SLEP000: SLEP and its workflow

Author:Adrin Jalali


This SLEP specifies details related to SLEP submission, review, and acceptance process.


Without a predefined workflow, the discussions around a SLEP can be long and consume a lot of energy for both the author(s) and the reviewers. The lack of a known workflow also results in the SLEPs to take months (if not years) before it is merged as Under Review. The purpose of this SLEP is to lubricate and ease the process of working on a SLEP, and make it a more enjoyable and productive experience. This SLEP borrows the process used in PEPs and NEPs which means there will be no Under Review status.

What is a SLEP?

SLEP stands for Scikit-Learn Enhancement Proposal, inspired from Python PEPs or Numpy NEPs. A SLEP is a design document providing information to the scikit-learn community, or describing a new feature for scikit-learn or its processes or environment. The SLEP should provide a concise technical specification of the proposed solution, and a rationale for the feature.

We intend SLEPs 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-learn. The SLEP author is responsible for building consensus within the community and documenting dissenting opinions.

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

SLEP Audience

The typical primary audience for SLEPs are the core developers of scikit-learn and technical committee, as well as contributors to the project. However, these documents also serve the purpose of documenting the changes and decisions to help users understand the changes and why they are made. The SLEPs are available under Scikit-learn enhancement proposals.

SLEP Types

There are three kinds of SLEPs:

1. A Standards Track SLEP describes a new feature or implementation for scikit-learn.

2. An Informational SLEP describes a scikit-learn design issue, or provides general guidelines or information to the scikit-learn community, but does not propose a new feature. Informational SLEPs do not necessarily represent a scikit-learn community consensus or recommendation, so users and implementers are free to ignore Informational SLEPs or follow their advice. For instance, an informational SLEP could be one explaining how people can write a third party estimator, one to explain the usual process of adding a package to the contrib org, or what our inclusion criteria are for scikit-learn and scikit-learn-extra.

3. A Process SLEP describes a process surrounding scikit-learn, or proposes a change to (or an event in) a process. Process SLEPs are like Standards Track SLEPs but apply to areas other than the scikit-learn library itself. They may propose an implementation, but not to scikit-learn’s codebase; they require community consensus. Examples include procedures, guidelines, changes to the decision-making process and the governance document, and changes to the tools or environment used in scikit-learn development. Any meta-SLEP is also considered a Process SLEP.

SLEP Workflow

A SLEP starts with an idea, which usually is discussed in an issue or a pull request on the main repo before submitting a SLEP. It is generally a good idea for the author of the SLEP to gauge the viability and the interest of the community before working on a SLEP, mostly to save author’s time.

A SLEP must have one or more champions: people who write the SLEP following the SLEP template, shepherd the discussions around it, and seek consensus in the community.

The proposal should be submitted as a draft SLEP via a GitHub pull request to a slepXXX directory with the name proposal.rst where XXX is an appropriately assigned three-digit number (e.g., slep000/proposal.rst). The draft must use the SLEP — Template and Instructions file.

Once the PR for the SLEP is created, a post should be made to the mailing list containing the sections up to “Backward compatibility”, with the purpose of limiting discussion there to usage and impact. Discussion on the pull request will have a broader scope, also including details of implementation.

The first draft of the SLEP needs to be approved by at least one core developer before being merged. Merging the draft does not mean it is accepted or is ready for the vote. To this end, the SLEP draft is reviewed for structure, formatting, and other errors. Approval criteria are:

  • The draft is sound and complete. The ideas must make technical sense.
  • The initial PR reviewer(s) should not consider whether the SLEP seems likely to be accepted.
  • The title of the SLEP draft accurately describes its content.

Reviewers are generally quite lenient about this initial review, expecting that problems will be corrected by the further reviewing process. Note: Approval of the SLEP draft is no guarantee that there are no embarrassing mistakes! Ideally they’re avoided, but they can also be fixed later in separate PRs. Once approved by at least one core developer, the SLEP draft can be merged. Additional PRs may be made by the champions to update or expand the SLEP, or by maintainers to set its status, discussion URL, etc.

Standards Track SLEPs (see bellow) 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 SLEP, as ideas that sound good in principle sometimes turn out to be impractical when subjected to the test of implementation. Often it makes sense for the prototype implementation to be made available as PR to the scikit-learn repo (making sure to appropriately mark the PR as a WIP).

Review and Resolution

SLEPs are discussed on the mailing list or the PRs modifying the SLEP. The possible paths of the status of SLEPs are as follows:

SLEP process flow diagram

All SLEPs should be created with the Draft status.

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

Once a SLEP 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. Since most SLEPs deal with a part of scikit-learn’s API, another way of viewing a SLEP as Final is when its corresponding API interface is considered stable.

To allow gathering of additional design and interface feedback before committing to long term stability for a feature or API, a SLEP 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 SLEPs, provisionally accepted SLEPs may still be Rejected or Withdrawn even after the related changes have been included in a scikit-learn 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 SLEPs), as this status can lead to version compatibility challenges in the wider scikit-learn ecosystem.

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

A SLEP 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 SLEP author themselves has decided that the SLEP is actually a bad idea, or has accepted that a competing proposal is a better alternative.

When a SLEP is Accepted, Rejected, or Withdrawn, the SLEP 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 thread in the mailing list archives or where the discussion happened.

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

Process SLEPs may also have a status of Active if they are never meant to be completed, e.g. SLEP 1 (this SLEP).

How a SLEP becomes Accepted

A SLEP is Accepted by the voting mechanism defined in the governance model. We need a concrete way to tell whether consensus has been reached. When you think a SLEP is ready to accept, create a PR changing the status of the SLEP to Accepted, then send an email to the scikit-learn mailing list with a subject like:

[VOTE] Proposal to accept SLEP #<number>: <title>

In the body of your email, you should:

  • link to the latest version of the SLEP, and a link to the PR accepting the SLEP.
  • briefly describe any major points of contention and how they were resolved,
  • include a sentence like: “The vote will be closed in a month i.e. on <the_date>.”

Generally the SLEP 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 SLEP is on the verge of acceptance, and give them a final chance 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 and voting period passes with the required majority, then the SLEP can officially be marked Accepted. The Resolution header should link to the PR accepting the SLEP.

If the vote does not achieve a required majority, then the SLEP 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, with the request of the author, the scikit-learn technical committee may be asked to decide whether a controversial SLEP is Accepted, put back to Draft with additional recommendation to try again to reach consensus or definitely Rejected. Please refer to the governance doc for more details.


In general, Standards track SLEPs 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. However, finalized Standards track SLEPs may be updated as needed.

Process SLEPs 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 SLEP being updated.

Format and Template

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

Header Preamble

Each SLEP 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 | Informational | Process>
  :Created: <date created on, in yyyy-mm-dd format>
* :Requires: <slep numbers>
* :scikit-learn-Version: <version number>
* :Replaces: <slep number>
* :Replaced-By: <slep number>
* :Resolution: <url>

The Author header lists the names, and optionally the email addresses of all the authors of the SLEP. 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.

References and Footnotes

[1]Open Publication License: