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.

You can see a list of all the current core developers on our @napari/core-devs GitHub team. You should now be on that list too.

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

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

All Contributors Are Treated The Same

As a core developer, 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.) It should also be considered best practice to leave a reasonable (24hr) time window after approval before merge to ensure that other core developers have a reasonable chance to weigh in.

We are also an international community, with contributors from many different time zones, some of whom will only contribute during their working hours, others who might only be able to contribute during nights and weekends. It is important to be respectful of other peoples schedules and working habits, even if it slows the project down slightly - we are in this for the long run. In the same vein you also shouldn’t feel pressured to be constantly available or online, and users or contributors who are overly demanding and unreasonable to the point of harassment will be directed to our Code of Conduct. We value sustainable development practices over mad rushes.

When merging, use GitHub’s Squash and Merge to ensure a clean git history.

You should also continue to make your own pull requests as before and in accordance with the general contributor guide. These pull requests still require the approval of another core developer before they can be merged.


How to Conduct A Good Review

Always be kind to contributors. Contributors are often doing 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.

napari 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 (many don’t realize that adding a commit automatically updates a pull request, for example). 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. Usability and generality: napari is a GUI application that strives to be accessible to both coding and non-coding users, and new features should ultimately be accessible to everyone using the app. napari targets the scientific user community broadly, and core features should be domain-agnostic and general purpose. Custom functionality is meant to be provided through our plugin ecosystem. If in doubt, consult back with our mission and values.

  2. Performance and benchmarks: As napari targets scientific applications that often involve large multidimensional datasets, high performance is a key value of napari. While every new feature won’t scale equally to all sizes of data, keeping in mind performance and our benchmarks during a review may be important, and you may need to ask for benchmarks to be run and reported or new benchmarks to be added.

  3. APIs and stability: Coding users and plugin developers will make extensive use of our APIs. The foundation of a healthy plugin ecosystem will be a fully capable and stable set of APIs, so as napari matures it will very important to ensure our APIs are stable. For now, while the project is still in an earlier stage, spending the extra time to consider names of public facing variables and methods, along side function signatures, could save us considerable trouble in the future. Right now we are still making breaking changes with minor version numbers 0.x and do not have a deprecation policy, but we will work to add one soon.

  4. Documentation and tutorials: All new methods should have appropriate doc strings following PEP257 and the NumPy documentation guide. For any major new features, accompanying changes should be made to our tutorials repository, that not only illustrates the new feature, but explains it.

  5. Implementations and algorithms: 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 in that order.

  6. Tests: All contributions 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 explore corner cases. It can be tempting not to review tests, but please do so.

Other changes may be nitpicky: spelling mistakes, formatting, etc. Do not insist contributors make these changes, but instead you should offer to make these changes by pushing to their branch, or using GitHub’s suggestion feature, and be prepared to make them yourself if needed. Using the suggestion feature 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, including a reference to the original pull request. In doing so, ensure you communicate that you are not throwing the contributor’s work away!

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 can consult 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 the social resources.

Where possible we prefer to point people towards asynchronous forms of communication like forum posts and github issues instead of realtime chat options as they are easier for a global community to consume.

We also have a private mailing list for core developers which is sparingly used for discussions that are required to be private, such as voting on new core members.

Inviting New Core Members

Any core member may nominate other contributors to join the core team. While there is no hard-and-fast rule about who can be nominated, ideally, they should have: been part of the project for at least two 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. After nomination voting will happen on a private mailing list. While it is expected that most votes will be unanimous, a two-thirds majority of the cast votes is enough.

Core developers can choose to become emeritus core developers and suspend their approval and voting rights until they become active again.

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!