Using mock objects without tying down unit tests

Question

I'm currently writing a set of unit tests for a Python microblogging library, and following advice received here have begun to use mock objects to return data as if from the service (identi.ca in this case).

However, surely by mocking httplib2 - the module I am using to request data - I am tying the unit tests to a specific implementation of my library, and removing the ability for them to function after refactoring (which is obviously one primary benefit of unit testing in the firt place).

Is there a best of both worlds scenario? The only one I can think of is to set up a microblogging server to use only for testing, but this would clearly be a large amount of work.

Ned Batchelder · Accepted Answer · 2011-02-06 18:37:38Z

1

You are right that if you refactor your library to use something other than httplib2, then your unit tests will break. That isn't such a horrible dependency, since when that time comes it will be a simple matter to change your tests to mock out the new library.

If you want to avoid that, then write a very minimal wrapper around httplib2, and your tests can mock that. Then if you ever shift away from httplib2, you only have to change your wrapper. But notice the number of lines you have to change is the same either way, all that changes is whether they are in "test code" or "non-test code".

answered Feb 6, 2011 at 18:37

Ned Batchelder

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Rafał Dowgird Over a year ago

I think that the wrapper might (at least in theory) decrease the size of changes required for swapping httplib2 for something else. Without wrapper you need to change the classes being tested and the mockup object. With wrapper, you only change the wrapper (the old mockup should still work.) This of course assumes that the wrapper's API is general enough not to change after swapping the library.

Rafał Dowgird · Accepted Answer · 2011-02-06 16:51:29Z

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Not sure what your problem is. The mock class is part of the tests, conceptually at least. It is ok for the tests to depend on particular behaviour of the mock objects that they inject into the code being tested. Of course the injection itself should be shared across unit tests, so that it is easy to change the mockup implementation.

answered Feb 6, 2011 at 16:51

Rafał Dowgird

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