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I (Apparently) Suck at Writing Tests

July 3rd, 2011 Posted in CPAN, Perl

(This probably looks familiar. Somehow, my WordPress install got rolled-back a few days. I don’t know how or why. I only know that this post disappeared, as did changes to a plug-in and changes to my “On Javascript and Flash” page. So apologies for seeing this post twice. I’m restoring it from Google’s cache. Clearly I need to spend what remains of the long holiday weekend setting up automated regular backups of my WP installation, something I should have already done were I not so lazy.)

As I go in to what is (for me) a 4-day weekend*, my plans are largely based around writing tests. Lots of tests, most likely. Because, it seems, I’m not very good at it.

That’s a pretty harsh statement for me to make, especially considering that I work in a QA organization. (Though, to be fair, I don’t actually write tests here— I work on the framework that my organization uses to develop their automated test suites.) And (to be fair) I’m probably being overly hard on myself. But I recently started playing around with Devel::Cover, and I was shocked to find out how much of my code is not covered by my tests.

The gist I show above is the sequence of commands that I use to generate coverage data. I adapted this from someone else’s blog post about Devel::Cover. It’s simple, but the part that had kept me from using this before now was not knowing how to apply it to the test suite as a whole, which is what setting HARNESS_PERL_SWITCHES does. (Turns out this is also covered in the man page for Devel::Cover, but for some reason I hadn’t seen that…)

The results have not been pretty.

My most-used module, Image::Size? Overall coverage is 57%, with statement coverage at 67.7%. My RPC::XML module is currently at 78.1% overall (84.1% statement) in my development copy, but that’s after many hours of work over the last week or so. It started out in the low 60s. And I haven’t even applied this to all of my modules.

It seems that my weakest area is in negative testing— I’m just not covering all of my error cases. In one module from RPC::XML, the statement coverage was barely over 50% because the module itself (RPC::XML::Parser) is meant as a common base-class with some code in new() to allow for backwards-compatibility with older versions of RPC::XML. I define the other methods that child classes should override, but I never tested the error-catching code that ensures that the methods are overridden. An hour and a new test-suite later, and this is one of only two modules that has 100% statement coverage (but not 100% overall… it’s only 66.7% covered on conditionals).

Of course, I’m being overly hard on myself. And I’m not really that serious about considering myself so poor at test development. What this (ongoing) exercise has shown me, is how much more I have to learn about it. I do try to practice test-driven development where I can, but I’m not sure that has yielded good coverage in all cases. I tend to practice defensive coding, trying to cover all the potential error cases where I can. But that is no guarantee that I have written tests for all of the potential error cases. Indeed, so far it’s pretty clear that I haven’t.

So if you haven’t looked in to Devel::Cover yet, give it a go. It’s been a great help to me!

* My employer is celebrating our ranking on Fortune’s Great Places to Work list by giving us an extra day off for the holiday weekend. There’s a reason why I like this place!

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