TYPE_CHECKING in ternary expression

首先使用 mypy 重现两个 Python 示例，并比较 TYPE_CHECKING 在三元表达式与 if/else 形式中的处理方式。跟踪三元表达式类型检查的入口点，以确定这种行为是 bug，还是应该记录在文档中。当示例得到正确验证，或不同的行为得到清晰记录，并且在行为发生变化时有回归覆盖，即视为完成。

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.