I cant understand what's wrong with my code

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):
        patternList = []
        stringList = string.split('-')
        for item in stringList:
            if item.lower() == 'dash':
                patternList.append('_')
            elif item.lower() == 'dot':
                patternList.append('.')    
        return cls(patternList)

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