[{'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:NULL_VAL = "NULL_VAL"\n\ndef fillState(state, country):\n    if state == NULL_VAL: \n        return country\n    \n    return state\n\nX_Train = df_train.loc[:, [\'State\', \'Country\', \'Date\', \'ConfirmedCases\', \'Fatalities\']]\nX_Train[\'State\'].fillna(NULL_VAL, inplace=True)\nX_Train[\'State\'] = X_Train.loc[:, [\'State\', \'Country\']].apply(lambda x : fillState(x[\'State\'], x[\'Country\']), axis=1)\nX_Train.loc[:, \'Date\'] = X_Train.Date.dt.strftime("%m%d")\nX_Train["Date"]  = X_Train["Date"].astype(int)\n\nX_Test = df_test.loc[:, [\'State\', \'Country\', \'Date\', \'ForecastId\']]\nX_Test[\'State\'].fillna(NULL_VAL, inplace=True)\nX_Test[\'State\'] = X_Test.loc[:, [\'State\', \'Country\']].apply(lambda x : fillState(x[\'State\'], x[\'Country\']), axis=1)\nX_Test.loc[:, \'Date\'] = X_Test.Date.dt.strftime("%m%d")\nX_Test["Date"]  = X_Test["Date"].astype(int)\n\nprint("*"*50)\nprint(X_Train.head())\nprint("*"*50)\nprint(X_Test.head())\nprint("*"*50)\''}, {'role': 'assistant', 'content': 'data_preprocessing'}]