Meaning of unittest.main() in Python unittest module

Question

I was trying to learn unit testing in Python, specifically the unittest module.

Consider the following lines:

import unittest

class abc(unittest.TestCase):
    def xyz():
      ...

if __name__ == "__main__":
    unittest.main()

I could see all my test cases running because of the call to unittest.main().

I was just curious to know how this call is making all the test cases run.

I know since I'm inheriting from unittest.TestCase for every test class, it is doing all the magic. Any insights?

Answer 1

main associated with unittest is actually an instance of TestProgram which when instantiated runs all your tests.

Below is the relevant code copied from unittest source at http://pythonhosted.org/gchecky/unittest-pysrc.html:

735  class TestProgram: 
752 -    def __init__(self, module='__main__', defaultTest=None, 
753                   argv=None, testRunner=None, testLoader=defaultTestLoader): 
754          if type(module) == type(''): 
755              self.module = __import__(module) 
756              for part in module.split('.')[1:]: 
757                  self.module = getattr(self.module, part) 
758          else: 
759              self.module = module 
760          if argv is None: 
761              argv = sys.argv 
762          self.verbosity = 1 
763          self.defaultTest = defaultTest 
764          self.testRunner = testRunner 
765          self.testLoader = testLoader 
766          self.progName = os.path.basename(argv[0]) 
767          self.parseArgs(argv) 
768          self.runTests() 
769   
770 -    def usageExit(self, msg=None): 
771          if msg: print msg 
772          print self.USAGE % self.__dict__ 
773          sys.exit(2) 
774   
775 -    def parseArgs(self, argv): 
776          import getopt 
777          try: 
778              options, args = getopt.getopt(argv[1:], 'hHvq', 
779                                            ['help','verbose','quiet']) 
780              for opt, value in options: 
781                  if opt in ('-h','-H','--help'): 
782                      self.usageExit() 
783                  if opt in ('-q','--quiet'): 
784                      self.verbosity = 0 
785                  if opt in ('-v','--verbose'): 
786                      self.verbosity = 2 
787              if len(args) == 0 and self.defaultTest is None: 
788                  self.test = self.testLoader.loadTestsFromModule(self.module) 
789                  return 
790              if len(args) > 0: 
791                  self.testNames = args 
792              else: 
793                  self.testNames = (self.defaultTest,) 
794              self.createTests() 
795          except getopt.error, msg: 
796              self.usageExit(msg) 
797   
798 -    def createTests(self): 
799          self.test = self.testLoader.loadTestsFromNames(self.testNames, 
800                                                         self.module) 
801   
802 -    def runTests(self): 
803          if self.testRunner is None: 
804              self.testRunner = TextTestRunner(verbosity=self.verbosity) 
805          result = self.testRunner.run(self.test) 
806          sys.exit(not result.wasSuccessful()) 
807   
808  main = TestProgram 

so when you execute unittest.main(), an object of TestProgram gets created which calls self.runTests() at line 768. The constructor also takes your current file as the default module containing the tests ( module='__main__'). When runTests() is called, it in turn calls self.testrunner.run(). When you refer to the "run" method of TextTestRunner class, you will find that it actually runs and reports all your test results. Test discovery is done by TestProgram.parseArgs at line 775 when you call unittest.main(). self.createTests at line 798 is actually responsible for discovering all your test cases and creating a test suite. This is all the magic.