How to create class object without __dict__

Question

class B(type):
    __slots__ = ()

class A(metaclass=B):
    pass

A.test = "test"

"A" is instance of metaclass "B" and "B" have __slots__ defined - why I have __dict__ at class A? How I can create class object without __dict__?

Answer 1

You cannot do this; classes always have a __dict__.

You can only use __slots__ on classes to produce instances without a __dict__, not on meta types. You'd normally only produce a few classes, so there is not much point in supporting __slots__ on metaclasses.

Don't use __slots__ to prevent attributes being set. Use __setattr__ for that instead:

class NoAttributesClassMeta(type):
    def __setattr__(cls, name, value):
        if name not in cls.__dict__:
            raise AttributeError("Cannot set attributes")
        type.__setattr__(cls, name, value)