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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_dictionary(self):
 """ Update the word frequency object """

        self._total_words = sum(self._dictionary.values())
        self._unique_words = len(self._dictionary.keys())
        self._letters = set()
        for key in self._dictionary:
            self._letters.update(key)