Python单元测试异常抛出失败：LoggableException捕获问题排查

我想编写单元测试，验证方法中捕获Exception后重抛LoggableException的逻辑。为触发异常，通过patch模拟logging.info抛出Exception。

当前测试表现 #

• 被patch的函数抛出Exception（符合预期）
• Exception被捕获并尝试抛出LoggableException（符合预期）
• 函数终止但异常未被测试用例的except块捕获（不符合预期）
• 单元测试失败

预期测试表现 #

• 触发被patch的Exception
• Exception被捕获并抛出LoggableException
• 测试用例捕获LoggableException，测试通过

相关代码 #

util.py（不可修改） #

class LoggableException(BaseException):
    def __init__(self, **kwargs):
        self.message = kwargs['message']

work.py（不可修改） #

import logging

from util import LoggableException
import logging

class Server:
    def __init__(self):
        logging.getLogger().setLevel(logging.INFO)

    def cat_work(self):
        try:
            logging.info("hi there")
        except Exception as ex:
            # addl. stuff
            raise LoggableException(message='cat was here')     

test.py（可修改） #

from util import LoggableException
from work import Server
from unittest.mock import Mock, patch

class TestingWork:
    def __init__(self):
        self.server = Server()

    def test_cat_work_thing(self):
        with patch("logging.info", Mock(side_effect=Exception())):
            try:
                self.server.cat_work()
                assert False
            except LoggableException:
                print("Caught the correct exception")
            except Exception:
                print("Caught the wrong exception")

tests = TestingWork()
tests.test_cat_work_thing()

测试输出 #

[Running] python3 -u "/Users/sudomeacat/Dev/experimental/mock_testing/tests.py"
Traceback (most recent call last):
  File "/Users/sudomeacat/Dev/experimental/mock_testing/work.py", line 12, in cat_work
    logging.info("hi there")
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/unittest/mock.py", line 1093, in __call__
    return self._mock_call(*args, **kwargs)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/unittest/mock.py", line 1097, in _mock_call
    return self._execute_mock_call(*args, **kwargs)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/unittest/mock.py", line 1152, in _execute_mock_call
    raise effect
Exception

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/sudomeacat/Dev/experimental/mock_testing/tests.py", line 18, in <module>
    tests.test_cat_work_thing()
  File "/Users/sudomeacat/Dev/experimental/mock_testing/tests.py", line 12, in test_cat_work_thing
    self.server.cat_work()
  File "/Users/sudomeacat/Dev/experimental/mock_testing/work.py", line 15, in cat_work
    raise LoggableException(msg='cat was here', word='not really')
  File "/Users/sudomeacat/Dev/experimental/mock_testing/util.py", line 3, in __init__
    self.msg = kwargs['message']
KeyError: 'message'

[Done] exited with code=1 in 0.122 seconds

问题分析与解决 #

从报错回溯可以明确问题根源：

1. 模拟logging.info抛出Exception的逻辑正常执行
2. 进入异常捕获块后，实际运行的work.py代码中，抛出LoggableException时传入的是msg='cat was here', word='not really'，而非你提供代码中的message='cat was here'
3. util.py中LoggableException的构造函数依赖kwargs['message']，缺少该参数直接引发KeyError
4. 这个KeyError是未被捕获的新异常，导致测试用例的except LoggableException块无法触发，最终测试失败

解决步骤 #

• 首先确认测试环境的代码版本：你提供的work.py代码与实际运行的代码存在参数不一致问题，需同步代码保证抛出LoggableException时传入正确的message参数
• 若work.py确实不可修改，需检查是否存在其他代码逻辑干扰LoggableException的初始化流程

内容的提问来源于stack exchange，提问作者sudomeacat