B024: ruff in pre-commit fails to recognise B024 with the presence of a class variable

[cmp0xff]

List of keywords I searched for before creating this issue

• B024

A minimal code snippet that reproduces the bug

from abc import ABC


class Foo(ABC):
    a: int
    def method(self) -> None:
        print("nothing")

The command you invoked

poetry run pre-commit run -a

The current Ruff settings

Relevant snippet of .pre-commit-config.yaml

  - repo: https://github.com/astral-sh/ruff/
    rev: 0.7.4
    hooks:
      # Run the linter.
      - id: ruff
        args:
          - --fix
          - --exit-non-zero-on-fix

Relevant snippet of pyproject.py

[tool.ruff]
line-length = 88
force-exclude = true # Recommended for pre-commit (https://github.com/charliermarsh/ruff#force-exclude)
# Exclude a variety of commonly ignored directories.
exclude = [
    ".bzr",
    ".direnv",
    ".eggs",
    ".git",
    ".git-rewrite",
    ".hg",
    ".mypy_cache",
    ".nox",
    ".pants.d",
    ".pytype",
    ".ruff_cache",
    ".svn",
    ".tox",
    ".venv",
    "__pypackages__",
    "_build",
    "buck-out",
    "build",
    "dist",
    "node_modules",
    "venv",
]
target-version = "py310"
# unsafe-fixes = true

[tool.ruff.lint.per-file-ignores]
"*ipynb*" = [
  "B018",  # https://docs.astral.sh/ruff/rules/useless-expression/
]

[tool.ruff.lint]
# Enable Pyflakes (`F`) and a subset of the pycodestyle (`E`)  codes by default.
select = ["ALL"]
ignore = [
  "ANN001",  # https://docs.astral.sh/ruff/rules/missing-type-function-argument/
  "ANN201",  # https://docs.astral.sh/ruff/rules/missing-return-type-undocumented-public-function/
  "ANN202",  # https://docs.astral.sh/ruff/rules/missing-return-type-private-function/
  "ANN401",  # https://docs.astral.sh/ruff/rules/any-type/
  "ARG",     # https://docs.astral.sh/ruff/rules/#flake8-unused-arguments-arg
  "B007",    # https://docs.astral.sh/ruff/rules/unused-loop-control-variable/
  "B023",    # https://docs.astral.sh/ruff/rules/function-uses-loop-variable/
  "B905",    # https://docs.astral.sh/ruff/rules/zip-without-explicit-strict/
  "C408",    # https://docs.astral.sh/ruff/rules/unnecessary-collection-call/
  "C901",    # https://docs.astral.sh/ruff/rules/complex-structure/
  "D",       # https://docs.astral.sh/ruff/rules/#pydocstyle-d
  "DTZ001",  # https://docs.astral.sh/ruff/rules/call-datetime-without-tzinfo/
  "E501",    # https://docs.astral.sh/ruff/rules/line-too-long/ (handled by black)
  "E722",    # https://docs.astral.sh/ruff/rules/bare-except/
  "E731",    # https://docs.astral.sh/ruff/rules/lambda-assignment/
  "F841",    # https://docs.astral.sh/ruff/rules/unused-variable/
  "G",       # https://docs.astral.sh/ruff/rules/#flake8-logging-format-g
  "S101",    # https://docs.astral.sh/ruff/rules/assert/
  "S301",    # https://docs.astral.sh/ruff/rules/suspicious-pickle-usage/
  "S608",    # https://docs.astral.sh/ruff/rules/hardcoded-sql-expression/
  "SIM114",  # https://docs.astral.sh/ruff/rules/if-with-same-arms/
  "BLE", "COM", "EM", "ERA", "FIX", "FBT", "FLY", "INP",
  "N",
  "PD",
  "PERF", "PGH",
  "PLC", "PLR", "PLW",
  "PT",
  "PTH",
  "PYI", "RET",
  "SLF",
  "T201",
  "TCH", "TD", "TID", "TRY", "UP", "W"
]

The current Ruff version

0.7.4

[dylwil3]

Thank you for submitting this issue!

This behavior is intentional - it is assumed that an unassigned, annotated class variable is intended to be abstract.

But that has certainly been a point of discussion/refinement. See this flake8-bugbear issue and the issues linked therein. Let us know if you think it should be further refined!

[cmp0xff]

Hi, thank you for the reply.

In such a case, the documentation needs to be updated. For example, in abstract-base-class-without-abstract-method, instead of

Checks for abstract classes without abstract methods.

we can write

Checks for abstract classes without abstract methods or class variables.

Please let me know if I can continue completing and implementing this idea.

[dylwil3]

That'd be great! I think the check only skips if the class variable is annotated and not assigned, though. So you may want to adjust the wording a bit, maybe:

Checks for abstract classes without abstract methods. Class variables which are annotated but not assigned are regarded as abstract.

[cmp0xff]

I am working on the documentation. Meanwhile, upon checking the Python documentation, it turns out that only when subscribed by typing.ClassVar is an annotate variable considered a class variable. Copied from the above-mentioned documentation:

class Starship:
    stats: ClassVar[dict[str, int]] = {} # class variable
    damage: int = 10                     # instance variable

What I had as an example in the original post is therefore an instance variable, not a class variable. This contradicts the discussion in PyCQA/flake8-bugbear#293, which only has to do with typing.ClassVar.

[dylwil3]

Hey @cmp0xff - you'll want to open a PR for ruff not docs. The relevant code is here:

ruff/crates/ruff_linter/src/rules/flake8_bugbear/rules/abstract_base_class.rs

Lines 13 to 19 in 3c52d2d

	/// ## What it does
	/// Checks for abstract classes without abstract methods.
	///
	/// ## Why is this bad?
	/// Abstract base classes are used to define interfaces. If an abstract base
	/// class has no abstract methods, you may have forgotten to add an abstract
	/// method to the class or omitted an `@abstractmethod` decorator.

Meanwhile, upon checking the Python documentation, it turns out that only when subscribed by typing.ClassVar is an annotate variable considered a class variable.

That's an interesting point... can you open a separate issue for this? For the present issue let's just change the documentation.

Thank you!