The basics

An overview on contributing to the Black project.

Technicalities

Development on the latest version of Python is preferred. You can use any operating system.

Install development dependencies inside a virtual environment of your choice, for example:

$ python3 -m venv .venv
$ source .venv/bin/activate # activation for linux and mac
$ .venv\Scripts\activate # activation for windows

(.venv)$ pip install -r test_requirements.txt
(.venv)$ pip install -e .[d]
(.venv)$ pre-commit install

Before submitting pull requests, run lints and tests with the following commands from the root of the black repo:

# Linting
(.venv)$ pre-commit run -a

# Unit tests
(.venv)$ tox -e py

# Optional Fuzz testing
(.venv)$ tox -e fuzz

# Format Black itself
(.venv)$ tox -e run_self

Development

Further examples of invoking the tests

# Run all of the above mentioned, in parallel
(.venv)$ tox --parallel=auto

# Run tests on a specific python version
(.venv)$ tox -e py39

# pass arguments to pytest
(.venv)$ tox -e py -- --no-cov

# print full tree diff, see documentation below
(.venv)$ tox -e py -- --print-full-tree

# disable diff printing, see documentation below
(.venv)$ tox -e py -- --print-tree-diff=False

Black has two pytest command-line options affecting test files in tests/data/ that are split into an input part, and an output part, separated by a line with# output. These can be passed to pytest through tox, or directly into pytest if not using tox.

--print-full-tree

Upon a failing test, print the full concrete syntax tree (CST) as it is after processing the input (“actual”), and the tree that’s yielded after parsing the output (“expected”). Note that a test can fail with different output with the same CST. This used to be the default, but now defaults to False.

--print-tree-diff

Upon a failing test, print the diff of the trees as described above. This is the default. To turn it off pass --print-tree-diff=False.

News / Changelog Requirement

Black has CI that will check for an entry corresponding to your PR in CHANGES.md. If you feel this PR does not require a changelog entry please state that in a comment and a maintainer can add a skip news label to make the CI pass. Otherwise, please ensure you have a line in the following format:

- `Black` is now more awesome (#X)

Note that X should be your PR number, not issue number! To workout X, please use Next PR Number. This is not perfect but saves a lot of release overhead as now the releaser does not need to go back and workout what to add to the CHANGES.md for each release.

Style Changes

If a change would affect the advertised code style, please modify the documentation (The Black code style) to reflect that change. Patches that fix unintended bugs in formatting don’t need to be mentioned separately though. If the change is implemented with the --preview flag, please include the change in the future style document instead and write the changelog entry under a dedicated “Preview changes” heading.

Docs Testing

If you make changes to docs, you can test they still build locally too.

(.venv)$ pip install -r docs/requirements.txt
(.venv)$ pip install -e .[d]
(.venv)$ sphinx-build -a -b html -W docs/ docs/_build/

Hygiene

If you’re fixing a bug, add a test. Run it first to confirm it fails, then fix the bug, run it again to confirm it’s really fixed.

If adding a new feature, add a test. In fact, always add a test. But wait, before adding any large feature, first open an issue for us to discuss the idea first.

Finally

Thanks again for your interest in improving the project! You’re taking action when most people decide to sit and watch.