mock

Mocking is a pretty common thing to do in unit tests covering OpenStack Nova code. Over the years we've used various mock libraries to do that, with the flavor de jour being unittest.mock. I must say that I strongly prefer unittest.mock to the old mox code we used to write, but I think I just accidentally found a fairly big bug. The problem is that python mocks are magical. Its an object where you can call any method name, and the mock will happily pretend it has that method, and return None. You can then later ask what "methods" were called on the mock. However, you use the same mock object later to make assertions about what was called. Herein is the problem -- the mock object doesn't know if you're the code under test, or the code that's making assertions. So, if you fat finger the assertion in your test code, the assertion will just quietly map to a non-existent method which returns None, and your code will pass. Here's an example: #!/usr/bin/python3 from unittest import mock class foo(object): def dummy(a, b): return a + b @mock.patch.object(foo, 'dummy') def call_dummy(mock_dummy): f = foo() f.dummy(1, 2) print('Asserting a call should…