How To Contribute

Thank you for considering contributing to attrs! It's people like you who make it such a great tool for everyone.

This document intends to make contribution more accessible by codifying tribal knowledge and expectations. Don't be afraid to open half-finished PRs, and ask questions if something is unclear!

Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms. Please report any harm to Hynek Schlawack in any way you find appropriate.


In case you‘d like to help out but don’t want to deal with GitHub, there's a great opportunity: help your fellow developers on Stack Overflow!

The official tag is python-attrs and helping out in support frees us up to improve attrs instead!


  • No contribution is too small! Please submit as many fixes for typos and grammar bloopers as you can!
  • Try to limit each pull request to one change only.
  • Since we squash on merge, it's up to you how you handle updates to the main branch. Whether you prefer to rebase on main or merge main into your branch, do whatever is more comfortable for you.
  • Always add tests and docs for your code. This is a hard rule; patches with missing tests or documentation won't be merged.
  • Make sure your changes pass our CI. You won‘t get any feedback until it’s green unless you ask for it.
  • For the CI to pass, the coverage must be 100%. If you have problems to test something, open anyway and ask for advice. In some situations, we may agree to add an # pragma: no cover.
  • Once you‘ve addressed review feedback, make sure to bump the pull request with a short note, so we know you’re done.
  • Don’t break backwards-compatibility.

Local Development Environment

You can (and should) run our test suite using tox. However, you’ll probably want a more traditional environment as well.

First, create a virtual environment so you don‘t break your system-wide Python installation. We recommend using the Python version from the .python-version-default file in project’s root directory.

If you‘re using direnv, you can automate the creation of a virtual environment with the correct Python version by adding the following .envrc to the project root after you’ve cloned it to your computer:

layout python python$(cat .python-version-default)

If you're using tools that understand .python-version files like pyenv does, you can make it a link to the .python-version-default file.

Then, fork the repository on GitHub.

Clone the fork to your computer:

$ git clone<your-username>/attrs.git

Or if you prefer to use Git via HTTPS:

$ git clone<your-username>/attrs.git

Then add the attrs repository as upstream remote:

$ git remote add -t main -m main --tags upstream

The next step is to sync your local copy with the upstream repository:

$ git fetch upstream

This is important to obtain eventually missing tags, which are needed to install the development version later on. See #1104 for more information.

Change into the newly created directory and after activating a virtual environment install an editable version of attrs along with its tests and docs requirements:

$ cd attrs
$ python -m pip install --upgrade pip wheel  # PLEASE don't skip this step
$ python -m pip install -e '.[dev]'

At this point,

$ python -m pytest

should work and pass. You can significantly speed up the test suite by passing -n auto to pytest which activates pytest-xdist and takes advantage of all your CPU cores.

For documentation, you can use:

$ tox run -e docs-serve

This will build the documentation, and then watch for changes and rebuild it whenever you save a file.

To just build the documentation and run doctests, use:

$ tox run -e docs

You will find the built documentation in docs/_build/html.

To file a pull request, create a new branch on top of the upstream repository's main branch:

$ git fetch upstream
$ git checkout -b my_topical_branch upstream/main

Make your changes, push them to your fork (the remote origin):

$ git push -u origin

and publish the PR in GitHub's web interface!

After your pull request is merged and the branch is no longer needed, delete it:

$ git checkout main
$ git push --delete origin my_topical_branch && git branch -D my_topical_branch

Before starting to work on your next pull request, run the following command to sync your local repository with the remote upstream:

$ git fetch upstream -u main:main

To avoid committing code that violates our style guide, we strongly advise you to install pre-commit and its hooks:

$ pre-commit install

This is not strictly necessary, because our tox file contains an environment that runs:

$ pre-commit run --all-files

and our CI has integration with But it's way more comfortable to run it locally and git catching avoidable errors.


  • Obey PEP 8 and PEP 257. We use the """-on-separate-lines style for docstrings:

    def func(x):
        Do something.
        :param str x: A very important parameter.
        :rtype: str
  • If you add or change public APIs, tag the docstring using .. versionadded:: 16.0.0 WHAT or .. versionchanged:: 16.2.0 WHAT.

  • We use isort to sort our imports, and we use Black with line length of 79 characters to format our code. As long as you run our full tox suite before committing, or install our pre-commit hooks (ideally you‘ll do both – see Local Development Environment above), you won’t have to spend any time on formatting your code at all. If you don't, CI will catch it for you – but that seems like a waste of your time!


  • Write your asserts as expected == actual to line them up nicely:

    x = f()
    assert 42 == x.some_attribute
    assert "foo" == x._a_private_attribute
  • To run the test suite, all you need is a recent tox. It will ensure the test suite runs with all dependencies against all Python versions just as it will in our CI. If you lack some Python versions, you can can always limit the environments like tox -e py38,py39, or make it a non-failure using tox --skip-missing-interpreters.

    In that case you should look into asdf or pyenv, which make it very easy to install many different Python versions in parallel.

  • Write good test docstrings.

  • To ensure new features work well with the rest of the system, they should be also added to our Hypothesis testing strategy, which can be found in tests/

  • If you've changed or added public APIs, please update our type stubs (files ending in .pyi).


  • Use semantic newlines in reStructuredText and Markdown files (files ending in .rst and .md):

    This is a sentence.
    This is another sentence.
  • If you start a new section, add two blank lines before and one blank line after the header, except if two headers follow immediately after each other:

    Last line of previous section.
    Header of New Top Section
    Header of New Section
    First line of new section.
  • If you add a new feature, demonstrate its awesomeness on the examples page!


If your change is noteworthy, there needs to be a changelog entry so our users can learn about it!

To avoid merge conflicts, we use the Towncrier package to manage our changelog. towncrier uses independent Markdown files for each pull request – so called news fragments – instead of one monolithic changelog file. On release, those news fragments are compiled into our

You don't need to install Towncrier yourself, you just have to abide by a few simple rules:

  • For each pull request, add a new file into changelog.d with a filename adhering to the pr#.(change|deprecation|breaking).md schema: For example, changelog.d/ for a non-breaking change that is proposed in pull request #42.

  • As with other docs, please use semantic newlines within news fragments.

  • Wrap symbols like modules, functions, or classes into backticks so they are rendered in a monospace font.

  • Wrap arguments into asterisks like in docstrings: Added new argument *an_argument*.

  • If you mention functions or other callables, add parentheses at the end of their names: attrs.func() or attrs.Class.method(). This makes the changelog a lot more readable.

  • Prefer simple past tense or constructions with “now”. For example:

    • Added attrs.validators.func().
    • attrs.func() now doesn't crash the Large Hadron Collider anymore when passed the foobar argument.
  • If you want to reference multiple issues, copy the news fragment to another filename. Towncrier will merge all news fragments with identical contents into one entry with multiple links to the respective pull requests.

Example entries:

Added `attrs.validators.func()`.
The feature really *is* awesome.


`attrs.func()` now doesn't crash the Large Hadron Collider anymore when passed the *foobar* argument.
The bug really *was* nasty.

tox -e changelog will render the current changelog to the terminal if you have any doubts.


attrs is maintained by team of volunteers that is always open to new members that share our vision of a fast, lean, and magic-free library that empowers programmers to write better code with less effort. If you'd like to join, just get a pull request merged and ask to be added in the very same pull request!

The simple rule is that everyone is welcome to review/merge pull requests of others but nobody is allowed to merge their own code.

Hynek Schlawack acts reluctantly as the BDFL and has the final say over design decisions.