Mocking a function to raise an Exception to test an except block


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:

import unittest
import mock
from apiclient.errors import HttpError

class FooTests(unittest.TestCase):
    def test_foo_shouldReturnResultOfBar_whenBarSucceeds(self, barMock):
        barMock.return_value = True
        result = foo()
        self.assertTrue(result)  # passes

    def test_foo_shouldReturnNone_whenBarRaiseHttpError404(self, barMock):
        barMock.side_effect = HttpError(mock.Mock(return_value={"status": 404}), "not found")
        result = foo()
        self.assertIsNone(result)  # fails, test raises HttpError

    def test_foo_shouldRaiseHttpError_whenBarRaiseHttpErrorNot404(self, barMock):
        barMock.side_effect = HttpError(mock.Mock(return_value={"status": 500}), "error")
        with self.assertRaises(HttpError):  # passes

def foo():
        result = bar()
        return result
    except HttpError as error:
        if error.resp.status == 404:
            print "404 - %s" % error.message
            return None

def bar():
    raise NotImplementedError()

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.

Why is setting the side_effect of barMock 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?

Answer rating: 161

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:

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

Additional keyword arguments to Mock() are set as attributes on the resulting object.

I put your foo and bar definitions in a my_tests module, added in the HttpError class so I could use it too, and your test then can be ran to success:

>>> from my_tests import foo, HttpError
>>> import mock
>>> with mock.patch("") as barMock:
...     barMock.side_effect = HttpError(mock.Mock(status=404), "not found")
...     result =
404 - 
>>> result is None

You can even see the print "404 - %s" % error.message line run, but I think you wanted to use error.content there instead; that"s the attribute HttpError() sets from the second argument, at any rate.