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

Author: serhiy-storchaka

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 @@ def test_fromlist_load_error_propagates(self):
  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

@@ -0,0 +1,2 @@
+Restored blocking "from package import module" by setting
+sys.modules["package.module"] to None.