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How To Contribute

First off, 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!


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 StackOverflow!

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 master branch. Whether you prefer to rebase on master or merge master 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 can'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.
  • 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 backward compatibility.


  • 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 follow the Black code style with a line length of 79 characters. As long as you run our full tox suite before committing, or install our pre-commit hooks (ideally you'll do both -- see below "Local Development Environment"), 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 py27,py35 (in that case you may want to look into pyenv, which makes 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 is 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 files (files ending in .rst):

    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 files for each pull request -- so called news fragments -- instead of one monolithic changelog file. On release, those news fragments are compiled into our CHANGELOG.rst.

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).rst schema: For example, changelog.d/42.change.rst 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 double backticks so they are rendered in a monospace font.
  • Wrap arguments into asterisks like in docstrings: these or attributes.
  • If you mention functions or other callables, add parentheses at the end of their names: attr.func() or attr.Class.method(). This makes the changelog a lot more readable.
  • Prefer simple past tense or constructions with "now". For example:
    • Added attr.validators.func().
    • attr.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 ``attr.validators.func()``.
The feature really *is* awesome.


``attr.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.

Local Development Environment

You can (and should) run our test suite using tox. However, you’ll probably want a more traditional environment as well. We highly recommend to develop using the latest Python 3 release because attrs tries to take advantage of modern features whenever possible.

First create a virtual environment. It’s out of scope for this document to list all the ways to manage virtual environments in Python, but if you don’t already have a pet way, take some time to look at tools like pew, virtualfish, and virtualenvwrapper.

Next, get an up to date checkout of the attrs repository:

$ git clone

or if you want to use git via https:

$ git clone

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

$ cd attrs
$ pip install -e '.[dev]'

At this point,

$ python -m pytest

should work and pass, as should:

$ cd docs
$ make html

The built documentation can then be found in docs/_build/html/.

To avoid committing code that violates our style guide, we strongly advise you to install pre-commit [1] hooks:

$ pre-commit install

You can also run them anytime (as our tox does) using:

$ pre-commit run --all-files
[1]pre-commit should have been installed into your virtualenv automatically when you ran pip install -e '.[dev]' above. If pre-commit is missing, it may be that you need to re-run pip install -e '.[dev]'.


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.

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.

Thank you for considering contributing to attrs!

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