unit testing failure makes no sense

[listservs at mac.com]

I have some unit testing code in one of my modules that appears to  
run without an error, but the unit test fails anyhow. Have a look at  
the output below -- the TestResult seems to have no errors and no  
failures, yet I get a system exit.

------------------------------------------------------------------------ 
---
exceptions.SystemExit                                Traceback (most  
recent call last)

/Users/chris/<ipython console>

/Library/Frameworks/Python.framework/Versions/2.4/lib/python2.4/ 
unittest.py in __init__(self=<unittest.TestProgram object at  
0x10ee670>, module='__main__', defaultTest=None, argv=['/usr/local/ 
bin/ipython'], testRunner=None, testLoader=<unittest.TestLoader  
object at 0x606290>)
     757         self.progName = os.path.basename(argv[0])
     758         self.parseArgs(argv)
--> 759         self.runTests()
         self.runTests = <bound method TestProgram.runTests of  
<unittest.TestProgram object at 0x10ee670>>
     760
     761     def usageExit(self, msg=None):

/Library/Frameworks/Python.framework/Versions/2.4/lib/python2.4/ 
unittest.py in runTests(self=<unittest.TestProgram object at 0x10ee670>)
     795             self.testRunner = TextTestRunner 
(verbosity=self.verbosity)
     796         result = self.testRunner.run(self.test)
--> 797         sys.exit(not result.wasSuccessful())
         global sys.exit = <built-in function exit>
         result.wasSuccessful = <bound method  
_TextTestResult.wasSuccessful of <unittest._TextTestResult run=3  
errors=0 failures=0>>
     798
     799 main = TestProgram

SystemExit: False
Type exit or quit to exit IPython (%Exit or %Quit do so  
unconditionally).

Any ideas what is going wrong here? Here is my testing code:

class MCMCTest(unittest.TestCase):

     def testCoalMiningDisasters(self):
         """Run coal mining disasters example sampler"""

         print 'Running coal mining disasters test case ...'

         # Create an instance of the sampler
         self.sampler = DisasterSampler()

         # Specify the nimber of iterations to execute
         iterations = 10000
         thin = 2
         burn = 5000
         chains = 2

         # Run MCMC simulation
         for i in range(chains):

             self.failUnless(self.sampler.sample(iterations,  
burn=burn, thin=thin, plot=True))

             # Run convergence diagnostics
             self.sampler.convergence()

             # Plot autocorrelation
             self.sampler.autocorrelation()

         # Goodness of fit
         x, n = self.sampler.goodness(iterations/10)['overall']
         self.failIf(x/n < 0.05 or x/n > 0.95)


--
Christopher Fonnesbeck
+ Atlanta, GA
+ fonnesbeck at mac.com
+ Contact me on AOL IM using email address