[{'role': 'system', 'content': "You are an expert in machine learning and your task is to classify code snippets into one of the following categories: ['helper_functions', 'load_data', 'data_exploration', 'data_preprocessing', 'evaluation', 'modelling', 'prediction', 'result_visualization', 'save_results', 'comment_only']."}, {'role': 'user', 'content': 'Identify the single most appropriate category to which the provided code belongs. Provide your response as a single category label without any additional information, commentary, or personal opinions.\\nPython Code:\n\ndef clean_sentences(df):\n    reviews = []\n\n    for sent in tqdm(df[\'Phrase\']):\n        \n        #remove html content\n        review_text = BeautifulSoup(sent).get_text()\n        \n        #remove non-alphabetic characters\n        review_text = re.sub("[^a-zA-Z]"," ", review_text)\n    \n        #tokenize the sentences\n        words = word_tokenize(review_text.lower())\n    \n        #lemmatize each word to its lemma\n        lemma_words = [lemmatizer.lemmatize(i) for i in words]\n    \n        reviews.append(lemma_words)\n\n    return(reviews)\n\ntrain_sentences = clean_sentences(train)\ntest_sentences = clean_sentences(test)\nprint(len(train_sentences))\nprint(len(test_sentences))\''}, {'role': 'assistant', 'content': 'data_preprocessing'}]