TCH002 false alarm when importing submodule with import..as

[jakkdl]

not using import..as

from __future__ import annotations

import libcst
import libcst.matchers

my_list: list[libcst.With] = []
dir(libcst.matchers)

$ ruff --select --isolated TCH002 foo.py

no errors, working as expected.

using import..as

from __future__ import annotations

import libcst
import libcst.matchers as m

my_list: list[libcst.With] = []
dir(m)

$ ruff --select --isolated TCH002 foo.py
foo.py:3:8: TCH002 Move third-party import `libcst` into a type-checking block
Found 1 error.

version

$ ruff --version
ruff 0.0.269
$ python --version
Python 3.11.3

flake8-type-checking handles both cases, so this is solely a bug in ruff.
Popped up in python-trio/flake8-async#196 (comment)

[dhruvmanila]

This seems to be similar to the problem faced in #4655. I'm guessing the submodule import (import libcst.matchers) is increasing the usage count of libcst import, thus not flagging for your first example. But, in your second example, the aliased import is not increasing the usage.

[jakkdl]

Oh interesting, it's related but also the reverse. So maybe there's two bugs here that cancel out in the first example, where I don't care to create a dedicated TYPE_CHECKING block with no performance improvement (since importing libcst.matchers will also import libcst)

[charliermarsh]

Guessing this is a bug in the "strictness" check, whereby we determine whether an import has some other runtime-required import of the same module.

[charliermarsh]

@dhruvmanila - Do you wanna take a look? I'd start by looking at is_implicit_import.

[dhruvmanila]

Yeah, sure!

[jakkdl]

Thanks for the quick fix! 🚀