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

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   import unittest import mock from apiclient errors import HttpError   class FooTests unittest TestCase        mock patch  my tests bar       def test foo shouldReturnResultOfBar whenBarSucceeds self  barMock           barMock return value   True         result   foo           self assertTrue result     passes       mock patch  my tests bar       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       mock patch  my tests bar       def test foo shouldRaiseHttpError whenBarRaiseHttpErrorNot404 self  barMock           barMock side effect   HttpError mock Mock return value   status   500     error           with self assertRaises HttpError      passes             foo    def foo        try          result   bar           return result     except HttpError as error          if error resp status    404              print  404 -  s    error message             return None         raise  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 amp 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 with Errno set to a given value   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

User · Accepted Answer

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('my_tests.bar') as barMock:
...     barMock.side_effect = HttpError(mock.Mock(status=404), 'not found')
...     result = my_test.foo()
... 
404 - 
>>> result is None
True

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.

[python] Mocking a function to raise an Exception to test an except block

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