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

[Neil Girdhar]

On Mon, Nov 20, 2017 at 9:41 PM Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 21 November 2017 at 12:34, Nick Coghlan <ncoghlan at gmail.com> wrote:
> > 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
> >     ...
> >     >>>
> >
> > 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)"), 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 do wonder if we might be able to make the error message here less
> cryptic by mentioning which *listed* base classes brought in the
> conflicting MRO entries.
>
> Consider the current:
>
>     >>> class Z(B, R): 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
>
> vs something like:
>
>     TypeError: Cannot create a consistent method resolution order
> (MRO) for bases C, E (inherited through B, R)
>
> (Note: I haven't actually checked how practical it would be to
> implement something like that)
>

I totally agree.  I actually proposed this on ideas a few months ago and
then wrote something here: https://github.com/NeilGirdhar/inheritance_graph

If you have any suggestions or improvements, please feel free to improve
the code.

Best,

Neil

> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20171121/87748a0e/attachment.html>