def test_pluginPackagesExcluded(self):
        """
        L{plugin.pluginPackagePaths} should exclude directories which are
        Python packages.  The only allowed plugin package (the only one
        associated with a I{dummy} package which Python will allow to be
        imported) will already be known to the caller of
        L{plugin.pluginPackagePaths} and will most commonly already be in
        the C{__path__} they are about to mutate.
        """
        root = FilePath(self.mktemp())
        foo = root.child('foo').child('dummy').child('plugins')
        foo.makedirs()
        foo.child('__init__.py').setContent(b'')
        sys.path = [root.child('foo').path, root.child('bar').path]
        self.assertEqual(
            plugin.pluginPackagePaths('dummy.plugins'),
            [root.child('bar').child('dummy').child('plugins').path])

        L{plugin.pluginPackagePaths} should exclude directories which are
        Python packages.  The only allowed plugin package (the only one
        associated with a I{dummy} package which Python will allow to be
        imported) will already be known to the caller of
        L{plugin.pluginPackagePaths} and will most commonly already be in
        the C{__path__} they are about to mutate.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_plugin.py