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)

@classmethod
def from_string(cls,string):
        answer = []
        tokens = string.split('-')  
        for char in tokens: 
            if char == 'dot':
                answer.append('.')
            elif char == 'dash':
                answer.append('_')
        return cls(answer)       
class S(Letter):
    def __init__(self):
         pattern = ['.', '.', '.']
         super().__init__(pattern)