python: problem with indentation

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_input):
        str_output=[]
        str_wip=str_input
        for str_proc in str_wip.split('-'):
            if str_proc == 'dash':
                str_output.append('_')
            elif str_proc == 'dot':
                str_output.append('.')
            else:
                pass
        return cls(str_output)




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