如何使用 assertRaises() 引发错误

Question

我正在学习如何在 Python 中使用 unittest 进行单元测试。我想做的是在转数为负的情况下引发 ValueError，并显示一条消息，例如 'turn cannot be negative'.

到目前为止我写的代码如下：

import unittest
from canvas import Game


class TestPlayer1(unittest.TestCase):

    def setUp(self):

        # Game objects
        self.game_turn_0 = Game()
        self.game_turn_5 = Game()
        self.game_turn_negative = Game()

        # values
        self.game_turn_0.turn = 0
        self.game_turn_5.turn = 5
        self.game_turn_negative = -2

    def test_get_turn(self):
        self.assertEqual(self.game_turn_0.get_turn(), 0)
        self.assertEqual(self.game_turn_5.get_turn(), 5)
        with self.assertRaises(ValueError):
            print('value error!')


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

但是结果并不如你所见，如你所见：

Testing started at 09:55 ...
C:\Users\oricc\PycharmProjects\practisingDrowing\venv\Scripts\python.exe "C:\Program Files\JetBrains\PyCharm Community Edition 2019.2.3\helpers\pycharm\_jb_unittest_runner.py" --target test_canvas.TestPlayer1.test_get_turn
Launching unittests with arguments python -m unittest test_canvas.TestPlayer1.test_get_turn in C:\Users\oricc\PycharmProjects\practisingDrowing

pygame 1.9.6
Hello from the pygame community. https://www.pygame.org/contribute.html
value error!

Failure
Traceback (most recent call last):
  File "C:\Users\oricc\AppData\Local\Programs\Python\Python37-32\lib\unittest\case.py", line 59, in testPartExecutor
    yield
  File "C:\Users\oricc\AppData\Local\Programs\Python\Python37-32\lib\unittest\case.py", line 628, in run
    testMethod()
  File "C:\Users\oricc\PycharmProjects\practisingDrowing\test_canvas.py", line 23, in test_get_turn
    print('value error!')
  File "C:\Users\oricc\AppData\Local\Programs\Python\Python37-32\lib\unittest\case.py", line 203, in __exit__
    self._raiseFailure("{} not raised".format(exc_name))
  File "C:\Users\oricc\AppData\Local\Programs\Python\Python37-32\lib\unittest\case.py", line 135, in _raiseFailure
    raise self.test_case.failureException(msg)
AssertionError: ValueError not raised


Assertion failed

Assertion failed


Ran 1 test in 0.034s

FAILED (failures=1)

Process finished with exit code 1

Assertion failed

Assertion failed

Assertion failed

Assertion failed

我现在已经看了一些视频，阅读了 python website 上的文档，并阅读了一些帖子。我刚好能掌握。

我的意思是，我不知道如何在我的案例中使用它。

任何人都可以向我解释一下这是如何工作的吗？谢谢

Answer 1

class TestPlayer1(unittest.TestCase):

    def setUp(self):

        # Game objects
        self.game_turn_0 = Game()
        self.game_turn_5 = Game()
        self.game_turn_negative = Game()

        # values
        self.game_turn_0.turn = 0
        self.game_turn_5.turn = 5
        self.game_turn_negative = -2

    def test_get_turn(self):
        self.assertEqual(self.game_turn_0.get_turn(), 0)
        self.assertEqual(self.game_turn_5.get_turn(), 5)
        with self.assertRaises(ValueError):
            print('value error!')


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

Answer 2

您在预期会引发错误的操作周围使用 assertRaises 上下文管理器。如果您查看您的测试代码，您能看到应该引发错误的行吗？我不能。

你很接近 - 你有一般的结构。但你需要的是

def test_get_turn(self):
    self.assertEqual(self.game_turn_0.get_turn(), 0)
    self.assertEqual(self.game_turn_5.get_turn(), 5)
    with self.assertRaises(ValueError):
        self.game_turn_negative.get_turn()

我只改了最后一行。看看您期望引发异常的操作是怎样的？

此外，您在 setUp 中有一个错误 - 您需要设置 self.game_turn_negative.turn = -2，而不是 self.game_turn_negative = -2。一定要解决这个问题。

现在要检查测试是否有效，请尝试运行测试并查看它是否通过，然后更改您的代码，使负转向值不会引发异常，然后运行再次测试。您应该会看到测试套件失败，并抱怨 ValueError 没有在预期的时候出现。

Answer 3

with assertRaises 是围绕 with 块中引发异常的预期而设计的。在这种情况下，with 块中唯一的代码运行是 print('value error!') - 它永远不会引发 ValueError。这会导致断言失败，因为未引发 ValueError。

要解决您的问题，您需要调整您的应用程序，以便在检测到无效条件时引发 ValueError。然后，您将能够在 assertRaises 块内捕获 ValueError。值得注意的是，您对 self.game_turn_negative 的分配似乎有问题。对于 game_turn_0 和 game_turn_5 值，您将整数值分配给 .turn 属性，而不是顶级变量。对于 game_turn_negative，您将其设置为 Game()，然后再将其设置为 -2（而不是设置 self.game_turn_negative.turn）。

要修复您的代码，这可能有效：

import unittest
from canvas import Game


class TestPlayer1(unittest.TestCase):

    def setUp(self):

        # Game objects
        self.game_turn_0 = Game()
        self.game_turn_5 = Game()
        self.game_turn_negative = Game()

        # values
        self.game_turn_0.turn = 0
        self.game_turn_5.turn = 5
        self.game_turn_negative.turn = -2

    def test_get_turn(self):
        self.assertEqual(self.game_turn_0.get_turn(), 0)
        self.assertEqual(self.game_turn_5.get_turn(), 5)
        with self.assertRaises(ValueError):
            self.game_turn_negative.get_turn()


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

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