[{'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:# timestamp_first_active\ntfa = np.vstack(\n    df_all.timestamp_first_active.astype(str).apply(\n        lambda x: list(map(int, [x[:4], x[4:6], x[6:8],\n                                 x[8:10], x[10:12],\n                                 x[12:14]]))\n        ).values\n    )\ndf_all['tfa_year'] = tfa[:,0]\ndf_all['tfa_month'] = tfa[:,1]\ndf_all['tfa_day'] = tfa[:,2]\ndf_all = df_all.drop(['timestamp_first_active'], axis=1)"}, {'role': 'assistant', 'content': 'data_preprocessing'}]