Code not working -construction zone

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, strinng):
        listt=stringg.split("-")
        pattern=[]
        for x in lits :
            if x =="dash":
                pattern.append("-")
            else:
                pattern.append(".")

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