jg-lint 0.5.0

Extensible Python linter written in Rust

jg-lint

Extensible Python linter with a Rust core.

Installation

Requires Python 3.14+.

uv add jg-lint

or with pip:

pip install jg-lint

or with Poetry:

poetry add jg-lint

Development install

To build from source, you need Maturin and a Rust toolchain:

git clone https://github.com/giacosoft/jg-lint.git
cd jg-lint
uv sync
uv run maturin develop

Usage

jg-lint check <paths...>

Lint one or more files or directories:

jg-lint check src/
jg-lint check src/ lib/ main.py
jg-lint check --config path/to/project src/

The --config flag points to the directory containing pyproject.toml (defaults to .).

Built-in rules

Code	Description
JG001	Imports must be at module top level
JG002	if statements are not allowed in test functions (any function or method whose name starts with test_)

Built-in rules are opt-in: they only run when you list them in select (see Configuration). For example, select = ["JG*"] enables every built-in rule, and select = ["JG001"] enables only JG001.

Output looks like:

src/app.py:12:1: MY001 TODO comments should be tracked as issues
src/app.py:45:1: MY001 TODO comments should be tracked as issues

Found 2 violation(s)

Exit code is 1 if any violations are found, 0 otherwise.

Inline suppression

Some rules opt in to per-line suppression via # noqa:

x = something()  # noqa: MY001
y = another()  # noqa: MY001, MY002

By design, # noqa is not honored by default — rules must explicitly set allow_noqa = True to allow it. This keeps suppression intentional and reserved for cases where the rule author has decided it's acceptable.

Configuration

All configuration lives in pyproject.toml under [tool.jg-lint]:

[tool.jg-lint]
select = ["MY001", "JG*"]    # rules to enable (see "Selecting rules" below)
ignore = ["MY002"]           # skip these rules globally
exclude = [".venv/**", "build/**"]
rules_path = "./rules"       # directory containing your custom rule modules

[tool.jg-lint.per-file-ignores]
"tests/**" = ["MY001"]       # skip MY001 in test files

Selecting rules

Each entry in select, ignore, and per-file-ignores is matched against a rule's code with the following pattern semantics:

• "*" — matches every code.
• "JG*" — matches every code starting with JG (e.g. JG001, JG042).
• "JG001" — exact match.

Defaults when select is empty:

• Built-in rules (e.g. JG001, JG002) are not enabled. List them explicitly with select = ["JG*"] or by code.
• Plugin rules from rules_path are enabled.

Writing custom rules

1. Create a rule module

Put it inside the directory you've configured as rules_path (e.g. ./rules/no_todo.py or ./rules/no_todo/__init__.py).

from jg_linter import Rule, Violation


class NoTodoComments(Rule):
    code = "MY001"
    message = "TODO comments should be tracked as issues"

    def check(self, file_path: str, content: str) -> list[Violation]:
        violations = []
        for i, line in enumerate(content.splitlines(), 1):
            if "# TODO" in line:
                violations.append(
                    Violation(file_path, i, 1, self.code, self.message)
                )
        return violations


def get_rules() -> list[Rule]:
    return [NoTodoComments()]

Each rule needs:

• code -- unique identifier (e.g. MY001)
• message -- human-readable description
• check(file_path, content) -- returns a list of Violation objects

Set test_only = True on a rule to run it only against test files (files named test_*.py, *_test.py, or inside a tests/ directory).

Set allow_noqa = True on a rule to allow inline # noqa: CODE suppression. Without this, the rule cannot be silenced per-line and can only be turned off project-wide via ignore or per-file-ignores.

2. Point jg-lint at the rules folder

[tool.jg-lint]
rules_path = "./rules"

Every top-level .py file and package inside rules_path is imported automatically; any module exposing get_rules() contributes rules. Files and folders starting with _ are skipped. The rules directory is prepended to sys.path during loading, so no PYTHONPATH setup is needed.

3. Run

jg-lint check src/

src/app.py:3:1: MY001 TODO comments should be tracked as issues
src/utils.py:17:1: MY001 TODO comments should be tracked as issues

Found 2 violation(s)

Contributing

You need a Rust toolchain (stable), Python 3.14+, and uv.

git clone https://github.com/giacosoft/jg-lint.git
cd jg-lint
uv sync
uv run maturin develop

Tests

cargo test                 # Rust unit tests
uv run pytest              # Python tests (rebuild with `maturin develop` if Rust changed)

Linting and formatting

Before opening a PR, run the same checks CI does:

# Rust
cargo fmt --all -- --check          # formatting
cargo clippy --all-targets -- -D warnings   # lints (fails on any warning)
cargo audit                          # CVEs in dependencies (install with `cargo install cargo-audit`)

# Python
uv run ruff check .                  # lint
uv run ruff format --check .         # format check

To auto-fix:

cargo fmt --all
cargo clippy --fix --all-targets
uv run ruff check --fix .
uv run ruff format .

License

MIT — see LICENSE.