two classes inheriting from one Higher level

Question

The following code seems to append ['next'] twice to the self.list_of, even though its acting on two independent objects.

class Higher:
    def __init__(self, list_of: list = ['item1', 'item2'],):

        self.list_of=list_of

        self.list_of += ['next']


class NumberOne(Higher):

    def __init__(self,**kwargs):

        super().__init__(**kwargs)


class NumberTwo(Higher):

    def __init__(self,**kwargs):

        super().__init__(**kwargs)


if __name__=="__main__":

    n = NumberOne()

    m = NumberTwo()

    print(m.list_of)

I would expect print(m.list_of) to return ['item1', 'item2', 'next'], instead of ['item1', 'item2', 'next', 'next']

I'm not sure whats causing this, can probably find a work around, but would rather understand whats going on.