Metaclasses in action

conflict.py

class MetaOne(type):
    def __new__(mcs, name, bases, attrs):
        kls = super().__new__(mcs, name, bases, attrs)
        kls._numbers = [1, 2, 3]
        return kls

class MetaTwo(type):
    def __new__(mcs, name, bases, attrs):
        kls = super().__new__(mcs, name, bases, attrs)
        kls._numbers = {1, 2, 3}
        return kls

class A(metaclass=MetaOne):
    pass

class B(metaclass=MetaOne):
    pass

assert hasattr(A, '_numbers')
assert hasattr(B, '_numbers')
assert A._numbers == B._numbers == [1, 2, 3]

try:
    class C(A, metaclass=MetaTwo):
        pass
except TypeError:
    print('Metaclass conflict exists between A and C')

class D:
    pass

class E:
    pass

class F(D, E, metaclass=MetaOne):
    pass

registry.py

class ModelRegistry:
    models = {}

class ModelMeta(type):
    @classmethod
    def __prepare__(mcs, name, bases):
        print('PRE', mcs, name, bases)
        return {'xyz': 4}

    def __new__(mcs, name, bases, attrs):
        print('NEW', mcs, name, bases, attrs)
        fields = {}
        for k, v in attrs.items():
            if isinstance(v, int):
                fields[k] = v
        for base in bases:
            if hasattr(base, '_fields'):
                fields.update(base._fields)
        attrs['_fields'] = fields
        kls = super().__new__(mcs, name, bases, attrs)
        if kls.__name__ != 'Model':
            ModelRegistry.models[name] = kls
        return kls

    def __init__(cls, *args, **kwargs):
        cls._jk = 'lol'

class CustomMeta(ModelMeta):
    pass

class Model(metaclass=ModelMeta):
    pass

class A(Model):
    foo = 1

class B(A):
    bar = 2

class C(metaclass=CustomMeta):
    reg = 3

print(A._fields)
print(B._fields)
print(B._jk)
print(ModelRegistry.models)

This code is a variation of an answer found in the following SO post: https://stackoverflow.com/questions/392160/what-are-some-concrete-use-cases-for-metaclasses