The basics#

An overview on contributing to the Black project.


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


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


All aspects of the Black style should be tested. Normally, tests should be created as files in the tests/data/cases directory. These files consist of up to three parts:

  • A line that starts with # flags: followed by a set of command-line options. For example, if the line is # flags: --preview --skip-magic-trailing-comma, the test case will be run with preview mode on and the magic trailing comma off. The options accepted are mostly a subset of those of Black itself, except for the --minimum-version= flag, which should be used when testing a grammar feature that works only in newer versions of Python. This flag ensures that we don’t try to validate the AST on older versions and tests that we autodetect the Python version correctly when the feature is used. For the exact flags accepted, see the function get_flags_parser in tests/ If this line is omitted, the default options are used.

  • A block of Python code used as input for the formatter.

  • The line # output, followed by the output of Black when run on the previous block. If this is omitted, the test asserts that Black will leave the input code unchanged.

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.

News / Changelog Requirement#

Black has CI that will check for an entry corresponding to your PR in 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 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/


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.


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