Linter workflow

Path to file (view file on GitHub)

The original file is located at the following path:

.github / workflows / CI_lint.yml

This GitHub Actions workflow enforces code style on every push and every pull request. It runs three independent checks: flake8 for PEP 8 compliance, black for formatting and isort for import ordering. All three run against the same two directories, app and rosemary.

Table of contents

  1. Workflow name
  2. Triggers
  3. Job
  4. Steps
    1. 1. Checkout
    2. 2. Set up Python
    3. 3. Install dependencies
    4. 4. Lint with flake8
    5. 5. Check formatting with black
    6. 6. Check import order with isort
  5. Configuration
  6. Reproducing the checks locally

Workflow name

  • name: Python Lint

Triggers 

on:
  push:
  pull_request:

Both triggers are bare, so the workflow runs on every branch and every pull request, not only on main. Style problems are caught on the branch, before review.

Job

  • lint: runs on ubuntu-24.04.

Steps

1. Checkout 

- name: Checkout
  uses: actions/checkout@v5

2. Set up Python 

- name: Set up Python
  uses: actions/setup-python@v6
  with:
    python-version: '3.13'

3. Install dependencies 

- name: Install Dependencies
  run: |
    python -m pip install --upgrade pip
    pip install flake8 flake8-pyproject black isort

This job does not install requirements.txt. It only needs the style tools, so it installs them directly and stays fast.

flake8-pyproject is what lets flake8 read its configuration from pyproject.toml. Without it, flake8 ignores the [tool.flake8] table and falls back to its 79 character default, which reports several hundred spurious E501 errors against a codebase formatted to 120 characters.

4. Lint with flake8 

- name: Lint with flake8
  run: flake8 app rosemary

5. Check formatting with black 

- name: Check formatting with black
  run: black --check app rosemary

--check makes black report and fail instead of rewriting files. The job never modifies your code.

6. Check import order with isort 

- name: Check import order with isort
  run: isort --check-only app rosemary

Same idea: --check-only reports and fails, it does not reorder anything.

Configuration

There is no .flake8 file and no setup.cfg. All three tools read the root pyproject.toml:

[tool.black]
line-length = 120
target-version = ["py313"]

[tool.isort]
line_length = 120
profile = "black"

[tool.flake8]
max-line-length = 120
extend-ignore = ["E203", "W503"]

The settings are deliberately consistent. isort uses profile = "black" and the same line length, so isort and black cannot disagree with each other, and flake8 ignores E203 and W503 because those two rules conflict with how black formats slices and line breaks around binary operators.

Reproducing the checks locally

Run the same three commands from inside web_app_container, which already has flake8, flake8-pyproject, black and isort installed and starts at /workspace. Without flake8-pyproject, flake8 will not pick up [tool.flake8] from pyproject.toml:

docker exec -it web_app_container flake8 app rosemary
docker exec -it web_app_container black --check app rosemary
docker exec -it web_app_container isort --check-only app rosemary

To fix the failures instead of just listing them, drop the check flags:

docker exec -it web_app_container isort app rosemary
docker exec -it web_app_container black app rosemary

rosemary also wraps this. rosemary linter runs flake8 over both directories, and rosemary linter:fix removes unused imports with autoflake, sorts imports with isort and formats with black:

docker exec -it web_app_container rosemary linter
docker exec -it web_app_container rosemary linter:fix

See Linting for more detail on the CLI commands.

rosemary linter is not the same as the CI job

rosemary linter only runs flake8. The CI job also runs black --check and isort --check-only, so a branch that passes rosemary linter can still fail the workflow. Run rosemary linter:fix before pushing, or run the three commands above directly.