“if False：from typing import Type”是什么意思？

Question

While going over pytest src code I found that peculiar statement, can someone please explain?

if False:  # TYPE_CHECKING
    from typing import Type

I'm actually looking over PEP 526 currently, and I think that this may somehow be connected to annotations, but not sure how. Is it possible that someone would annotate False in a way that it would evaluate to True ?

After a quick look in the typing src, I found that:

# Constant that's True when type checking, but False here.
TYPE_CHECKING = False

So from what I gather so far, @chepner is right. This statement is evaluated to True when running a type check, and then the actual import is probably needed. Though that still looks really weird to my eyes ( #if DBG in C was way clearer)

Answer 1

if False: 
    ...

was an old way of writing what's now (since Python 3.5.2) done with

from typing import TYPE_CHECKING

if TYPE_CHECKING: 
    ...

TYPE_CHECKING is

A special constant that is assumed to be True by 3rd party static type checkers. It is False at runtime.