Core team member guide

CORE_DEV_GUIDE.md

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+> **_Note:_**  This Core Developer Guide was adapted from the [napari project's Core Developer Guide](https://napari.org/stable/developers/core_dev_guide.html).
+
+# 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
+[@pydata/xarray](https://github.com/orgs/pydata/teams/xarray)
+GitHub team. You should now be on that list too.
+This document offers guidelines for your new role.
+
+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](#reviewing) and especially
+[Merge Only Changes You Understand](#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](https://github.com/pydata/xarray/tree/main/CODE_OF_CONDUCT.md).
+We value sustainable development practices over mad rushes.
+
+When merging, we automatically use GitHub's
+[Squash and Merge](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/merging-a-pull-request#merging-a-pull-request)
+to ensure a clean git history.
+
+You should also continue to make your own pull requests as before and in accordance
+with the [general contributing guide](https://docs.xarray.dev/en/stable/contributing.html). These pull requests still
+require the approval of another core developer before they can be merged.
+
+## Tasks
+
+Xarray values a wide range of contributions, only some of which involve writing code.
+As such, we do not currently make a distinction between a "core developer", "maintainer",
+or "triage team member" as some projects do (e.g. [pandas](https://pandas.pydata.org/docs/development/maintaining.html)).
+
+Xarray is mostly a volunteer project, so these tasks shouldn’t be read as “expectations”.
+**There are no strict expectations**, other than to adhere to our [Code of Conduct](https://github.com/pydata/xarray/tree/main/CODE_OF_CONDUCT.md).
+Rather, the tasks that follow are general descriptions of what it might mean to be a core dev/maintainer.
+
+- Triage newly filed issues,
+- Review newly opened pull requests,
+- Respond to updates on existing issues and pull requests,
+- Drive discussion and decisions on stalled issues and pull requests,
+- Provide experience / wisdom on API design questions to ensure consistency and maintainability,
+- Project organization (run / attend developer meetings, represent xarray),
+- Project evangelism (advertise xarray to new users / collaborate with maintainers of other packages which could interface with xarray),
+- Project fundraising (help write and administrate grants that will support xarray),
+- Improve documentation or tutorials,
+- Maintain xarray's various Continuous Integration Workflows,
+- Write code that goes into xarray `main`.
+
+Matt Rocklin's post on [the role of a maintainer](https://matthewrocklin.com/blog/2019/05/18/maintainer) may be interesting background reading,
+but should not be taken to strictly apply to the Xarray project.
+
+It is natural that your available time and enthusiasm for the project will wax and wane - this is fine and expected!
+It is also common for core developers to have a "niche" - a particular part of the codebase they have specific expertise
+with, or certain type of task above which they primarily perform.
+
+If however you feel that is unlikely you will be able to be actively contribute in the foreseeable future
+(or especially if you won't be available to answer questions about pieces of code that you wrote previously)
+then you may want to consider letting us know you would rather be listed as an "Emeritus Core Developer",
+as this would help us in evaluating the overall health of the project.
+
+## Reviewing
+
+### 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.
+
+`xarray` 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:** `xarray` is a user-facing package that strives to be accessible
+to both novice and advanced users, and new features should ultimately be
+accessible to everyone using the package. `xarray` 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 various types of interoperability.
+
+2. **Performance and benchmarks:** As `xarray` targets scientific applications that often involve
+large multidimensional datasets, high performance is a key value of `xarray`. While
+every new feature won't scale equally to all sizes of data, keeping in mind performance
+and our [benchmarks](https://github.com/pydata/xarray/tree/main/asv_bench) 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 developers will make
+extensive use of our APIs. The foundation of a healthy ecosystem will be
+a fully capable and stable set of APIs, so as `xarray` matures it will
+very important to ensure our APIs are stable. Spending the extra time to consider names of public facing
+variables and methods, alongside function signatures, could save us considerable
+trouble in the future. We do our best to provide [deprecation cycles](https://docs.xarray.dev/en/stable/contributing.html#backwards-compatibility)
+when making backwards-incompatible changes.
+
+4. **Documentation and tutorials:** All new methods should have appropriate doc
+strings following [PEP257](https://peps.python.org/pep-0257/) and the
+[NumPy documentation guide](https://numpy.org/devdocs/dev/howto-docs.html#documentation-style).
+For any major new features, accompanying changes should be made to our
+[tutorials](https://tutorial.xarray.dev). These should 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](#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](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/committing-changes-to-a-pull-request-branch-created-from-a-fork), or using GitHub’s [suggestion](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/commenting-on-a-pull-request)
+[feature](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/incorporating-feedback-in-your-pull-request), 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:
+
+- Our [contributor guide](https://docs.xarray.dev/en/stable/contributing.html).
+- Our [code of conduct](https://github.com/pydata/xarray/tree/main/CODE_OF_CONDUCT.md).
+- Our [philosophy and development roadmap](https://docs.xarray.dev/en/stable/roadmap.html).
+- [PEP8](https://peps.python.org/pep-0008/) for Python style.
+- [PEP257](https://peps.python.org/pep-0257/) and the
+   [NumPy documentation guide](https://numpy.org/devdocs/dev/howto-docs.html#documentation-style)
+   for docstring conventions.
+- [`pre-commit`](https://pre-commit.com) hooks for autoformatting.
+- [`black`](https://github.com/psf/black) autoformatting.
+- [`flake8`](https://github.com/PyCQA/flake8) linting.
+- [python-xarray](https://stackoverflow.com/questions/tagged/python-xarray) on Stack Overflow.
+- [@xarray_dev](https://twitter.com/xarray_dev) on Twitter.
+- [xarray-dev](https://discord.gg/bsSGdwBn) discord community (normally only used for remote synchronous chat during sprints).
+
+You are not required to monitor any of the social resources.
+
+Where possible we prefer to point people towards asynchronous forms of communication
+like github issues instead of realtime chat options as they are far easier
+for a global community to consume and refer back to.
+
+We also have a private mailing list for core developers
+`xarray-core-team@googlegroups.com` 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.
+
+## Conclusion
+
+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!