How can I make mock.mock_open raise an IOError?

Question

I need to test the instance method that calls open . In the first test case, I set mock.mock_open to return a string, as expected. It works great.

However, I also need to check the case when an IOError selected from this function. How can I make mock.mock_open raise an arbitrary exception?

This is my approach:

 @mock.patch.object(somemodule, 'generateDefaultKey') def test_load_privatekey(self, genkey) mo = mock.mock_open(read_data=self.key) mo.side_effect = IOError with mock.patch('__main__.open', mo, create=True): self.controller.loadPrivkey() self.assertTrue(genkey.called, 'Key failed to regenerate') 

Answer 1

Assign the mock.mock_open.side_effect exception:

 mock.mock_open.side_effect = IOError 

From the mock.Mock.side_effect documentation :

This can be either a function called when mock is called, or an exception (class or instance).

Demo:

 >>> mock = MagicMock() >>> mock.mock_open.side_effect = IOError() >>> mock.mock_open() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/Users/mj/Development/venvs/stackoverflow-2.7/lib/python2.7/site-packages/mock.py", line 955, in __call__ return _mock_self._mock_call(*args, **kwargs) File "/Users/mj/Development/venvs/stackoverflow-2.7/lib/python2.7/site-packages/mock.py", line 1010, in _mock_call raise effect IOError 

Using patch() as a context manager creates a new layout; assign an object to this layout:

 with mock.patch('__main__.open', mo, create=True) as mocked_open: mocked_open.side_effect = IOError() self.controller.loadPrivkey()