Stuck on the @classmethod

class Letter:
    def __init__(self, pattern=None):
        self.pattern = pattern

    def __iter__(self):
        yield from self.pattern   

    def __str__(self):
        output = []
        for blip in self:
            if blip == '.':
                output.append('dot')
            else:
                output.append('dash')
        return '-'.join(output)

    @classmethod
    def from_string(cls, string):
        a = cls(string).pattern
        new_list = []
        for x in a.split('-'):
            if x == 'dot':
                new_list.append('.')
            elif x == 'dash':
                new_list.append('_')
        return cls(new_list)



class S(Letter):
    def __init__(self):
         pattern = ['.', '.', '.']
        super().__init__(pattern)