4.3.3 Test-driven implementation

With the test written, we begin with the implementation. For illustration purposes, we will intentionally write some incorrect implementations to see how the test behaves.

The easiest way to run tests defined with the unittest framework, is to put a call to its main method at the end of the test module:

% python test_gray.py -v
Check that only one bit changes in successive codewords ... FAIL
Check that all codewords occur exactly once ... FAIL
<trace backs not shown>

As expected, this fails completely. Let us try an incorrect implementation, that puts the lsb of in the input on the output:

def bin2gray(B, G, width):
    ### INCORRECT - DEMO PURPOSE ONLY! ###

    @always_comb
    def logic():
        G.next = B[0]

    return logic

% python test_gray.py -v
Check that only one bit changes in successive codewords ... ok
Check that all codewords occur exactly once ... FAIL

======================================================================
FAIL: Check that all codewords occur exactly once
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_gray.py", line 109, in testUniqueCodeWords
    sim.run(quiet=1)
...
  File "test_gray.py", line 104, in test
    self.assertEqual(actual, expected)
  File "/usr/local/lib/python2.2/unittest.py", line 286, in failUnlessEqual
    raise self.failureException, \
AssertionError: [0, 0, 1, 1] != [0, 1, 2, 3]

----------------------------------------------------------------------
Ran 2 tests in 0.785s

Now the test passes the first requirement, as expected, but fails the second one. After the test feedback, a full traceback is shown that can help to debug the test output.

Finally, if we use the correct implementation as in section 2.4.1, the output is:

% python test_gray.py -v
Check that only one bit changes in successive codewords ... ok
Check that all codewords occur exactly once ... ok

----------------------------------------------------------------------
Ran 2 tests in 6.364s

OK