cls(ptrn) just recreates the initial string since cls() runs the original letter init code

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, str_pattern):
        ptrn = []
        for val in str_pattern.split('-'):
            if val == 'dash':
                ptrn.append('-')
            elif val == 'dot':
                ptrn.append('.')
        return cls(ptrn)

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