[{'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:df = pd.merge(df, t_copy[['PROPERTY_NUMBER','mdate']] \\\n.groupby('PROPERTY_NUMBER').count(), how='left', left_on='parcel', right_index=True)\ndf['mdate'] = df['mdate'].fillna(0)\ndf = df.rename(columns = {'mdate':'t_num'})\nper = ((df.Date - dt.datetime(2006,1,1)).astype(pd.Timedelta).apply(lambda x: x.days)/365.0)\ndf['t_num'] = df['t_num']/per"}, {'role': 'assistant', 'content': 'data_preprocessing'}]