TC003: Allow TYPE_CHECKING = False

[hugovk]

Description

TC003:

Checks for standard library imports that are only used for type annotations, but aren't defined in a type-checking block.

https://docs.astral.sh/ruff/rules/typing-only-standard-library-import/#typing-only-standard-library-import-tc003

TYPE_CHECKING = False

from __future__ import annotations

TYPE_CHECKING = False
if TYPE_CHECKING:
    import os


def thing(locale: os.PathLike[str]) -> None: ...

Ruff doesn't allow it:

❯ ruff --version
ruff 0.9.6
❯ ruff check --select TC --isolated 1.py
1.py:5:12: TC003 Move standard library import `os` into a type-checking block
  |
3 | TYPE_CHECKING = False
4 | if TYPE_CHECKING:
5 |     import os
  |            ^^ TC003
  |
  = help: Move into type-checking block

Found 1 error.

But upstream https://pypi.org/project/flake8-type-checking/ does:

❯ flake8 --version
7.1.1 (flake8-2020: 1.8.1, flake8-implicit-str-concat: 0.5.0, flake8-type-checking: 3.0.0, mccabe: 0.7.0, pycodestyle: 2.12.1, pyflakes: 3.2.0) CPython 3.13.2 on Darwin
❯ flake8 1.py
❯ 

from typing import TYPE_CHECKING

Both are fine with this:

from __future__ import annotations

from typing import TYPE_CHECKING
if TYPE_CHECKING:
    import os

❯ ruff check --select TC --isolated 1.py
All checks passed!

❯ flake8 1.py --statistics
❯ 

But the whole point of this typing-only-standard-library-import TC003 check is to avoid imports that are only needed for type checking:

Unused imports add a performance overhead at runtime, and risk creating import cycles. If an import is only used in typing-only contexts, it can instead be imported conditionally under an if TYPE_CHECKING: block to minimize runtime overhead.

See also:

• TC003/TC002/TC001 False positive on manual TYPE_CHECKING with __future__.annotations #15681
• Recognize all symbols named TYPE_CHECKING for in_type_checking_block #15719

Which look like it should have fixed it in 0.9.5, but I can repro in 0.9.4-0.9.6.

[AlexWaygood]

Does this still repro if you use --preview? #15719 only changed the behaviour in preview mode for now, since it's technically a breaking change. We might be able to stabilise it in v0.10; if not, it'll probably be stabilised in v0.11 :-)

[hugovk]

Hooray, it passes!

❯ ruff --version && ruff check --select TC --isolated 1.py --preview
ruff 0.9.6
All checks passed!

Thanks!

[AlexWaygood]

no worries!