What is a PEP?

PEP stands for Python Enhancement Proposal. A PEP is a design document providing information to the Python community, or describing a new feature for Python or its processes or environment. The PEP should provide a concise technical specification of the feature and a rationale for the feature.

We intend PEPs 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 Python. The PEP author is responsible for building consensus within the community and documenting dissenting opinions.

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

PEP Audience

The typical primary audience for PEPs are the core developers of the CPython reference interpreter and their elected Steering Council, as well as developers of other implementations of the Python language specification.

However, other parts of the Python community may also choose to use the process (particularly for Informational PEPs) to document expected API conventions and to manage complex design coordination problems that require collaboration across multiple projects.

PEP Types

There are three kinds of PEP:

1. A Standards Track PEP describes a new feature or implementation for Python. It may also describe an interoperability standard that will be supported outside the standard library for current Python versions before a subsequent PEP adds standard library support in a future version.
2. An Informational PEP describes a Python design issue, or provides general guidelines or information to the Python community, but does not propose a new feature. Informational PEPs do not necessarily represent a Python community consensus or recommendation, so users and implementers are free to ignore Informational PEPs or follow their advice.
3. A Process PEP describes a process surrounding Python, or proposes a change to (or an event in) a process. Process PEPs are like Standards Track PEPs but apply to areas other than the Python language itself. They may propose an implementation, but not to Python's codebase; they often require community consensus; unlike Informational PEPs, they are more than recommendations, and users are typically not free to ignore them. Examples include procedures, guidelines, changes to the decision-making process, and changes to the tools or environment used in Python development. Any meta-PEP is also considered a Process PEP.

PEP Workflow

PEP Review & Resolution

Once the authors have completed a PEP, they may request a review for style and consistency from the PEP editors.

However, content review and final acceptance of the PEP must be requested of the core developers, usually via an email to the python-dev mailing list.

To expedite the process in selected cases (e.g. when a change is clearly beneficial and ready to be accepted, but the PEP hasn't been formally submitted for review yet), the Steering Council may also initiate a PEP review, first notifying the PEP author(s) and giving them a chance to make revisions.

The final authority for PEP approval is the Steering Council. However, whenever a new PEP is put forward, any core developer that believes they are suitably experienced to make the final decision on that PEP may offer to serve as the BDFL-Delegate for that PEP, and they will then have the authority to approve (or reject) that PEP. Individuals taking on this responsibility are free to seek additional guidance from the Steering Council at any time, and are also expected to take the advice and perspectives of other core developers into account.

The designated decision maker for each PEP is recorded in the "BDFL-Delegate" header in the PEP.

Such self-nominations are accepted by default, but may be explicitly declined by the Steering Council. Possible reasons for the Steering Council declining a self-nomination as BDFL-Delegate include, but are not limited to, perceptions of a potential conflict of interest (e.g. working for the same organisation as the PEP submitter), or simply considering another potential BDFL-Delegate to be more appropriate. If core developers (or other community members) have concerns regarding the suitability of a BDFL-Delegate for any given PEP, they may ask the Steering Council to review the delegation.

If no volunteer steps forward, then the Steering Council will approach core developers (and potentially other Python community members) with relevant expertise, in an attempt to identify a candidate that is willing to serve as BDFL-Delegate for that PEP. If no suitable candidate can be found, then the PEP will be marked as Deferred until one is available.

Previously appointed BDFL-Delegates may choose to step down, or be asked to step down by the Council, in which case a new BDFL-Delegate will be appointed in the same manner as for a new PEP (including deferral of the PEP if no suitable replacement can be found). In the event that a BDFL-Delegate is asked to step down, this will overrule any prior acceptance or rejection of the PEP, and it will revert to Draft status.

With the approval of the Steering Council, PEP review and resolution may also occur on a list other than python-dev (for example, distutils-sig for packaging related PEPs that don't immediately affect the standard library). In these cases, the "Discussions-To" heading in the PEP will identify the appropriate alternative list where discussion, review and pronouncement on the PEP will occur.

When such standing delegations are put in place, the Steering Council will maintain sufficient public records to allow subsequent Councils, the core developers, and the wider Python community to understand the delegations that currently exist, why they were put in place, and the circumstances under which they may no longer be needed.

For a PEP to be accepted it must meet certain minimum criteria. It must be a clear and complete description of the proposed enhancement. The enhancement must represent a net improvement. The proposed implementation, if applicable, must be solid and must not complicate the interpreter unduly. Finally, a proposed enhancement must be "pythonic" in order to be accepted by the Steering Council. (However, "pythonic" is an imprecise term; it may be defined as whatever is acceptable to the Steering Council. This logic is intentionally circular.) See PEP 2 [2] for standard library module acceptance criteria.

Once a PEP 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 PEP 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 PEPs, provisionally accepted PEPs may still be Rejected or Withdrawn even after the related changes have been included in a Python 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 PEPs), as this status can lead to version compatibility challenges in the wider Python ecosystem. PEP 411 provides additional details on potential use cases for the Provisional status.

