如何在python中存根S3Transfer

import unittest
from unittest.mock import Mock, patch
import boto3

class MyTestCase(unittest.TestCase):
    @patch('boto3.s3.transfer.S3Transfer')
    def test_s3_interaction(self, MockS3Transfer):
        # 创建一个模拟的S3Transfer实例
        mock_transfer = MockS3Transfer.return_value
        
        # 设置模拟方法的返回值
        mock_transfer.upload_file.return_value = None
        mock_transfer.download_file.return_value = None
        
        # 使用模拟的S3Transfer实例进行测试
        s3_client = boto3.client('s3')
        s3_client.upload_file('local_file.txt', 'my-bucket', 'remote_file.txt')
        s3_client.download_file('my-bucket', 'remote_file.txt', 'local_file_downloaded.txt')
        
        # 断言模拟方法被正确调用
        mock_transfer.upload_file.assert_called_once_with('local_file.txt', 'my-bucket', 'remote_file.txt')
        mock_transfer.download_file.assert_called_once_with('my-bucket', 'remote_file.txt', 'local_file_downloaded.txt')

if __name__ == '__main__':
    unittest.main()