TYPE_CHECKING in ternary expression

まず、両方の Python の例を mypy で再現し、三項式と if/else 形式で TYPE_CHECKING がどのように扱われるかを比較します。三項式の型チェックのエントリーポイントを追跡して、その動作がバグなのか、ドキュメント化すべきものなのかを判断します。例が正しく検証されるか、異なる動作が明確にドキュメント化され、動作が変更される場合は回帰テストによるカバレッジが追加されれば完了です。

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.