One of my TDD weaknesses: mock objects for complex objects

April 27, 2009

I am not really tuned in to test driven development yet, although I've written some programs that way (and found them quite valuable). As a result, I have some weaknesses, some TDD things that I just haven't acclimatized to yet. One of them is mock objects, specifically mock versions of complex objects; I don't like them and thus I don't use them. In trying to understand my dislike of them, I think that they seem too complex and fragile for at least two reasons.

First, how do I know that I've got the behavior of the mock objects correct? By definition, the complex objects generally have complex behavior, which can easily mean that the mock versions also need relatively complex behavior, which opens up the possibility of bugs. Second, how do I know that the mock objects are still behaving the same as the real objects? If I change the behavior of the real ones (complete with changing their unit tests to match), I may or may not remember the mock versions, and I may or may not realize that the change I made should change the mock version's responses in some way.

Both of these make me feel that mock versions of complex objects are fragile. I don't like fragile tests; they're a recipe for problems and frustrations, and at least in my state of TDD awareness, frustrations can easily lead to abandoning tests entirely. My current solution is to use more or less real objects but to have a unittest test ordering that matches the bottom up dependencies of the program. If early unittests fail, I know that it is pointless to go on; higher levels of code would just see a whole series of cascade failures as things malfunction underneath them.

(It is possible that part of my problem here is that I am confusing unit tests with some other sort of testing, functional or integration or end to end tests, as a result of having only an informal exposure to TDD.)

Written on 27 April 2009.
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Last modified: Mon Apr 27 22:05:33 2009
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