The same applies to pyexpat.model.

It seems like pyexpat is the only builtin module which has submodules (errors, model).

Normally, as I understand it, the module gets imported given a spec and the import machinery ensures that this spec ends up in the __spec__ attribute of the module.  But in this case only pyexpat gets imported by importlib.  The submodules are added when initializing the module.

Also, importlib's BuiltinImporter assumes that a builtin module is never a package.  Is this reasonable in this case?