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

[Martin Teichmann]

Hi again,

staring at the code for hours, I just realized that there is
a very simple yet powerful solution to the metaclass merging
problem. The changes to be made are so simple there isn't
even a need to change the documentation!

The current algorithm to find a proper metaclass looks for
a class all other metaclasses are a subtype of. For that
it uses PyType_IsSubtype. We could simply change that
to PyObject_IsSubclass. This would give a great hook into
the system: we just need to intercept the __subclasshook__,
and we have a hook into the metaclass algorithm!

Backwards compatibility should not be a problem: how many
metaclasses are out there whose metaclass (so the
meta-meta-class) overwrites the __subclasshook__?

I changed the code appropriately, and also added a library
that uses this hook. It defines a class (called Dominant for
technical reasons, I'm waiting for suggestions for a better
name), which acts as a baseclass for metaclasses. All
metaclasses inheriting from this baseclass are combined
automatically (the algorithm doing that could still be improved,
but it works).

While I was already at it, I also added my variant of PEP 422,
which fits already into this metaclass scheme.

The code is at
https://github.com/tecki/cpython/commits/metaclass-issubclass

Greetings

Martin