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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def update_placeholder_formats(self, format_string, placeholder_formats):
 """ Update a format string adding formats if they are not already present. """

        # Tokenize the format string and process them
        output = []
        for token in self.tokens(format_string):
            if (
                token.group("placeholder")
                and (not token.group("format"))
                and token.group("key") in placeholder_formats
            ):
                output.append(
                    "{%s%s}"
                    % (token.group("key"), placeholder_formats[token.group("key")])
                )
                continue
            value = token.group(0)
            output.append(value)
        return u"".join(output)