def test_unimportablePackageWalkModules(self):
        """
        If a package has been explicitly forbidden from importing by setting a
        L{None} key in sys.modules under its name, L{modules.walkModules} should
        still be able to retrieve an unloaded L{modules.PythonModule} for that
        package.
        """
        existentPath = self.pathEntryWithOnePackage()
        self.replaceSysPath([existentPath.path])
        self.replaceSysModules({"test_package": None})

        walked = list(modules.walkModules())
        self.assertEqual([m.name for m in walked],
                          ["test_package"])
        self.assertFalse(walked[0].isLoaded())

        If a package has been explicitly forbidden from importing by setting a
        L{None} key in sys.modules under its name, L{modules.walkModules} should
        still be able to retrieve an unloaded L{modules.PythonModule} for that
        package.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_modules.py