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

[Nick Coghlan]

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)

Cheers,
Nick.

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