Python Unittest Discover即使失败也返回退出代码0

我读过几篇文章说,如果用unittest.main()调用unittests,如果失败,它们将以失败代码退出。我用命令调用unittests: python -m unittest discover -v . 我正在使用Python3.6.6。unittest示例如下:

from server import app
import unittest

class ServerTestCase(unittest.TestCase):
    """
    Unittesting for the server application.
    """


    def setUp(self):
        """
        Create a test client
        """
        self.app = app.test_client()
        self.app.testing = True 

    def tearDown(self):
        pass

    def test_root_endpoint(self):
        """
        Testing the root endpoint
        """

        result = self.app.get('/') 
        self.assertEqual(result.status_code, 200) 

    def test_health_endpoint(self):
        """
        Testing the health endpoint
        """

        result = self.app.get('/health')
        assert b'UP' in result.data

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

即使其中一个测试失败,我在检查退出代码时也会得到:

$ echo $?
0

我做错什么了?