python - self.assertRaises 作为上下文管理器，msg 参数未按预期工作

Question

请检查以下代码:

import unittest


CORRECT_MESSAGE = 'Correct message'
WRONG_MESSAGE = 'Wrong message'


def fn():
    raise KeyError(CORRECT_MESSAGE)


class Test(unittest.TestCase):
    def test(self):
        # I am expecting this test to fail as the msg I am
        # checking is WRONG_MESSAGE, and not CORRECT_MESSAGE.
        with self.assertRaises(KeyError, msg=WRONG_MESSAGE):
            fn()


unittest.main()

---

如评论中所述，我预计此测试会失败，因为我正在检查的消息 (WRONG_MESSAGE) 不正确，但测试通过了。

我错过了什么？我检查过:assertRaises(exception, *, msg=None) .

Answer 1

我会回答我自己的问题。

---

我误解了msg的用法。来自 Python's official documentation :

All the assert methods accept a msg argument that, if specified, is used as the error message on failure.

用于调试目的。

例如

import unittest


CORRECT_MESSAGE = 'Correct message'
WRONG_MESSAGE = 'Wrong message'


def fn():
    return
    # Don't raise an exception.
    # raise KeyError(CORRECT_MESSAGE)


class Test(unittest.TestCase):
    def test(self):
        with self.assertRaises(KeyError, msg=WRONG_MESSAGE):
            fn()


unittest.main()

在输出中我们将得到:

AssertionError: KeyError not raised : Wrong message

msg 在未引发断言错误时使用。

---

实际的解决方案是使用assertRaisesRegex如果我们想检查异常消息。

例如

import unittest


CORRECT_MESSAGE = 'Correct message'
WRONG_MESSAGE = 'Wrong message'


def fn():
    raise KeyError(CORRECT_MESSAGE)


class Test(unittest.TestCase):
    def test_will_pass(self):
        # This will check if the error message is CORRECT_MESSAGE.
        with self.assertRaisesRegex(KeyError, CORRECT_MESSAGE):
            fn()

    def test_will_fail(self):
        # This will check if the error message is WRONG_MESSAGE.
        with self.assertRaisesRegex(KeyError, WRONG_MESSAGE):
            fn()

unittest.main()