TYPE_CHECKING in ternary expression

This issue should be marked with "topic-ternary-expression" label.

Mypy doesn't understand TYPE_CHECKING checked in a ternary expression.

Example 1

from typing import TYPE_CHECKING

from redis.asyncio import ConnectionPool
from redis.asyncio import Redis as OGRedis

from . import settings

Redis = OGRedis[bytes] if TYPE_CHECKING else OGRedis  # Redis is Generic in stubs


async def func() -> None:
    connection_pool = ConnectionPool.from_url(settings.redis_url)
    async with Redis(connection_pool=connection_pool) as redis:
        await redis.set(
            name="test",
            value="Hello World",
        )

$ mypy project/
project/code.py:13: error: "object" has no attribute "__aenter__"  [attr-defined]
project/code.py:13: error: "object" has no attribute "__aexit__"  [attr-defined]
Found 2 errors in 1 file (checked 3 source files)

Example 2

from typing import TYPE_CHECKING

from redis.asyncio import ConnectionPool
from redis.asyncio import Redis as OGRedis

from . import settings

if TYPE_CHECKING:  # these
    Redis = OGRedis[bytes]  # 4 lines
else:  # are
    Redis = OGRedis  # the only difference


async def func() -> None:
    connection_pool = ConnectionPool.from_url(settings.redis_url)
    async with Redis(connection_pool=connection_pool) as redis:
        await redis.set(
            name="test",
            value="Hello World",
        )

$ mypy project/
Success: no issues found in 3 source files

That's either a bug or an expected behavior that can be documented. I can imagine a programmer spending an hour debugging and searching why his/her code isn't validated properly.

Inizia riproducendo entrambi gli esempi Python con mypy e confronta come viene gestito TYPE_CHECKING nell’espressione ternaria rispetto alla forma if/else. Segui il punto di ingresso del controllo dei tipi dell’espressione ternaria per determinare se il comportamento è un bug o se dovrebbe essere documentato. Il lavoro è completato quando l’esempio viene convalidato correttamente oppure il comportamento diverso viene documentato chiaramente, con una copertura di regressione se il comportamento cambia.