1.py

    def _bfs(self):
        word_queue = [(self.begin_word, 1)]
        word_enqueued = {self.begin_word: True}

        while word_queue:
            word, level = word_queue.pop(0)
            self.word_levels[word] = level

            if word == self.end_word: return level

            for next_word in self._get_next_words(word):
                if next_word not in word_enqueued:
                    word_queue.append((next_word, level + 1))
                    word_enqueued[next_word] = True

        return 0