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KataMinesweeper
An example solution using Python and TextTest
Test Driven Development (TDD) is a way of programming where you start be specifying a (failing) test, write code until it passes, then refactor. Repeat. This technique has been hugely valuable and adopted widely since it was introduced to the world by Kent Beck and others. The classic way to perform TDD is to specify the tests using the same programming language as the code under test, with the help of a little framework called xUnit. (Replace “x” with the language of choice). But how would TDD look if you used another tool instead of xUnit? Is there a way of capturing the advantages of the approach, but using tests that are easier to write and maintain?
About TDD with TextTest
I begain using xUnit in 2000 and have been improving my skill at TDD ever since. Recently though I have been working less and less with xUnit tests, and more and more with using TextTest tests to drive development. This approach seems to preserve the original spirit of TDD, ie the tests are still driving development, but there are some differences and advantages.
When you use TextTest to drive development, your tests all end up being written in a representation that is separate from your code, ie a kind of Domain Specific Language (DSL). Your tests can be read and understood by anybody who understands the domain of the system under test, that is, they don't have to be able to read or write program code. You typically don't write new code when you write a new test, you just specify new desired program behaviour using this DSL. This makes it quick to add new tests, and easy to refactor your code later, since the test doesn't place any demands on the internal structure of your program (its classes and methods).
With xUnit tests, each test is only working with a small section of code, whereas a TextTest typically drives a path through a whole system or subsystem. This means that you need to use another technique to get fine grained visibility of what your code is doing. Instead of turning to a debugger, the approach with TextTest is to insert log statements into your code. Having your application write a log is not a new idea and there are a number of excellent logging frameworks out there. TextTest allows you to not only specify the behaviour of your application in terms your customer will understand, but also control the application logging behaviour. When you make a code change, and your program no longer behaves the way the customer wants, (you introduce a bug), you can turn to the recorded application log to pinpoint exactly where program behaviour has deviated from previously. So a test case consists basically of program input, expected user-readable output, and an application log.
Another aspect of TDD with xUnit, is that it in some sense forces you to make good low level design decisions, and you don't generally get the same direct effect with TextTest. With xUnit, your code must exhibit loose coupling and high cohesion before you can extract units of code to test. This is desireable, and TextTest does not force you to do it, since it tests at a higher level of granularity. (On the other hand, if you are starting with poorly structured legacy code, where easily extractable units are hard to come by, you can get going with TextTest more or less immediately just by adding some (hopefully) harmless log statements. But I am not talking about legacy code in this article).
The other thing that TDD with xUnit does to help with low level design, is that when you define your test, you are effectively designing the interface to your class. I'm talking about choice of method names, and what arguments you give them. This is useful because it forces you to think about the external interface to your class before you consider the internal implementation. TextTest doesn't help much with this either. It will force you to start defining the interface to your whole program or subsystem, but it won't help on the class level. So when developing with TextTest we suggest something called “usage first” design. Start with the external interface to your program, and write code as if the classes and methods you want are already there. Then fill in the implementations, always working top down, and defining the usage of a class or method before your implement it. This takes some discipline, but leads to a similar effect to using xUnit.
About Code Katas
A code Kata is an idea introduced by Dave (Pragmatic) Thomas. He points out that specialists in other disciplines do a lot more practicing than programmers do, in order to improve their skills. Others have further developed this concept into the idea of a “Coding dojo”, or a place where programmers meet to practice code Kata together. The website www.codingdojo.org is a resource for these communities, and amongst other things has a catalogue of various code Kata exercises various individuals and groups have worked on.
About the KataMinesweeper screencast
So that is all the theory, how does it work in practice? In this screencast, I demonstrate a Prepared Kata of KataMinesweeper, using TextTest to drive development, in python. During development, I aim to work in a Test Driven manner, and I want to develop top down, usage first, in small steps. The idea is that although in real life you may not be so strictly test driven, or exclusively top down, or use such small steps, that it is important to know how to do these things, and they are worth practicing on a toy problem like this.
My aim is to demonstrate what it is like to do TDD with TextTest, and how you might solve this code Kata using the python programming language. If you would like to comment and give me constructive feedback on this screencast, I would love to hear from you. Please comment on www.codingdojo.org or the TextTest mailing list, or the python mailing list.
The screencast is in four parts, and I estimate each part will take you about 10 – 15 minutes to view.
(Note: This screencast was created using Wink. In it I use Windows Vista Home Edition, Easy Eclipse for LAMP version 1.2.2 and TextTest version 3.9.1.)
Here is the final code that I end up with: (Click to enlarge)
And here it is again in plain text in case you can't read that screenshot:

     import sys
     def log(message):
         print >> sys.stderr, message
     
     MINE = "*"
     END_OF_INPUT = "0 0"
     
     class Minefield:
         def __init__(self, m, n, minefield):
             self.m = m
             self.n = n
             self.minefield = minefield

         def clues_to_str(self, clues):
             clues_str = ""
             for i in range(self.m):
                 for j in range(self.n):
                     clues_str += str(clues[(i, j)])
                 clues_str += "\n"
             return clues_str

         def clues(self):
             clues = dict.fromkeys([ (x, y) for x in range(self.m)\
                  for y in range(self.n)], 0)
             log("initialized clues dictionary %s" % clues)
             for x, row in enumerate(self.minefield.splitlines()):
                 for y, cell in enumerate(row):
                     if cell == MINE:
             clues[ (x, y) ] = MINE
             self.increment_adjacent(clues, x, y)
             log("calculated clues %s" % clues)
             clues_str = self.clues_to_str(clues)
             return clues_str

         def increment_adjacent(self, clues, x, y):
             for i in (x-1, x, x+1):
                 for j in (y-1, y, y+1):
                     if clues.get( (i, j) ) not in [None, MINE]:
             clues[ (i, j) ] += 1

     def MinefieldReader(inputfile):
         next = iter(inputfile).next
         while 1:
             shape = next().strip()
             if shape == END_OF_INPUT:
                 raise StopIteration()
             m, n = map(int, shape.split())
             log("found minefield shape %s %s" % (m, n))
             minefield = ""
             for i in range(m):
                 minefield += next()
             yield m, n, minefield

     def main(inputfile, outputfile):
         counter = 0
         for m, n, minefield in MinefieldReader(inputfile):
             counter += 1
             print >> outputfile, "Field #%d:" % counter
             print >> outputfile, Minefield(m, n, minefield).clues()

     if __name__ == "__main__":
         inputfile = sys.stdin
         outputfile = sys.stdout
         main(inputfile, outputfile)


Last updated: 05 October 2012