SKIP 0 — Purpose and Process#

Author:: Jarrod Millman <millman@berkeley.edu>
Author:: Juan Nunez-Iglesias <juan.nunez-iglesias@monash.edu>
Author:: Stéfan van der Walt <stefanv@berkeley.edu>
Status:: Active
Type:: Process
Created:: 2019-07-30

What is a SKIP?#

SKIP stands for scikit-image proposal. A SKIP is a design document providing information to the community, or describing a new feature for scikit-image or its processes or environment. The SKIP should provide a rationale for the proposed change as well as a concise technical specification, if applicable.

We intend SKIPs to be the primary mechanisms for proposing major new features, for collecting community input on an issue, and for documenting the design decisions that have gone into scikit-image. The SKIP author is responsible for building consensus within the community and documenting dissenting opinions.

Because the SKIPs are maintained as text files in a versioned repository, their revision history is the historical record of the feature proposal [1].

Types#

There are three kinds of SKIPs:

A Standards Track SKIP describes a new feature or implementation for scikit-image.
An Informational SKIP describes a scikit-image design issue, or provides general guidelines or information to the Python community, but does not propose a new feature. Informational SKIPs do not necessarily represent a scikit-image community consensus or recommendation, so users and implementers are free to ignore Informational SKIPs.
A Process SKIP describes a process surrounding scikit-image, or proposes a change to (or an event in) a process. Process SKIPs are like Standards Track SKIPs but apply to areas other than the scikit-image library itself. They may propose an implementation, but not to scikit-image’s codebase; they require community consensus. Examples include procedures, guidelines, changes to the decision-making process, and changes to the tools or environment used in scikit-image development. Any meta-SKIP is also considered a Process SKIP.

SKIP Workflow#

The SKIP process begins with a new idea for scikit-image. A SKIP should contain a single key proposal or new idea. Small enhancements or patches often don’t need a SKIP and can be injected into the scikit-image development workflow with a pull request to the scikit-image repo. The more focused the SKIP, the more likely it is to be accepted.

Each SKIP must have a champion—someone who writes the SKIP using the style and format described below, shepherds the discussions in the appropriate forums, and attempts to build community consensus around the idea. The SKIP champion (a.k.a. Author) should first attempt to ascertain whether the idea is suitable for a SKIP. Posting to the scikit-image developer forum is the best way to do this.

The proposal should be submitted as a draft SKIP via a GitHub pull request to the doc/source/skips directory with the name skip-<n>.rst where <n> is an appropriately assigned number (e.g., skip-35.rst). The draft must use the SKIP X — Template and Instructions file.

Once the PR is in place, the SKIP should be announced on the developer forum for discussion (comments on the PR itself should be restricted to minor editorial and technical fixes).

At the earliest convenience, the PR should be merged (regardless of whether it is accepted during discussion). A SKIP that outlines a coherent argument and that is considered reasonably complete should be merged optimistically, regardless of whether it is accepted during discussion. Additional PRs may be made by the author to update or expand the SKIP, or by maintainers to set its status, discussion URL, etc.

Standards Track SKIPs consist of two parts, a design document and a reference implementation. It is generally recommended that at least a prototype implementation be co-developed with the SKIP, as ideas that sound good in principle sometimes turn out to be impractical. Often it makes sense for the prototype implementation to be made available as PR to the scikit-image repo, as long as it is properly marked as WIP (work in progress).

Review and Resolution#

SKIPs are discussed on the developer forum. The possible paths of the status of SKIPs are as follows:

All SKIPs should be created with the Draft status.

Eventually, after discussion, there may be a consensus that the SKIP should be accepted – see the next section for details. At this point the status becomes Accepted.

Once a SKIP has been Accepted, the reference implementation must be completed. When the reference implementation is complete and incorporated into the main source code repository, the status will be changed to Final.

To allow gathering of additional design and interface feedback before committing to long term stability for a language feature or standard library API, a SKIP may also be marked as “Provisional”. This is short for “Provisionally Accepted”, and indicates that the proposal has been accepted for inclusion in the reference implementation, but additional user feedback is needed before the full design can be considered “Final”. Unlike regular accepted SKIPs, provisionally accepted SKIPs may still be Rejected or Withdrawn even after the related changes have been included in a release.

