How to test type compatibility in Python 3.7?

[Kimi-Arthur]

With the new fixes done in Python 3.7. We cannot test type compatibility like:

type_info = List[int]
issubclass(type_info, list)

# Or

issubclass(type_info, List)

This is expected as mentioned in PEP560. So I'm not saying I should continue to write code like that.

But since it's still useful to inspect whether type is a list or not (e.g. when you inspect the object's type hints and generate a default value for it), I wonder whether these is already a known way to test such compatibility.

Thanks!

[ilevkivskyi]

One possible solution is to use typing_inspect library on PyPI, it has a get_origin() function which you can use as issubclass(get_origin(type_info), list).

But I think maybe it is time consider adding some basic introspection to typing? From all recent similar requests and general experience with typing_inspect it looks like people are interested mostly in two things:

• Get an "erased" version of a type, i.e. a runtime class equivalent of a given type
• Get type arguments from a given type

@gvanrossum Does it make sense to add such two helper functions that would be essentially public wrappers around internal APIs like .__origin__ and .__args__?

[gvanrossum]

Yes. I like that we make these things functions in typing rather than special attributes or methods -- it's much easier for a function to do the right thing when the type given to it is some built-in or user-defined type rather than one of the special one from typing. It should also deal with forward reference strings.

[Kimi-Arthur]

.__origin__ is already very helpful. Thanks! Didn't know that.

Besides the two features you suggest, I guess testing type (either actual type or typing's type variable, like _GenericAlias) compatibility would also be useful. If this would be automatically supported by those two features, it would still be worth mentioning in that particular release's notes. Thanks!

[ilevkivskyi]

get_origin() (an official public API for .__origin__) got into typing in Python 3.8, so I think this can be closed now.