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March 29, 2012

Filed under: tech»coding

Test Driven Testing

Next week a new quarter of Intro to JavaScript will start, and while I'm almost sick of writing about it, it's been constantly on my mind. I've talked a little here about some of the changes I'll be making--primarily I'll be teaching jQuery first--but there's another big experiment I'm going to try: using test-driven development tools to give students feedback on assignments and quizzes throughout the class. I did this in a limited way during the Winter quarter, with an assignment and an exam built using unit tests, but now I want to make it much more widespread. Why use testing this way? Two reasons: to force students to code, and to encourage them to think more critically about their mistakes.

The first reason may seem odd--they're in a programming class, shouldn't that force them to code already?--but it is really an extension of drill learning. I am not good at teaching via drills: I appreciate the value of repetition when I'm learning myself, but I hate standing in class and going through the same exercise again and again. I'd much rather give lectures about theory and application than do drills. But it's clear from last quarter that many students do not get enough practice from lectures. They may be too busy to write code at home, and assignments don't usually force them to use all the constructs of the language.

Drill and repetition are important because, just as with a spoken language, it's not enough to learn the rules of grammar. You need to go out and practice, so that the mechanics become natural and you can concentrate on expressing yourself. That's the motivation behind Zed Shaw's Learn Python the Hard Way series, and I tend to agree. Take the humble for loop, for example. I've written loops for so long I could do them in my sleep, but if you're just starting out there are a lot of moving parts that need to be grasped: for (var i = 0; i < list.length; i++) { //loop goes here } We've written a keyword (for), a series of semicolon-delimited statements inside parentheses, a block (represented by the curly braces), and we haven't even gotten to the actual loop. Just within this boilerplate, we're initializing a counter with the arbitrary name of "i" and giving it an initial value, testing it, and incrementing it. All of this is expressed in a peculiar syntax (the test and increment are tossed in after the setup statement, not placed in the order where they actually get executed), with a lot of very particular punctuation, plus a lot of bookkeeping to track mentally.

Writing for loops (or conditionals, or functions, or almost anything else) is therefore as much about learning a very specific arrangement of characters as it is about the overarching concepts. Languages can make this slightly easier to grasp (as with Python's comprehension-style loops and no-braces indentation), but I don't think you ever get away from it entirely. It is my goal this quarter to make students write as many of any given construct as possible in class, so that the low-level syntax becomes automatic, while using tests to keep the high-level goals in sight.

Which brings us to the second reason for using constant testing: students need to learn how to find mistakes by thinking critically about their output. Part of that process involves teaching them to use the developer tools: how to read a Firebug log, understand an interpreter error, and examine values in the debugger. But it also means they need to learn to check their code constantly, and to have thought about what they expect to get out of any given block. While I'll be providing tests on quizzes and homework assignments, so that students can immediately see if their answer was correct, the ultimate goal is to have them write their own simple tests to check their own code.

It's my belief that most of my students know what they want to do at the very highest level, but they're not able to break those goals into smaller pieces, and they don't know how to figure out when the smaller pieces stop working. My hypothesis is that writing tests may not be enough to create critical thinking where none exists, but it will serve as a good starting place for discussions of problem-solving strategy: what caused this test to fail? What were our expectations, and how were the results different? Was this test actually a good test to begin with, even if it "passes?" These are questions that good programmers ask, perhaps naturally, but I refuse to believe that they can't be encouraged in any student.

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