Fix #3174: [Napoleon] Defers autodoc-skip-member to other extensions if Napoleon doesn't care if the member is skipped

doc/ext/autodoc.rst

@@ -446,6 +446,11 @@ member should be included in the documentation by using the following event:
    documentation.  The member is excluded if a handler returns ``True``.  It is
    included if the handler returns ``False``.
 
+   If more than one enabled extension handles the ``autodoc-skip-member``
+   event, autodoc will use the first non-``None`` value returned by a handler.
+   Handlers should return ``None`` to fall back to the skipping behavior of
+   autodoc and other enabled extensions.
+
    :param app: the Sphinx application object
    :param what: the type of the object which the docstring belongs to (one of
       ``"module"``, ``"class"``, ``"exception"``, ``"function"``, ``"method"``,

sphinx/ext/napoleon/__init__.py

@@ -453,4 +453,4 @@ def _skip_member(app, what, name, obj, skip, options):
                     (is_private and inc_private) or
                     (is_init and inc_init)):
                 return False
-    return skip
+    return None

tests/test_ext_napoleon.py

@@ -123,19 +123,19 @@ class SetupTest(TestCase):
 
 
 class SkipMemberTest(TestCase):
-    def assertSkip(self, what, member, obj, expect_skip, config_name):
-        skip = 'default skip'
+    def assertSkip(self, what, member, obj, expect_default_skip, config_name):
+        skip = True
         app = mock.Mock()
         app.config = Config()
         setattr(app.config, config_name, True)
-        if expect_skip:
-            self.assertEqual(skip, _skip_member(app, what, member, obj, skip,
+        if expect_default_skip:
+            self.assertEqual(None, _skip_member(app, what, member, obj, skip,
                                                 mock.Mock()))
         else:
             self.assertFalse(_skip_member(app, what, member, obj, skip,
                                           mock.Mock()))
         setattr(app.config, config_name, False)
-        self.assertEqual(skip, _skip_member(app, what, member, obj, skip,
+        self.assertEqual(None, _skip_member(app, what, member, obj, skip,
                                             mock.Mock()))
 
     def test_namedtuple(self):