[{'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:# concat (outer join)\ntrain_station_1 =  train_station_1.reset_index(drop= True)\ntrain_station_2 = train_station_2.reset_index(drop = True)\ntrain_merge = pd.concat([train_station_1, train_station_2], axis= 1)\n\ntest_station_1 =  test_station_1.reset_index(drop= True)\ntest_station_2 = test_station_2.reset_index(drop = True)\ntest_merge = pd.concat([test_station_1, test_station_2], axis= 1)'}, {'role': 'assistant', 'content': 'data_preprocessing'}]