Can someone please advise where did I get wrong in my class method definition?

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, output):
            stringList = output.split('-')
            pattern = []
            for item in stringList:
                if item == 'dash':
                    pattern.append('-')
                elif item == 'dot':
                    pattern.append('.')
                else:
                    pass
            return cls(pattern)


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