Contributing to PyStan

PyStan provides a featherweight interface to C++ functions in the Stan library. The interface provided is one which is familiar to Python users.


  • Power-to-weight. Using a minimal amount of code, PyStan lets Python users access the most frequently-used features of the Stan C++ library.

  • Timely releases. PyStan releases appear within 72 hours of a Stan release.

  • Minimize toil. In order to guarantee timely releases and rapid bug fixing, maintaining PyStan should require as little time as possible. Contributions which make the software easier to maintain are welcome.


  • Adding new features. PyStan prioritizes reliability over ease of use. Contributions which make the software more difficult to maintain are not welcome. A plug-in interface allows third-party packages to extend the software.

  • Supporting new platforms. Adding new platforms and hardware architectures will only be considered when PyStan has an established track record of timely releases.

If these goals and non-goals strike you as restrictive, we kindly remind you that PyStan is open source software which you are free to fork and customize.

How to Make a Code Contribution

Code contributions must be readable and easy to understand.

PyStan adopts virtually all the conventions and procedures discussed in Astropy’s How to make a code contribution.

In general, contributions which follow the project’s coding style, have tests, and solve a specific problem will be merged. Here the project follows the spirit of the Collective Code Construction Contract (C4).

Coding Style

PyStan code is PEP 8 compliant. The project uses the code formatter black with a maximum line-length of 119 characters. Documentation, comments, and docstrings should be wrapped at 79 characters, even though PEP 8 suggests 72.

PyStan code is also checked by mypy.

Commit Messages

git commit messages must be formatted carefully in order to allow the automatic generation of release notes.

You should also use the first line of your commit message to indicate the commit’s “type”. If it is a bugfix, the commit message should start with “fix:”. If it is a new feature, the commit message should start with “feat:”. This information makes reviewing patches and generating release notes easier. For a full list of common commit “types”, consult the Conventional Commits specification.

Commit messages must have a body. The body should elaborate on the summary. It should also explain why the change is valuable. Our working hypothesis is that requiring commit message bodies will make it more difficult to inadvertently introduce code which makes maintaining the software more difficult.

In general, follow the Astropy guidelines for git. Important reminders:

  • Make frequent commits, and always include a commit message. Each commit should represent one logical set of changes.

  • Never merge changes from httpstan/main into your feature branch.

Contributor Covenant Code of Conduct

Our Pledge

In the interest of fostering an open and welcoming environment, we as contributors and maintainers pledge to making participation in our project and our community a harassment-free experience for everyone, regardless of age, body size, disability, ethnicity, sex characteristics, gender identity and expression, level of experience, education, socio-economic status, nationality, personal appearance, race, religion, or sexual identity and orientation.

Our Standards

Examples of behavior that contributes to creating a positive environment include:

  • Using welcoming and inclusive language

  • Being respectful of differing viewpoints and experiences

  • Gracefully accepting constructive criticism

  • Focusing on what is best for the community

  • Showing empathy towards other community members

Examples of unacceptable behavior by participants include:

  • The use of sexualized language or imagery and unwelcome sexual attention or advances

  • Trolling, insulting/derogatory comments, and personal or political attacks

  • Public or private harassment

  • Publishing others’ private information, such as a physical or electronic address, without explicit permission

  • Other conduct which could reasonably be considered inappropriate in a professional setting

Our Responsibilities

Project maintainers are responsible for clarifying the standards of acceptable behavior and are expected to take appropriate and fair corrective action in response to any instances of unacceptable behavior.

Project maintainers have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, or to ban temporarily or permanently any contributor for other behaviors that they deem inappropriate, threatening, offensive, or harmful.


This Code of Conduct applies both within project spaces and in public spaces when an individual is representing the project or its community. Examples of representing a project or community include using an official project e-mail address, posting via an official social media account, or acting as an appointed representative at an online or offline event. Representation of a project may be further defined and clarified by project maintainers.


Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting the project team at Alternatively, you may use the NumFOCUS Code of Conduct reporting tool available at All complaints will be reviewed and investigated and will result in a response that is deemed necessary and appropriate to the circumstances. The project team is obligated to maintain confidentiality with regard to the reporter of an incident. Further details of specific enforcement policies may be posted separately.

Project maintainers who do not follow or enforce the Code of Conduct in good faith may face temporary or permanent repercussions as determined by other members of the project’s leadership.


This Code of Conduct is adapted from the Contributor Covenant, version 1.4, available at

For answers to common questions about this code of conduct, see