Treat alternative syntax for unions (and other "new-semantics-old-syntax" typing features) in explicit `TypeAlias`es inside `if TYPE_CHECKING` blocks as stringized annotations.

The Problem

At runtime, TYPE_CHECKING is always False, only type checkers assume it is True. Therefore, if TYPE_CHECKING blocks are only parsed, but never evaluated by the Python interpreter. Since A = int | str is a valid syntax for all still-supported Python versions at the moment (all versions ever since typing was introduced, actually), it would make sense if type checkers were allowed to accept usage of "new-semantics-old-syntax" features (like the alternative union syntax) in explicitly-annotated TypeAliases inside of if TYPE_CHECKING blocks as long as the Python version specified [project.requires-python] (or it's equivalent in tools) can successfully parse that syntax. This is essentially the same as treating explicitly-annotated TypeAliases as stringized annotations by-default.

Examples

# Run on 3.9

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    A = int | str  # Alternative syntax for unions requires Python 3.10 or newer # pyright
                   # Invalid type alias: expression is not a valid type          # mypy
                   # Unsupported left operand type for | ("type[int]")           # mypy

# With proposal
from typing import TYPE_CHECKING
from typing_extensions import TypeAlias

if TYPE_CHECKING:
    A: TypeAlias = int | str  # Treated as a stringized annotation because `: TypeAlias`
                              # is explicit, therefore valid.

pyright-play
mypy-play

Commencez par les exemples liés de pyright-play et mypy-play, en vous concentrant sur Python 3.9, TYPE_CHECKING et les TypeAliases explicitement annotés. Aucun fichier du dépôt ni aucun test n’est identifié ; le travail serait terminé lorsqu’une spécification établie sur la manière dont ces cas doivent être interprétés, ainsi que l’accord ou le travail d’implémentation correspondant dans les outils de vérification de types concernés, seraient disponibles.