Over-engineering

def test_next_visitor(visitor):
    Visitor = visitor.make_visitor()

    @Visitor.mark
    class Test1:
        pass

    @Visitor.mark
    class Test2(Test1):
        pass

    class Visitor1(Visitor):
        pass

    class Visitor2(Visitor1):
        pass

    @Visitor2.bind(Test2)
    def Visitor2(self, node, next_visitor):
        return 'a' + next_visitor('f')

    @Visitor1.bind(Test2)
    def Visitor1(self, node, next_visitor):
        return 'b' + next_visitor('f')

    @Visitor2.bind(Test1)
    def Visitor2(self, node, next_visitor):
        return 'c' + next_visitor('f')

    @Visitor1.bind(Test1)
    def Visitor1(self, node, next_visitor):
        return 'd' + next_visitor('e')

    assert Visitor2().visit(Test2()) == 'abcde'