Why is it wrong?

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, in_here):
        pattern = []
        for i in in_here.split('-'):
            if i == 'dot':
                pattern.append('.')
            else:
                pattern.append('_')
        return (pattern)


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