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+++
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title = "Things I Learnt The Hard Way - Testing Every Function Creates Dead Code"
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date = 2019-06-21
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[taxonomies]
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tags = ["en-au", "books", "things i learnt", "unit tests", "dead code"]
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+++
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If you write a test for every single function on your system, and your system
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keeps changing, how will you know when a function is not necessary anymore?
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<!-- more -->
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Writing a test for every single function on your system may come from the
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"100% Coverage Syndrome", which afflicts some managers, thinking that the only
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way to be completely sure your system is "bug free" is to write tests for
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every single line of code, till you reach the magical "100% coverage" in all
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the tests.
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I do believe you can reach 100% coverage, as long as you're willing to
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_delete_ your code.
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(Cue the universal grasps here.)
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But how do you know which pieces of code can be deleted?
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When I mentioned [integration
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tests](/books/things-i-learnt/integration-tests), I mentioned how much more
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sense it made to me reading them instead of the "unit" tests, because they
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were describing exactly how the system would operate in normal (and some
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abnormal) conditions. If you write tests that go through the system, assuming
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it is a black box with an input point and an output, and you can get tests for
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all the normal cases -- and some "abnormal", like when things go wrong -- then
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you know that, if you run those tests and they mark some lines as "not
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tested", it's because you don't need them.
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"But Julio, you're forgetting the error control!" I do agree, specially when
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you're talking with project owners or some other expert, that people will
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forget to tell you what to do in case of things going wrong -- say, the user
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typing their name in the age field -- but _you_ can see those and _you_ know
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that you need error control so _you_ can add the error control and describe
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the situation where that error control would trigger.
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If, on the other hand, you write a test for every function, when you do a
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short/simple check, you'll find that the function is still being used in the
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system by the tests, not actually, "value to the user" code. Sure, you can
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use your IDE to go back and forth between code and test and see if it points a
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use beyond the test, but it won't do it for yourself.
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There is one other weird thing about using integration tests for error
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controls: Sometimes, you can't reach the control statement. It's true! I did
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wrote control checks for every function once but, when running in the
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integration tests, there was no way to produce an input at the input layer of
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the system that would reach the error control in that function 'cause the
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other functions, which would run before the one I was trying to test, would
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catch the error before it. If that's a design problem or not -- it probably
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was -- it's a different discussion, but the fact is that that function didn't
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need error control, something that I wouldn't see if I wrote test specifically
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for it, but it was clear in an integration test run.
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{{ chapters(prev_chapter_link="/books/things-i-learnt/integration-tests", prev_chapter_title="Unit Tests Are Good, Integration Tests Are Gooder", next_chapter_title="Tests Make Better APIs", next_chapter_link="/books/things-i-learnt/tests-apis") }}
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