bpo-31642: Restore blocking "from" import by setting None in sys.modules. (#3834)

Lib/importlib/_bootstrap.py

@@ -1019,7 +1019,8 @@ def _handle_fromlist(module, fromlist, import_):
                     # Backwards-compatibility dictates we ignore failed
                     # imports triggered by fromlist for modules that don't
                     # exist.
-                    if exc.name == from_name:
+                    if (exc.name == from_name and
+                        sys.modules.get(from_name, _NEEDS_LOADING) is not None):
                         continue
                     raise
     return module

Lib/test/test_importlib/import_/test_api.py

@@ -82,6 +82,20 @@ class APITest:
                     self.__import__(PKG_NAME,
                                     fromlist=[SUBMOD_NAME.rpartition('.')[-1]])
 
+    def test_blocked_fromlist(self):
+        # If fromlist entry is None, let a ModuleNotFoundError propagate.
+        # issue31642
+        mod = types.ModuleType(PKG_NAME)
+        mod.__path__ = []
+        with util.import_state(meta_path=[self.bad_finder_loader]):
+            with util.uncache(PKG_NAME, SUBMOD_NAME):
+                sys.modules[PKG_NAME] = mod
+                sys.modules[SUBMOD_NAME] = None
+                with self.assertRaises(ModuleNotFoundError) as cm:
+                    self.__import__(PKG_NAME,
+                                    fromlist=[SUBMOD_NAME.rpartition('.')[-1]])
+                self.assertEqual(cm.exception.name, SUBMOD_NAME)
+
 
 class OldAPITests(APITest):
     bad_finder_loader = BadLoaderFinder

Misc/NEWS.d/next/Core and Builtins/2017-10-08-10-00-55.bpo-31642.1IKqgs.rst

+Restored blocking "from package import module" by setting
+sys.modules["package.module"] to None.