Python:如何对子目录中的所有源文件运行unittest.main()？

import unittest
from os.path import dirname, join, abspath
from os import walk

def get_test_suite(test_dir):
    test_loader = unittest.TestLoader()
    test_suite = unittest.TestSuite()
    for root, dirs, files in walk(test_dir):
        for file in files:
            if file.endswith('_test.py'):
                module_name = file[:-3]
                module = __import__(module_name, fromlist=[module_name])
                test_suite.addTest(test_loader.loadTestsFromModule(module))
    return test_suite

if __name__ == '__main__':
    test_dir = join(dirname(abspath(__file__)), 'subdirectory')
    unittest.TextTestRunner().run(get_test_suite(test_dir))