Wherever possible, it is considered preferable to reduce the scope of a proposal to avoid the need to rely on the “Provisional” status (e.g. by deferring some features to later SKIPs), as this status can lead to version compatibility challenges in the wider ecosystem.

A SKIP can also be assigned status Deferred. The SKIP author or a core developer can assign the SKIP this status when no progress is being made on the SKIP.

A SKIP can also be Rejected. Perhaps after all is said and done it was not a good idea. It is still important to have a record of this fact. The Withdrawn status is similar—it means that the SKIP author themselves has decided that the SKIP is actually a bad idea, or has accepted that a competing proposal is a better alternative.

When a SKIP is Accepted, Rejected, or Withdrawn, the SKIP should be updated accordingly. In addition to updating the status field, at the very least the Resolution header should be added with a link to the relevant post on the discussion forum.

SKIPs can also be Superseded by a different SKIP, rendering the original obsolete. The Replaced-By and Replaces headers should be added to the original and new SKIPs respectively.

Process SKIPs may also have a status of Active if they are never meant to be completed, e.g. SKIP 0 (this SKIP).

How a SKIP becomes Accepted#

A SKIP is Accepted by consensus of all interested contributors. We need a concrete way to tell whether consensus has been reached. When you think a SKIP is ready to accept, start a topic on the developer forum with a subject like:

Proposal to accept SKIP #<number>: <title>

In the body of your email, you should:

link to the latest version of the SKIP,
briefly describe any major points of contention and how they were resolved,
include a sentence like: “If there are no substantive objections within 7 days from this email, then the SKIP will be accepted; see SKIP 0 for more details.”

For an equivalent example in the NumPy library, see: https://mail.python.org/pipermail/numpy-discussion/2018-June/078345.html

After you send the email, you should make sure to link to the email thread from the Discussion section of the SKIP, so that people can find it later.

Generally the SKIP author will be the one to send this email, but anyone can do it – the important thing is to make sure that everyone knows when a SKIP is on the verge of acceptance, and give them a final chance to respond. If there’s some special reason to extend this final comment period beyond 7 days, then that’s fine, just say so in the email. You shouldn’t do less than 7 days, because sometimes people are travelling or similar and need some time to respond.

In general, the goal is to make sure that the community has consensus, not provide a rigid policy for people to try to game. When in doubt, err on the side of asking for more feedback and looking for opportunities to compromise.

If the final comment period passes without any substantive objections, then the SKIP can officially be marked Accepted. You should send a followup email notifying the list (celebratory emoji optional but encouraged 🎉✨), and then update the SKIP by setting its :Status: to Accepted, and its :Resolution: header to a link to your followup email.

If there are substantive objections, then the SKIP remains in Draft state, discussion continues as normal, and it can be proposed for acceptance again later once the objections are resolved.

In unusual cases, when no consensus can be reached between core developers, the scikit-image Steering Council may be asked to decide whether a controversial SKIP is Accepted.

Maintenance#

In general, Standards track SKIPs are no longer modified after they have reached the Final state, as the code and project documentation are considered the ultimate reference for the implemented feature. They may, however, be updated under special circumstances.

Process SKIPs may be updated over time to reflect changes to development practices and other details. The precise process followed in these cases will depend on the nature and purpose of the SKIP being updated.

Format and Template#

SKIPs are UTF-8 encoded text files using the reStructuredText format. Please see the SKIP X — Template and Instructions file and the reStructuredTextPrimer for more information. We use Sphinx to convert SKIPs to HTML for viewing on the web [2].

Header Preamble#

Each SKIP must begin with a header preamble. The headers must appear in the following order. Headers marked with * are optional. All other headers are required.

  :Author: <list of authors' real names and optionally, email addresses>
  :Status: <Draft | Active | Accepted | Deferred | Rejected |
           Withdrawn | Final | Superseded>
  :Type: <Standards Track | Process>
  :Created: <date created on, in dd-mmm-yyyy format>
* :Requires: <skip numbers>
* :skimage-Version: <version number>
* :Replaces: <skip number>
* :Replaced-By: <skip number>
* :Resolution: <url>

The Author header lists the names, and optionally the email addresses of all the authors of the SKIP. The format of the Author header value must be

Random J. User <address@dom.ain>

if the email address is included, and just

Random J. User

if the address is not given. If there are multiple authors, each should be on a separate line.