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Core Developer Guide

Welcome, new core developer! The core team appreciate the quality of your work, and enjoy working with you; we have therefore invited you to join us. Thank you for your numerous contributions to the project so far.

This document offers guidelines for your new role. First and foremost, you should familiarize yourself with the project’s mission, vision, and values. When in doubt, always refer back here.

As a core team member, you gain the responsibility of shepherding other contributors through the review process; here are some guidelines.

All Contributors Are Treated The Same

You now have the ability to push changes directly to the master branch, but should never do so; instead, continue making pull requests as before and in accordance with the general contributor guide.

As a core contributor, you gain the ability to merge or approve other contributors’ pull requests. Much like nuclear launch keys, it is a shared power: you must merge only after another core has approved the pull request, and after you yourself have carefully reviewed it. (See Reviewing and especially Merge Only Changes You Understand below.) To ensure a clean git history, use GitHub’s Squash and Merge feature to merge, unless you have a good reason not to do so.


How to Conduct A Good Review

Always be kind to contributors. Nearly all of scikit-image is volunteer work, for which we are tremendously grateful. Provide constructive criticism on ideas and implementations, and remind yourself of how it felt when your own work was being evaluated as a novice.

scikit-image strongly values mentorship in code review. New users often need more handholding, having little to no git experience. Repeat yourself liberally, and, if you don’t recognize a contributor, point them to our development guide, or other GitHub workflow tutorials around the web. Do not assume that they know how GitHub works (e.g., many don’t realize that adding a commit automatically updates a pull request). Gentle, polite, kind encouragement can make the difference between a new core developer and an abandoned pull request.

When reviewing, focus on the following:

  1. API: The API is what users see when they first use scikit-image. APIs are difficult to change once released, so should be simple, functional (i.e. not carry state), consistent with other parts of the library, and should avoid modifying input variables. Please familiarize yourself with the project’s deprecation policy.
  2. Documentation: Any new feature should have a gallery example, that not only illustrates but explains it.
  3. The algorithm: You should understand the code being modified or added before approving it. (See Merge Only Changes You Understand below.) Implementations should do what they claim, and be simple, readable, and efficient.
  4. Tests: All contributions to the library must be tested, and each added line of code should be covered by at least one test. Good tests not only execute the code, but explores corner cases. It is tempting not to review tests, but please do so.

Other changes may be nitpicky: spelling mistakes, formatting, etc. Do not ask contributors to make these changes, and instead make the changes by pushing to their branch, or using GitHub’s suggestion feature. (The latter is preferred because it gives the contributor a choice in whether to accept the changes.)

Unless you know that a contributor is experienced with git, don’t ask for a rebase when merge conflicts arise. Instead, rebase the branch yourself, force-push to their branch, and advise the contributor to force-pull. If the contributor is no longer active, you may take over their branch by submitting a new pull request and closing the original. In doing so, ensure you communicate that you are not throwing the contributor’s work away!

Please add a note to a pull request after you push new changes; GitHub does not send out notifications for these.

Merge Only Changes You Understand

Long-term maintainability is an important concern. Code doesn’t merely have to work, but should be understood by multiple core developers. Changes will have to be made in the future, and the original contributor may have moved on.

Therefore, do not merge a code change unless you understand it. Ask for help freely: we have a long history of consulting community members, or even external developers, for added insight where needed, and see this as a great learning opportunity.

While we collectively “own” any patches (and bugs!) that become part of the code base, you are vouching for changes you merge. Please take that responsibility seriously.

Further resources

As a core member, you should be familiar with community and developer resources such as:

You are not required to monitor all of the social resources.

Inviting New Core Members

Any core member may nominate other contributors to join the core team. Nominations happen on a private email list, As of this writing, there is no hard-and-fast rule about who can be nominated; at a minimum, they should have: been part of the project for at least six months, contributed significant changes of their own, contributed to the discussion and review of others’ work, and collaborated in a way befitting our community values.

Contribute To This Guide!

This guide reflects the experience of the current core developers. We may well have missed things that, by now, have become second nature—things that you, as a new team member, will spot more easily. Please ask the other core developers if you have any questions, and submit a pull request with insights gained.


We are excited to have you on board! We look forward to your contributions to the code base and the community. Thank you in advance!