'Ensure that the correct list of _subclasses (subclasses) is
assembled when importing part of the model.'

def test_external_subclasses(self):

    class Animal(Base, ):
        pass
    class Fish(Animal, ):
        pass
    class Guppy(Fish, ):
        pass
    class Mammal(Animal, ):
        pass
    class Dog(Mammal, ):
        pass
    class Human(Mammal, ):
        pass
    self.assertEqual(Animal._subclasses, ('Base.Animal', 'Base.Animal.Fish', 'Base.Animal.Fish.Guppy', 'Base.Animal.Mammal', 'Base.Animal.Mammal.Dog', 'Base.Animal.Mammal.Human'))
    self.assertEqual(Fish._subclasses, ('Base.Animal.Fish', 'Base.Animal.Fish.Guppy'))
    self.assertEqual(Guppy._subclasses, ('Base.Animal.Fish.Guppy',))
    self.assertEqual(Mammal._subclasses, ('Base.Animal.Mammal', 'Base.Animal.Mammal.Dog', 'Base.Animal.Mammal.Human'))
    self.assertEqual(Human._subclasses, ('Base.Animal.Mammal.Human',))
