Replace Loader with PEP 451 _Loader protocol in _importlib_modulespec.pyi annotations.

[brandtbucher]

For built-in imports, ModuleSpec.loader is an uninstantiated BuiltinImporter class.

[brandtbucher]

Same for frozen imports and FrozenImporter.

[srittau]

I am not sure I like this change. The importlib documentation states that loaders are defined as objects having a load_module() function. I am not an importlib expert, but it sounds to me that replacing the Loader defined in importlib (but not that in importlib.abc) with a protocol and using that in the annotation of ModuleSpec.loader is the better option. isinstance() checks for asserting the correct type seem reasonable in user code in this instance.

[brandtbucher]

Hi, thanks for the feedback @srittau.

I'm a bit confused with your suggestion - it looks to me like the only Loader in importlib is the one defined in importlib.abc. Are you saying we should create a new one-off _Loader protocol for this specific case?

[srittau]

Sorry, I was a bit confused from the circular import prevention going on in the stubs. But yes I was suggesting to add a _Loader protocol, which just has a load_module() function (per PEP 451) and use that instead of the concrete Loader implementation in ModuleSpec.

[brandtbucher]

I think that's a great solution; I actually wasn't familiar with Protocol before this. I also saw that I forgot to update the corresponding annotation in ModuleType.__loader__.

Here's a patch.

[srittau]

Thank you!