[{'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:sa_copy = pd.merge(sa, tci[['parcel','Date']].set_index('parcel'), how='left',left_on='parcel',right_index=True)\nsa_copy = sa_copy.loc[sa_copy.salesdt < sa_copy.Date]\nrecent_sa = sa_copy.sort_values('salesdt').groupby('parcel').last().reset_index()\nsa_days = pd.DataFrame(recent_sa[['parcel','salesdt','Date']])\nsa_days['sa_days'] = (sa_days.Date-sa_days.salesdt).astype(pd.Timedelta).apply(lambda x: x.days)\nsa_days = sa_days.set_index('parcel')\ndf = pd.merge(df, sa_days[['sa_days']], left_on='parcel',right_index=True, how='left')\ndf.loc[df.sa_days.isnull(),'sa_days'] = max(df[df.sa_days.notnull()].sa_days)+1"}, {'role': 'assistant', 'content': 'data_preprocessing'}]