def test_requiresDISPLAY(self):
        """
        On X11, L{gtk3reactor} is unimportable if the C{DISPLAY} environment
        variable is not set.
        """
        display = os.environ.get("DISPLAY", None)
        if display is not None:
            self.addCleanup(os.environ.__setitem__, "DISPLAY", display)
            del os.environ["DISPLAY"]
        with SetAsideModule("twisted.internet.gtk3reactor"):
            exc = self.assertRaises(ImportError,
                                    __import__, "twisted.internet.gtk3reactor")
            self.assertEqual(
                exc.args[0],
                "Gtk3 requires X11, and no DISPLAY environment variable is set")

        On X11, L{gtk3reactor} is unimportable if the C{DISPLAY} environment
        variable is not set.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_gireactor.py