test.py

class Foo:
    @classmethod
    def my_list(cls):
        if hasattr(cls, 'mylist'):
            return cls.mylist
        cls.mylist = []
        return cls.mylist
  
class Bar(Foo):
    pass
  
class Baz(Foo):
    pass
  
a = Bar()
a.my_list().append(1)
b = Baz()
print(b.my_list())