def test_testmoduleOnNonexistentFile(self):
        """
        Check that --testmodule displays a meaningful error message when
        passed a non-existent filename.
        """
        buffy = NativeStringIO()
        stderr, sys.stderr = sys.stderr, buffy
        filename = 'test_thisbetternoteverexist.py'
        try:
            self.config.opt_testmodule(filename)
            self.assertEqual(0, len(self.config['tests']))
            self.assertEqual("File %r doesn't exist\n" % (filename,),
                                 buffy.getvalue())
        finally:
            sys.stderr = stderr

        Check that --testmodule displays a meaningful error message when
        passed a non-existent filename.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_script.py