[Python-ideas] A way out of Meta-hell (was: A (meta)class algebra)

[Martin Teichmann]

> You're asking us to assume covariant metaclasses in a subset of cases,
> while retaining the assumption of contravariance otherwise. That isn't
> going to happen - we expect type designers to declare metaclass
> covariance by creating an explicit subclass and using it where
> appropriate..

How do you solve my example with ABCs? That means if a
QObject (from PyQt) tries to implement an ABC by inheritance,
then one first has to create a new metaclass inheriting from
ABCMeta and whatever the PyQt metaclass is.

Sure, one could register the class to the ABC, but then you
loose the option of inheriting stuff from the ABC. One could
also write an adaptor - so using composition. But to me that
defeats the point of the concept of ABCs: that you can make
anything fit appropriately. In general, that smells to me like
a lot of boilerplate code.

> PEP 422 bypasses the default assumption of metaclass contravariance by
> operating directly on type instances, without touching any other part
> of the metaclass machinery.

So there is a problem with the metaclass machinery, and we
solve that by making one certain special case work? When is the
next special case going to show up? Will it also be included
into type?

Greetings

Martin