Python, mock: raise exception

Question

I'm having issues raising an exception from a function in my test:

### Implemetation
def MethodToTest():
    myVar = StdObject()
    try:
        myVar.raiseError() # <--- here
        return True
    except Exception as e:
        # ... code to test
        return False

### Test file
@patch('stdLib.StdObject', autospec=True)
def test_MethodeToTest(self, mockedObjectConstructor):
    mockedObj = mockedObjectConstructor.return_value
    mockedObj.raiseError.side_effect = Exception('Test') # <--- do not work
    ret = MethodToTest()
    assert ret is False

I would like to raiseError() function to raise an error.

I found several examples on SO, but none that matched my need.

maybe this could help you http://stackoverflow.com/questions/2052390/manually-raising-throwing-an-exception-in-python — Alex L, Nov 22 '16 at 07:52
Are you sure you're patching in the right place (where it's imported, not where it's imported *from*)? — jonrsharpe, Nov 22 '16 at 07:54
I _think_ that I patch in the right place, as my other tests are working as expected. — user4780495, Nov 22 '16 at 08:26
[side_effect()](https://docs.python.org/3/library/unittest.mock.html#unittest.mock.Mock.side_effect) supports raising an exception. Maybe you patch the wrong object. — guettli, May 03 '21 at 14:26

score 78 · Accepted Answer · edited Feb 07 '19 at 14:27

78

I changed

@patch('stdLib.StdObject', autospec=True)

to

@patch('stdLib.StdObject', **{'return_value.raiseError.side_effect': Exception()})

and removed the # <--- do not work line.

It's now working.

This is a good example.

EDIT:

mockedObj.raiseError.side_effect = Mock(side_effect=Exception('Test'))

also works.

edited Feb 07 '19 at 14:27

ischenkodv

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answered Nov 22 '16 at 09:48

user4780495

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This work for me: mockedObj.side_effect = Exception('Test') – Hai Nguyen Dec 24 '18 at 06:18
14

arguments to `@patch` are passed onto the constructor of `Mock`, so using `@patch('stdLib.StdObject', side_effect=Exception('Test'))` works as well and is shorter. – luto Jan 20 '19 at 14:41
1

@luto comment worked for me. Patching `mockedObj.raiseError.side_effect` did not actually raise an error. – verboze Jun 01 '22 at 22:43

score 17 · Answer 2 · answered Nov 22 '16 at 12:45

17

Ok, your answer you provided is valid, but you changed how you did it (which is fine. To fix your original problem, you need to assign a function to side_effect, not the results or an object:

def my_side_effect():
    raise Exception("Test")

@patch('stdLib.StdObject', autospec=True)
def test_MethodeToTest(self, mockedObjectConstructor):
    mockedObj = mockedObjectConstructor.return_value
    mockedObj.raiseError.side_effect = my_side_effect # <- note no brackets, 
    ret = MethodToTest()
    assert ret is False

Hope that helps. Note, if the target method takes args, the side effect needs to take args as well (I believe).

answered Nov 22 '16 at 12:45

Jmons

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You do not need to assign a function; that's just one of the options, but it is not the best here. Just assign the exception to `side_effect` instead: `mockedObj.raiseError.side_effect = Exception("Test")`. – Martijn Pieters Feb 10 '18 at 18:03
1

You don't have to no: and his edit has a third way of doing it where he is making a Mock with the side effect set, but its still a valid, and good thing to know how to do in testing. Theres nothing wrong with doing it this way, especially as it lets you extend your test later? – Jmons Feb 17 '18 at 20:10
1

Did the OP edit their question? It looks like they are already assigning to `side_effect`, and the suggestion from @MartijnPieters looks exactly like the line that they commented "do not work". – Davos Nov 19 '18 at 00:31
I don't think the question was edited. I think at the time (bering in mind this was Nov 2016, and its now Nov 2018), my understanding of side_effect was that it was called as a function - which isn't true, You should be able to do things like `side_effect = 12`, Given that the OP give a radically different answer that they then accepted, I wonder if there were other, unpasted, code bugs. If i remember rightly I copied this code into a shell script and tested and also saw it 'not working' before I patched it to use a function. But that was two years ago! Now a better python programmer every day – Jmons Nov 23 '18 at 09:41

Python, mock: raise exception

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