[{'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:features4 = ['Email_Campaign_Type', 'Email_Source_Type', 'Email_Type',\n'Subject_Hotness_Score', 'Time_Email_sent_Category', 'Total_Images',\n'Total_Links', 'Total_Past_Communications', 'Word_Count']\ndata4 = data3[features4]\nimputer = Imputer(missing_values='NaN', strategy='mean', axis=0)\nimputed_data = imputer.fit_transform(data4)\ndata5 = pd.DataFrame(imputed_data, index=data4.index, columns=data4.columns)"}, {'role': 'assistant', 'content': 'data_preprocessing'}]