def test_first(self):
                """
                Start a second test run which will have a default working
                directory which is the same as the working directory of the
                test run already in progress.
                """
                # Change the working directory to the value it had before this
                # test suite was started.
                where['concurrent'] = subsequentDirectory = os.getcwd()
                os.chdir(runDirectory.path)
                self.addCleanup(os.chdir, subsequentDirectory)

                secondRunner.run(ConcurrentCase('test_second'))

                Start a second test run which will have a default working
                directory which is the same as the working directory of the
                test run already in progress.
                
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_runner.py