PEP 695 TypeAliasType.__value__ not supporting recursion intentional?

[randolf-scholz]

I was moving some code to PEP695 and just realized an issue. Consider the following example:

from typing import TypeAlias

StringScalar: TypeAlias = str | bytes
NumericalScalar: TypeAlias = bool | int | float | complex
Scalar: TypeAlias = StringScalar | NumericalScalar

assert isinstance("foo", StringScalar)  # ✅
assert isinstance(3.14, NumericalScalar)  # ✅
assert isinstance(42, Scalar)  # ✅

and the supposed PEP 695 analogue fails:

type StringScalar = str | bytes
type NumericalScalar = bool | int | float | complex
type Scalar = StringScalar | NumericalScalar

assert isinstance("foo", StringScalar.__value__)  # ✅
assert isinstance(3.14, NumericalScalar.__value__)  # ✅
assert isinstance(42, Scalar.__value__)  # ❌ arg 2 must be a type, a tuple of types, or a union

The issue is that Scalar.__value__ is the Union of two TypeAliasType members. PEP 695 only states that "__value__ is the evaluated value of the type alias", which under certain interpretations could be considered to mean that the result shouldn't contain any TypeAliasType. The PEP also mentions __evaluate_value__ in the context of PEP649, i.e. the resolution of Forward References.

What's the recommended way of getting the value of the TypeAliasType recursively? I guess for the moment, I could expand the unions manually. Will this be covered by __evaluate_value__ in the future?

[JelleZijlstra]

I would recommend against using the type statement when you need to use the "type alias" in a runtime context.

I am not planning to change any of this behavior in the future (but that decision is not solely up to me).