>> class Z(B, R): pass
>     ...
>     >>>
>

Sure, but like I mentioned, that's really ugly because B doesn't care about
C and shouldn't have to mention it as a base class.  This solution can
quickly spiral out of control so that changes in the inheritance graph
require you to hunt down extraneous base classes.


>
> If you wanted to pick up cases where two classes generate inconsistent
> MROs that will prevent mutual subclasses (like "class B(S, E)" vs
> "class R(E, C)"),


yes, exactly.


> that feels like a job for a type checker, since it
> isn't obvious whether it's the definition of B or the definition of R
> that should be changed to reorder their MRO.
>

I don't know what the "type checker" means here.  I figured that the
easiest user specification would be precedence relationships between
classes that could be applied (in the way I described).  And I figured that
that could be processed for given classes when their MRO is generated.

The proposal of having a custom MRO generator would also be able to solve
this problem.


> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
>
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