Bummer: Didn't get the right string output. Got: dot-dot-dot for `S()`

class Letter:
    def __init__(self, pattern=None):
        self.pattern = pattern

    def __str__(self):
        string=''
        for item in self.pattern:
            if item=='.':
                if len(string)==0:
                    string+='dot'
                else:
                    string+='-dot'

            elif item=='_':
                if len(string)==0:
                    string+='underscore'
                else:
                    string+='-underscore'
        return "{}".format(string)


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