class AnnotationSupport(type):
    '''Preload a class with a deferred reference to itself.
    After the class is created, scan function annotations
    and replace the deferred references with the actual class.

    '''

    @classmethod
    def __prepare__(mcl, name, bases):
        return {name: type(name, bases, {})}

    def __new__(mcl, name, bases, mapping):
        deferred = mapping.pop(name)
        cls = super().__new__(mcl, name, bases, mapping)
        for attrname, attrvalue in vars(cls).items():
            annotations = getattr(attrvalue, '__annotations__', {})
            for var, kind in annotations.items():
                if kind is deferred:
                    annotations[var] = cls
        return cls


if __name__ == '__main__':

    class Set(metaclass=AnnotationSupport):
        'Example of class that needs self-referencing function annotations'
        def union(self: Set, other: Set) -> Set:
            return Set(list(self) + list(other))

    print('Set id:', id(Set))
    print('Set vars:', list(vars(Set)))
    print('Set.union.__annotations:', Set.union.__annotations__)
    print('Id of return type:', id(Set.union.__annotations__['return']))