My code is returning the same 'dot-dash' string that I'm passing in.

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_list = []
        string_list = string.split('-')
        for blip in string_list:
            if blip == 'dash':
                pattern_list.append('-')
            elif blip == 'dot':
                pattern_list.append('.')
            else:
                pass
        return cls(pattern_list)

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