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):
        pattern = []
        split_string = string.split('-')
        for i in split_string:
            if i == 'dot':
                pattern.append('.')
            elif i == 'dash':
                pattern.append('_')
        return cls(pattern)

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