No method named `from_string`

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,pattern):
        pattern1 = pattern.split('-')
        newpattern = []
        for item in pattern1:
            if item == 'dot':
                newpattern.append('.')
            else:
                newpattern.append('_')
        newpattern = newpattern.join('-')
        return cls(newpattern)



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