Clean code that works, in Ron Jeffries' pithy phrase,
is the goal of TestDriven Development (TDD). Clean code that works
is a worthwhile goal for a whole bunch of reasons.
- It is a predictable way to develop. You know when you are
finished, without having to worry about a long bug trail.
- It gives you a chance to learn all of the lessons that the code
has to teach you. If you only slap together the first thing you
think of, then you never have time to think of a second, better
thing.
- It improves the lives of the users of your software.
- It lets your teammates count on you, and you on them.
- It feels good to write it.
But how do we get to clean code that works? Many forces drive us
away from clean code, and even from code that works. Without taking
too much counsel of our fears, here's what we do: we drive
development with automated tests, a style of development called
TestDriven Development (TDD). In TestDriven Development, we
- Write new code only if an automated test has failed
- Eliminate duplication
These are two simple rules, but they generate complex individual
and group behavior with technical implications such as the
following.
- We must design organically, with running code providing
feedback between decisions.
- We must write our own tests, because we can't wait 20 times per
day for someone else to write a test.
- Our development environment must provide rapid response to
small changes.
- Our designs must consist of many highly cohesive, loosely
coupled components, just to make testing easy.
The two rules imply an order to the tasks of programming.
- RedWrite a little test that doesn't work, and perhaps doesn't
even compile at first.
- GreenMake the test work quickly, committing whatever sins
necessary in the process.
- RefactorEliminate all of the duplication created in merely
getting the test to work.
Red/green/refactorthe TDD mantra.
Assuming for the moment that such a programming style is
possible, it further might be possible to dramatically reduce the
defect density of code and make the subject of work crystal clear
to all involved. If so, then writing only that code which is
demanded by failing tests also has social implications.
- If the defect density can be reduced enough, then quality
assurance (QA) can shift from reactive work to proactive work.
- If the number of nasty surprises can be reduced enough, then
project managers can estimate accurately enough to involve real
customers in daily development.
- If the topics of technical conversations can be made clear
enough, then software engineers can work in minutebyminute
collaboration instead of daily or weekly collaboration.
- Again, if the defect density can be reduced enough, then we can
have shippable software with new functionality every day, leading
to new business relationships with customers.
So the concept is simple, but what's my motivation? Why would a
software engineer take on the additional work of writing automated
tests? Why would a software engineer work in tiny little steps when
his or her mind is capable of great soaring swoops of design?
Courage.
Courage
Testdriven development is a way of managing fear during
programming. I don't mean fear in a bad waypow widdle
prwogwammew needs a pacifiewbut fear in the legitimate,
thisisahardproblemandIcan'tseetheendfromthebeginning sense. If pain
is nature's way of saying "Stop!" then fear is nature's way of
saying "Be careful." Being careful is good, but fear has a host of
other effects.
- Fear makes you tentative.
- Fear makes you want to communicate less.
- Fear makes you shy away from feedback.
- Fear makes you grumpy.
None of these effects are helpful when programming, especially
when programming something hard. So the question becomes how we
face a difficult situation and,
- Instead of being tentative, begin learning concretely as
quickly as possible.
- Instead of clamming up, communicate more clearly.
- Instead of avoiding feedback, search out helpful, concrete
feedback.
- (You'll have to work on grumpiness on your own.)
Imagine programming as turning a crank to pull a bucket of water
from a well. When the bucket is small, a freespinning crank is
fine. When the bucket is big and full of water, you're going to get
tired before the bucket is all the way up. You need a ratchet
mechanism to enable you to rest between bouts of cranking. The
heavier the bucket, the closer the teeth need to be on the
ratchet.
The tests in testdriven development are the teeth of the
ratchet. Once we get one test working, we know it is working, now
and forever. We are one step closer to having everything working
than we were when the test was broken. Now we get the next one
working, and the next, and the next. By analogy, the tougher the
programming problem, the less ground that each test should
cover.
Readers of my book Extreme Programming Explained will
notice a difference in tone between Extreme Programming (XP) and
TDD. TDD isn't an absolute the way that XP is. XP says, "Here are
things you must be able to do to be prepared to evolve further."
TDD is a little fuzzier. TDD is an awareness of the gap between
decision and feedback during programming, and techniques to control
that gap. "What if I do a paper design for a week, then testdrive
the code? Is that TDD?" Sure, it's TDD. You were aware of the gap
between decision and feedback, and you controlled the gap
deliberately.
That said, most people who learn TDD find that their programming
practice changed for good. Test Infected is the phrase
Erich Gamma coined to describe this shift. You might find yourself
writing more tests earlier, and working in smaller steps than you
ever dreamed would be sensible. On the other hand, some software
engineers learn TDD and then revert to their earlier practices,
reserving TDD for special occasions when ordinary programming isn't
making progress.
There certainly are programming tasks that can't be driven
solely by tests (or at least, not yet). Security software and
concurrency, for example, are two topics where TDD is insufficient
to mechanically demonstrate that the goals of the software have
been met. Although it's true that security relies on essentially
defectfree code, it also relies on human judgment about the methods
used to secure the software. Subtle concurrency problems can't be
reliably duplicated by running the code.
Once you are finished reading this book, you should be ready
to
- Start simply
- Write automated tests
- Refactor to add design decisions one at a time
This book is organized in three parts.
- Part I, The Money ExampleAn example of typical model code
written using TDD. The example is one I got from Ward Cunningham
years ago and have used many times since: multicurrency arithmetic.
This example will enable you to learn to write tests before code
and grow a design organically.
- Part II, The xUnit ExampleAn example of testing more
complicated logic, including reflection and exceptions, by
developing a framework for automated testing. This example also
will introduce you to the xUnit architecture that is at the heart
of many programmeroriented testing tools. In the second example,
you will learn to work in even smaller steps than in the first
example, including the kind of selfreferential hooha beloved of
computer scientists.
- Part III, Patterns for TestDriven DevelopmentIncluded are
patterns for deciding what tests to write, how to write tests using
xUnit, and a greatesthits selection of the design patterns and
refactorings used in the examples.
I wrote the examples imagining a pair programming session. If
you like looking at the map before wandering around, then you may
want to go straight to the patterns in Part III and use the
examples as illustrations. If you prefer just wandering around and
then looking at the map to see where you've been, then try reading
through the examples, referring to the patterns when you want more
detail about a technique, and using the patterns as a reference.
Several reviewers of this book commented they got the most out of
the examples when they started up a programming environment,
entered the code, and ran the tests as they read.
A note about the examples. Both of the examples, multicurrency
calculation and a testing framework, appear simple. There are (and
I have seen) complicated, ugly, messy ways of solving the same
problems. I could have chosen one of those complicated, ugly, messy
solutions, to give the book an air of "reality." However, my goal,
and I hope your goal, is to write clean code that works. Before
teeing off on the examples as being too simple, spend 15 seconds
imagining a programming world in which all code was this clear and
direct, where there were no complicated solutions, only apparently
complicated problems begging for careful thought. TDD can help you
to lead yourself to exactly that careful thought.
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