[ Python 常見問題 ] Mocking a function to raise an Exception to test an except block

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Question 
I have a function (foo) which calls another function (bar). If invoking bar() raises an HttpError, I want to handle it specially if the status code is 404, otherwise re-raise. I am trying to write some unit tests around this foo function, mocking out the call to bar(). Unfortunately, I am unable to get the mocked call to bar() to raise an Exception which is caught by my except block. 

Here is my code which illustrates my problem: 
- test.py 

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1. import unittest  
2. from unittest import mock  
3.   
4. class HttpError(Exception):  
5.     def __init__(self, result, msg):  
6.         super(HttpError, self).__init__(msg)  
7.         self.result = result  
8.   
9. class FooTests(unittest.TestCase):  
10.     @mock.patch('test.bar')  
11.     def test_foo_shouldReturnResultOfBar_whenBarSucceeds(self, barMock):  
12.         barMock.return_value = True  
13.         result = foo()  
14.         self.assertTrue(result)  # passes  
15.   
16.     @mock.patch('test.bar')  
17.     def test_foo_shouldReturnNone_whenBarRaiseHttpError404(self, barMock):  
18.         barMock.side_effect = HttpError(mock.Mock(return_value={'status': 404}), 'not found')  
19.         result = foo()  
20.         self.assertIsNone(result)  # fails, test raises HttpError  
21.   
22.     @mock.patch('test.bar')  
23.     def test_foo_shouldRaiseHttpError_whenBarRaiseHttpErrorNot404(self, barMock):  
24.         barMock.side_effect = HttpError(mock.Mock(return_value={'status': 500}), 'error')  
25.         with self.assertRaises(HttpError):  # passes  
26.             foo()  
27.   
28. def foo():  
29.     try:  
30.         result = bar()  
31.         return result  
32.     except HttpError as error:  
33.         if error.resp.status == 404:  
34.             print('404 - %s' % error.message)  
35.             return None  
36.         raise  
37.   
38. def bar():  
39.     raise NotImplementedError()  

Execution output: 
# pytest -s -v test.py 

... 
collected 3 items 

test.py::FooTests::test_foo_shouldRaiseHttpError_whenBarRaiseHttpErrorNot404 FAILED 
test.py::FooTests::test_foo_shouldReturnNone_whenBarRaiseHttpError404 FAILED 
test.py::FooTests::test_foo_shouldReturnResultOfBar_whenBarSucceeds PASSED 
...

I followed the Mock docs which say that you should set the side_effect of a Mock instance to an Exception class to have the mocked function raise the error. I also looked at some other related StackOverflow Q&As, and it looks like I am doing the same thing they are doing to cause and Exception to be raised by their mock. 

* https://stackoverflow.com/a/10310532/346561 
* How to use Python Mock to raise an exception - but...ith Errno set to a given value

Why is setting the side_effect of bar Mock not causing the expected Exception to be raised? If I am doing something weird, how should I go about testing logic in my except block? 

How-To 
Your mock is raising the exception just fine, but the error.resp.status value is missing. Rather than use return_value, just tell Mock that status is an attribute: 

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1. barMock.side_effect = HttpError(mock.Mock(status=404), 'not found')  

Additional keyword arguments to Mock() are set as attributes on the resulting object. The modified test.py will look like: 

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1. import unittest  
2. from unittest import mock  
3.   
4. class HttpError(Exception):  
5.     def __init__(self, resp, msg):  
6.         super(HttpError, self).__init__(msg)  
7.         self.resp = resp  
8.         self.message = msg  
9.   
10. class FooTests(unittest.TestCase):  
11.     @mock.patch('test.bar')  
12.     def test_foo_shouldReturnResultOfBar_whenBarSucceeds(self, barMock):  
13.         barMock.return_value = True  
14.         result = foo()  
15.         self.assertTrue(result)  # passes  
16.   
17.     @mock.patch('test.bar')  
18.     def test_foo_shouldReturnNone_whenBarRaiseHttpError404(self, barMock):  
19.         barMock.side_effect = HttpError(mock.Mock(status=404), 'not found')  
20.         result = foo()  
21.         self.assertIsNone(result)  # fails, test raises HttpError  
22.   
23.     @mock.patch('test.bar')  
24.     def test_foo_shouldRaiseHttpError_whenBarRaiseHttpErrorNot404(self, barMock):  
25.         barMock.side_effect = HttpError(mock.Mock(status=500), 'error')  
26.         with self.assertRaises(HttpError):  # passes  
27.             foo()  
28.   
29. def foo():  
30.     try:  
31.         result = bar()  
32.         return result  
33.     except HttpError as error:  
34.         if error.resp.status == 404:  
35.             print('404 - %s' % error.message)  
36.             return None  
37.         raise  
38.   
39. def bar():  
40.     raise NotImplementedError()  

The execution output: 

# pytest -s -v test.py 
... 
test.py::FooTests::test_foo_shouldRaiseHttpError_whenBarRaiseHttpErrorNot404 PASSED 
test.py::FooTests::test_foo_shouldReturnNone_whenBarRaiseHttpError404 404 - not found 
PASSED 
test.py::FooTests::test_foo_shouldReturnResultOfBar_whenBarSucceeds PASSED 
...