[Python-ideas] Making TYPE_CHECKING builtin.

Jan. 18, 2021

      Hi, all.

I want to write type hints without worrying about runtime overhead.
Current best practice is:

```
from __future__ import annotations

import typing

if typing.TYPE_CHECKING:
    import xxx # modules used only in type hints.
```

But it would be nice if I can avoid importing even "typing" module.
How about adding TYPE_CHECKING builtin that is False?

```
from __future__ import annotations

if TYPE_CHECKING:
    from typing import Any, Optional

# We can use Any, Optional, etc here.
```

I wonder if we can make TYPE_CHECKING constant like True, False, and None.
But it will break existing `from typing import TYPE_CHECKING` codes.

Regards,

-- 
Inada Naoki  <songofacandy@gmail.com>