[{'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:## create 1-year lag value of store sales (can't use 1-week as test set doesn't have any `Weekly_Sales` ##\n## X_train ends at 2012-10-26 and X_test ends at 2013-07-26, so no NaN values for `Weekly_Sales_Lag` in X_test ##"}, {'role': 'assistant', 'content': 'helper_functions'}]