guys need help im stuck

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


    def __iter__(self):
        yield from self.pattern

    def  from_string (cls ,str) :
        split_str=str.split ('-')
        list = []
        for e in split_str:
            if e =='dash':
                list.append('_')
            else: 
                list.append('.')
        return cls(list)        



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


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