[{'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:dftrain['Dayofyear'] = dftrain['Date'].dt.dayofyear\ndftest['Dayofyear'] = dftest['Date'].dt.dayofyear\ndftest['Expo'] = dftest['Dayofyear']-baseday\nprint(dftrain.tail(5))\ndftest = dftest.merge(dftrain[['Country_Region','Province_State','Date','ConfirmedCases','Fatalities']]\n                      , on=['Country_Region','Province_State','Date'], how='left', indicator=True)\n"}, {'role': 'assistant', 'content': 'data_preprocessing'}]