Cubecumber

Alongside my work on Karl Broman’s visualizations, I have continued to add features and tests for my Data Cube generator. As a result of some conversations with other members of the development community, we decided to change the way missing values are handled, which entailed a fair amount of refactoring, so I needed to make sure a good set of tests was in place. I’ve built out the Rspec tests to cover some of these situations, but we were also asked to create some tests using Cucumber, a higher level Behavior Driven Development focused testing tool. I’ve always just used Rspec, since it’s favored by some of the people in Madison who helped get me into Ruby. Many people are not fond of the more magic (seeming) way in which cucumber works, but I thought it’d at least be good to learn the basics even if I didn’t use it much beyond that.

Sadly this is not from a bar catering primarily to Rubyists (source)

Turns out, I really like Cucumber. I may even grow to love it one day, although I don’t want to be too hasty with such pronouncements since our relationship is barely a week old. With Rspec, I like the simplicity of laying everything out in one file and organizing instance variables by context, but I still find myself frustrated by little things like trying to test slight variations on an object or procedure. I know Rspec allows for a lot of modularity if you know how to use it, but personally I just find them a little too clunky most of the time. Cucumber, by contrast, is all about reusability, of methods and objects, and the interface it presents just seems to click a lot more with me.

Cucumber is built around describing behaviors, or “features”, of your application in a human readable manner, and tying this description to code that actually tests the features. You create a plaintext description, and provide a list of steps to take to achieve it. The magic comes in the step definitions, which are bound using regular expressions to allow the reuse of individual steps in different scenarios.

Here’s a simple scenario:

Most of this is just descriptive information. The main thing to notice is the “Scenario”, which contains “Given” and “Then” keywords. These, in addition to the “When” keyword (which isn’t used in this example) are the primary building blocks of a Cucumber feature. They are backed up by step definitions in a separate file, written in Ruby.

Cucumber will pull in these step definitions and use them for the “Given” and “Then” calls in the main feature. So far, so good, but this really isn’t worth the added complexity over Rspec yet. If I had a more complicated feature such as this one:

I could add “Given” definitions to cover the new Scenarios, but that would be a waste of the tools Cucumber offers (and the powers of Ruby). Instead, I can put parentheses around an element of the regular expression in the step definition to make it an argument for the step. Then with just a little Ruby cleverness I can reuse one step definition for all of these scenarios:

Nothing too complicated here, but it always makes me smile when I can do something simply in Ruby that I’d only barely feel comfortable hacking together with reflection in Java even after an entire undergrad education in the language.

I’ve written some steps to test the presence of various Data Cube elements in the output of a generator (check the repository if you’re interested), and I’m also working on moving my Integrity Constraints tests over from Rspec. This will make it easy to test my code on your code, or random csv files, so will help with adding features and investigating edge cases tremendously.

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