not sure what I am missing here

class Letter:
    def __init__(self, pattern=None):
        self.pattern = pattern
    def __str__(self):
        output = "" #empty string to add items inside
        for letter in self.pattern: # looping through each letter in the pattern attribute
            if letter == ".": # checking if letter is "."
                output.append("dot") # If check is true, add it to the empty list
            elif letter == "_": # checking if lettet is "_"
                output.append("dash") # check is true, add it to the empty list
            return  "_".join("output")  # join the two results to come up with the desired output     

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