No error when importing typing.type_check_only

[KotlinIsland]

from typing import type_check_only

Success: no issues found in 1 source file

[sobolevn]

Can you please explain why do you think this should be an error?
Do you mean older versions of python that don't have this in typing?

[KotlinIsland]

https://docs.python.org/3/library/typing.html#typing.type_check_only

This decorator is itself not available at runtime.

It doesn't actually exist.

> python test.py
Traceback (most recent call last):
  File "/tmp/test.py", line 1, in <module>
    from typing import type_check_only
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ImportError: cannot import name 'type_check_only' from 'typing' (/usr/lib/python3.11/typing.py)

[AlexWaygood]

We want things like this to be legal, though:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing import type_check_only

This will not fail at runtime, so a blanket "mypy should raise an error if you ever try to import type_check_only in a non-stub file" rule wouldn't be good enough.

[KotlinIsland]

The docs specify that:

It is mainly intended to mark classes that are defined in type stub files if an implementation returns an instance of a private class

And if currently using it in a "runtime" context is a huge false negative, wouldn't it be better to always error when used in a py file (except for explicit TYPE_CHECKING blocks.) and when you do want to use explicitly put a type ignore on it.

[KotlinIsland]

related: #9531

[hauntsaninja]

Duplicate of #9531