[{'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:def union_features(features_to_use, X_encode, X_vectorized, target, nrow_train):\n    X_origin = merge[features_to_use]\n    X_origin.fillna(0 ,inplace=True)\n    X = hstack((X_origin, X_encode, X_vectorized)).tocsr()\n    y = merge[target]\n\n    X_train_all = X[:nrow_train]\n    X_test = X[nrow_train:]\n    y_train_all = y[:nrow_train]\n    # y_test = y[nrow_train:]\n\n\n    X_train, X_val, y_train, y_val = train_test_split(X_train_all, y_train_all, test_size=0.2 , random_state=10)\n    return X_train, X_test, X_val, y_train, y_val \n\nX_train, X_test, X_val, y_train, y_val =  union_features(features_to_use, X_encode, X_vectorized, target, nrow_train)'}, {'role': 'assistant', 'content': 'data_preprocessing'}]