So after two weeks of preparations, I spent last week migrating the core importing functionality to ImportEngine.

Since there the hierarchy of classes in importlib involved in __import__ functionality has to be pretty deep to maximise code reuse, the changes are spread over quite a few places:

• @module_for_loader
• BuiltinImporter.find_module
• BuiltinImporter.load_module
• FrozenImporter.find_module
• FrozenImporter.load_module
• _LoaderBasics._load_module
• _SourcelessFileLoader.load_module
• _ExtensionFileLoader.load_module
• PathFinder._path_hooks
• PathFinder._path_importer_cache
• PathFinder.find_module
• _FileFinder.find_module
• _DefaultPathFinder._path_hooks

In short, the engine argument has to be passed from the __import__ function down to the loaders. This is particularly tricky in case of path hooks (which are in fact implemented as a meta hook). All this results in substantial changes in the code and there are still a few places I need to iron out, and afterwards it will need solid testing.