PEP 781: Make ``TYPE_CHECKING`` a built-in constant

[ehomrich (Emerson)]

@thejcannon

Why not just use module __getattr__? Am I missing something?

I asked this same question the other day in another thread and Michael gave some insights in the answer.

Specify `TYPE_CHECKING = False` without typing import

Regarding import times, currently (read, 3.13) typing has a module-level __getattr__ to generate some aliases on-the-fly. Wouldn’t moving the special forms implementations to an “internal” module, something like _typing.py or _typing_internals.py, and using typing’s __getattr__ to get them from that module help reduce load times? Or would module resolution merge everything anyway?

Specify `TYPE_CHECKING = False` without typing import

probably not. The moment someone has a module level import of from typing import SomeAlias the __getattr__ will be invoked. For this to have any benefit, you’d need import typing as t and then use of t.SomeAlias only in places where evaluation of the attribute access is deferred, such as type alias statements or annotations (3.13 and earlier with a future import, 3.14 forward with 649) This could still be done, but it would require adoption across “foundational libraries” to make it likely that…