Getting correct output on my machine but it says wrong output here. What could be wrong?

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,patt):
        patt = patt.split('-')
        patt2=[]
        for i in patt:
            patt2.append("." if i=="dot" else "-")
        return cls(patt2)

class S(Letter):
    def __init__(self,pattern=None):
         # pattern = ['.', '.', '.']
         super().__init__(pattern)