How To Contribute

First off, thank you for considering contributing to argon2_cffi! 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!


  • 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 on Travis 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.


  • 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 your change is noteworthy, add an entry to the changelog. Use semantic newlines, and add a link to your pull request:

    - Added ``argon2_cffi.func()`` that does foo.
      It's pretty cool.
      [`#1 <>`_]
    - ``argon2_cffi.func()`` now doesn't crash the Large Hadron Collider anymore.
      That was a nasty bug!
      [`#2 <>`_]

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 argon2_cffi 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 argon2_cffi 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 argon2_cffi along with its tests and docs requirements:

  • First you have to make sure, that our git submodules are up to date and the Argon2 extension is built:

    1. git submodule init (to initialize git submodule mechanics)

    2. git submodule update (to update the vendored Argon2 C library to the version argon2_cffi is currently packaging)

    3. python build (to build the CFFI module)

    One of the environments requires a system-wide installation of Argon2. On macOS, it’s available in Homebrew (brew install argon2, but you also will have to update your LDFLAGS so you compiler finds it) and recent Ubuntus (zesty and later) ship it too.

  • Next (re-)install argon2_cffi along with its developement requirements:

    $ pip install -e '.[dev]'

Whenever the Argon2 C code changes: you will have to perform the steps above again except of git submodule init.

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

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 to contribute!