根据我做的循环，数据打印得很好，但是在它完成写入txt后，我收到了文件I/O错误。我没有删除all_true，因为它计算总价值。有人能告诉我什么是错误的，为什么系统抛出值I/O错误吗？
错误：文件“C：\Users\User\anaconda 3\lib\site-packages\ipykernel\kernelbase.py”，行277，在dispatch_shell sys.stdout.flush（）
ValueError：I/O operation on closed file.

import sys

with open('testing.txt', 'w') as f:
  sys.stdout = f
  for i in range(len(train_idx)):
    training_data, testing_data = dataset.iloc[train_idx[i]], dataset.iloc[test_idx[i]]
    tree = CART(training_data, training_data, training_data.columns[:-1])

    y_pred = test(testing_data, tree)
    y_true = testing_data["class"]

    y_pred = np.array(y_pred).astype(str)
    y_true = np.array(y_true).astype(str)

    all_true.append(list(y_true))
    all_pred.append(list(y_pred))

    print("----------------- Fold {} --------------".format(i+1))

    # calculate precision, recall and f1-score
    calculate_metrics(y_true, y_pred)

    # plot confusion matrix
    plot_confusion_matrix(y_true, y_pred)

  all_true = [v for item in all_true for v in item]
  all_pred = [v for item in all_pred for v in item]

  calculate_metrics(all_true, all_pred)

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