This will not pass. Help please!

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, self):
        split_list = self.split('-')
        output = []
        for blip in split_list:
            if blip == "dot":
                output.append(".")
            else:
                output.append("_")
        return cls(output)   

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