Question

I’m trying to unit test some code that looks like this:

def main():     parser = optparse.OptionParser(description='This tool is cool', prog='cool-tool')     parser.add_option('--foo', action='store', help='The foo option is self-explanatory')     options, arguments = parser.parse_args()     if not options.foo:         parser.error('--foo option is required')     print 'Your foo is %s.' % options.foo     return 0  if __name__ == '__main__':    sys.exit(main()) 

With code that looks like this:

@patch('optparse.OptionParser') def test_main_with_missing_p4clientsdir_option(self, mock_optionparser):     #     # setup     #     optionparser_mock = Mock()     mock_optionparser.return_value = optionparser_mock     options_stub = Mock()     options_stub.foo = None     optionparser_mock.parse_args.return_value = (options_stub, sentinel.arguments)     def parser_error_mock(message):         self.assertEquals(message, '--foo option is required')         sys.exit(2)     optionparser_mock.error = parser_error_mock      #     # exercise & verify     #     self.assertEquals(sut.main(), 2) 

I’m using Michael Foord’s Mock, and nose to run the tests.

When I run the test, I get:

  File '/Users/dspitzer/Programming/Python/test-optparse-error/tests/sut_tests.py', line 27, in parser_error_mock     sys.exit(2) SystemExit: 2  ---------------------------------------------------------------------- Ran 1 test in 0.012s  FAILED (errors=1) 

The problem is that OptionParser.error does a sys.exit(2), and so main() naturally relies on that. But nose or unittest detects the (expected) sys.exit(2) and fails the test.

I can make the test pass by adding ‘return 2’ under the parser.error() call in main() and removing the sys.exit() call from parser_error_mock(), but I find it distasteful to modify the code under test to allow a test to pass. Is there a better solution?

Update: df‘s answer works, although the correct call is ‘self.assertRaises(SystemExit, sut.main)’.

Which means the test passes whatever the number is in the sys.exit() in parser_error_mock(). Is there any way to test for the exit code?

BTW, the test is more robust if I add:

self.assertEquals(optionparser_mock.method_calls, [('add_option', ('--foo',), {'action': 'store', 'help': 'The foo option is self-explanatory'}), ('parse_args', (), {})]) 

at the end.

Update 2: I can test for the exit code by replacing ‘self.assertRaises(SystemExit, sut.main)’ with:

try:     sut.main() except SystemExit, e:     self.assertEquals(type(e), type(SystemExit()))     self.assertEquals(e.code, 2) except Exception, e:     self.fail('unexpected exception: %s' % e) else:     self.fail('SystemExit exception expected') 

Answer 1

As noted in my updates to my question, I had to modify dF‘s answer to:

self.assertRaises(SystemExit, sut.main) 

…and I came up with a few longer snippet to test for the exit code.

[Note: I accepted my own answer, but I will delete this answer and accept dF‘s if he updates his.]