Any idea why this code is not passing?

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, pattern):
        result = []
        fix_me = Letter(['_','.'])
        for item in pattern:
            if item =='dash':
                result.append("_")
            else:
                result.append("dot")
        return ".".join(result)




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