def test_defaultEnviron(self):
        """
        If the process environment is not explicitly passed to
        L{Environment.__init__}, the real process environment dictionary is
        used.
        """
        self.patch(os, 'environ', {
                'LISTEN_PID': str(os.getpid()),
                'LISTEN_FDS': '5'})
        sddaemon = ListenFDs.fromEnvironment()
        self.assertEqual(list(range(3, 3 + 5)),
            sddaemon.inheritedDescriptors())

        If the process environment is not explicitly passed to
        L{Environment.__init__}, the real process environment dictionary is
        used.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/test/test_systemd.py