Test-driven Development in Agile Projects
Hi, I'm Giv Parvaneh, and I'm a senior PHP web developer at the BBC. When I started working here earlier this year, I quickly realised that I would have to re-think the way I develop web applications. As a public service and one of the most visited websites in the UK, the BBC has an obligation to deliver quality products; the websites we build not only have to conform to web standards and be fully accessible, they need to be scalable to handle millions of users. Quickly hacking things together isn't really an option here...
The need for speed
Developers at the BBC tend to use Agile methodologies as a way to quickly release iterations of products. But where does rigorous code testing fit in with the short development and release cycles? How can we maintain the quality of our code when things need to change so fast?
A confession
OK: I'm guilty of releasing untested code in the past, where I've needed to meet deadlines. I'm probably not alone. And if you're a developer who's worked in a big team, you'll know that often you'll have to work with code that was written by your predecessors or contractors who didn't create 'test-friendly' code. Often this means having to waste time re-writing the code so that it can be tested. Naturally, this eats into your valuable development time during sprint cycles.
The Agile Manifesto states that we should value "Working software over comprehensive documentation". Some might see a problem here, because this seems to place a higher importance on "working software" than the quality of that software. The way I see it, you can't truly maintain "working software" if it's not well-tested... and the truth remains that many developers compromise good test coverage in order to complete tasks by the deadline. So what can we do?
A compromise
One thing I love about Python is its built-in support for documentation and testing. Even if you're not in the habit of writing unit tests for your code, you should at least comment your code so that you and other developers know what your classes and methods do. Python's "doctest" kills two birds with one stone: as you write your comments, you can optionally specify how the code should be used and what the expected outcome should be. Comments now serve as valuable documentation for your code, and can be executed as a test to ensure your logic is intact.
This method is not supposed to replace unit tests. In fact, there's only so much you can do with doctests, and your code can quickly become a huge mess if you try to write too many. But the idea is that they can be quickly used when you don't have time to write a full test suite. Some tests is better than no tests.
Let's have a look at an example in Python first:
def greetings(your_name): return 'hello, %s!' % (your_name)This function takes a person's name as an argument and returns a greetings message. Let's add this description and a test to the docs:
def greetings(your_name): """ This function takes a person's name and returns a greetings message >>> greetings('Auntie') 'hello, Auntie!' """ return 'hello, %s!' % (your_name)When you execute this script, Python is smart enough to know ">>>" means the comment needs to be interpreted as code and what the returned value should be.
We use PHP at the BBC, so I did some research and came across a PHP equivalent called PHPDT, which can be installed via Pear. Here's the PHP version of the above function:
/**
* This function takes a person's name and * returns a greetings message
*
* greetings('Auntie');
* // expects:
* // 'hello, Auntie!'
*
*/
function greetings($your_name)
{ return 'hello, ' . $your_name . '!';
}You can test this by running $ phpdt example.php in your terminal. The comments inside the <code> tags will be interpreted as PHP code.
Testing times
These are simple examples, but you can see how easily you could combine your comments with some testing to make sure that any accidental changes in your code are caught. Again, doctests aren't meant to replace unit tests, but they might help to keep things working when you're faced with the need to write code faster than your tests can keep up.
I plan to try adding doctests to my code, as well as continue writing proper unit tests in PHPUnit. I'd be really interested to hear from anyone who has tried this approach: on large applications, is this a good way to move fast without letting your test coverage slip?
