construction zone

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)


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

@classmethod
def from_string(cls, my_string):
        my_list = my_string.split("_")
        output = []
        for element in my_list:
            if element == "dash":
                output.append("_")
            elif element =="dot":
                output.append(".")
        return cls(output)