Why does the class definition's metaclass keyword argument accept a callable?

Question

Background

The Python 3 documentation clearly describes how the metaclass of a class is determined:

• if no bases and no explicit metaclas

Answer 1

Well, the type is of course MyMetaClass. metaclass_callable is initially 'selected' as the metaclass since it's been specified in the metaclass kwarg and as such, it's __call__ (a simple function call) is going to be performed.

It just so happens that calling it will print and then invoke MyMetaClass.__call__ (which calls type.__call__ since __call__ hasn't been overridden for MyMetaClass). There the assignment of cls.__class__ is made to MyMetaClass.

metaclass_callable is called once and then appears to be unrecoverable

Yes, it is only initially invoked and then hands control over to MyMetaClass. I'm not aware of any class attribute that keeps that information around.

derived classes do not use (as far as I can tell) metaclass_callable in any way.

Nope, if no metaclass is explicitly defined, the best match for the metaclasses of bases (here MyClass) will be used (resulting in MyMetaClass).

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As for question 2, pretty sure everything you can do with a callable is also possible by using an instance of type with __call__ overridden accordingly. As to why, you might not want to go full blown class-creation if you simply want to make minor changes when actually creating a class.