A PEP can also be assigned the status "Deferred". The PEP author or an editor can assign the PEP this status when no progress is being made on the PEP. Once a PEP is deferred, a PEP editor can re-assign it to draft status.

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

When a PEP is Accepted, Rejected or Withdrawn, the PEP 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 in the python-dev mailing list archives.

PEPs can also be superseded by a different PEP, rendering the original obsolete. This is intended for Informational PEPs, where version 2 of an API can replace version 1.

The possible paths of the status of PEPs are as follows:

While not shown in the diagram, "Accepted" PEPs may technically move to "Rejected" or "Withdrawn" even after acceptance. This will only occur if the implementation process reveals fundamental flaws in the design that were not noticed prior to acceptance of the PEP. Unlike Provisional PEPs, these transitions are only permitted if the accepted proposal has not been included in a Python release - released changes must instead go through the regular deprecation process (which may require a new PEP providing the rationale for the deprecation).

Some Informational and Process PEPs may also have a status of "Active" if they are never meant to be completed. E.g. PEP 1 (this PEP).

What belongs in a successful PEP?

Each PEP should have the following parts/sections:

1. Preamble -- RFC 822 style headers containing meta-data about the PEP, including the PEP number, a short descriptive title (limited to a maximum of 44 characters), the names, and optionally the contact info for each author, etc.

2. Abstract -- a short (~200 word) description of the technical issue being addressed.

3. Motivation -- The motivation is critical for PEPs that want to change the Python language, library, or ecosystem. It should clearly explain why the existing language specification is inadequate to address the problem that the PEP solves. PEP submissions without sufficient motivation may be rejected outright.

4. Rationale -- The rationale fleshes out the specification by describing why particular design decisions were made. It should describe alternate designs that were considered and related work, e.g. how the feature is supported in other languages.

   The rationale should provide evidence of consensus within the community and discuss important objections or concerns raised during discussion.

5. Specification -- The technical specification should describe the syntax and semantics of any new language feature. The specification should be detailed enough to allow competing, interoperable implementations for at least the current major Python platforms (CPython, Jython, IronPython, PyPy).

6. Backwards Compatibility -- All PEPs that introduce backwards incompatibilities must include a section describing these incompatibilities and their severity. The PEP must explain how the author proposes to deal with these incompatibilities. PEP submissions without a sufficient backwards compatibility treatise may be rejected outright.

7. Security Implications -- If there are security concerns in relation to the PEP, those concerns should be explicitly written out to make sure reviewers of the PEP are aware of them.

8. How to Teach This -- For a PEP that adds new functionality or changes language behavior, it is helpful to include a section on how to teach users, new and experienced, how to apply the PEP to their work.

   This section may include key points and recommended documentation changes that would help users adopt a new feature or migrate their code to use a language change.

9. Reference Implementation -- The reference implementation must be completed before any PEP is given status "Final", but it need not be completed before the PEP is accepted. While there is merit to the approach of reaching consensus on the specification and rationale before writing code, the principle of "rough consensus and running code" is still useful when it comes to resolving many discussions of API details.

   The final implementation must include test code and documentation appropriate for either the Python language reference or the standard library reference.

10. Rejected Ideas -- Throughout the discussion of a PEP, various ideas will be proposed which are not accepted. Those rejected ideas should be recorded along with the reasoning as to why they were rejected. This both helps record the thought process behind the final version of the PEP as well as preventing people from bringing up the same rejected idea again in subsequent discussions.

    In a way this section can be thought of as a breakout section of the Rationale section that is focused specifically on why certain ideas were not ultimately pursued.

11. Open Issues -- While a PEP is in draft, ideas can come up which warrant further discussion. Those ideas should be recorded so people know that they are being thought about but do not have a concrete resolution. This helps make sure all issues required for the PEP to be ready for consideration are complete complete and reduces people duplicating prior discussion.

12. References -- A collection of URLs used as references through the PEP.

13. Copyright/license -- Each new PEP must be placed under a dual license of public domain and CC0-1.0-Universal [8] (see this PEP for an example).

PEP Formats and Templates

PEPs are UTF-8 encoded text files using the reStructuredText [9] format. ReStructuredText [9] allows for rich markup that is still quite easy to read, but also results in good-looking and functional HTML. PEP 12 contains instructions and a template [4] for reStructuredText PEPs.

The PEP text files are automatically converted to HTML [6] for easier online reading.

PEP Header Preamble

Each PEP must begin with an RFC 822 style header preamble. The headers must appear in the following order. Headers marked with "*" are optional and are described below. All other headers are required.

  PEP: <pep number>
  Title: <pep title>
  Author: <list of authors' real names and optionally, email addrs>
* Sponsor: <real name of sponsor>
* BDFL-Delegate: <PEP czar's real name>
* Discussions-To: <email address>
  Status: <Draft | Active | Accepted | Provisional | Deferred | Rejected |
           Withdrawn | Final | Superseded>
  Type: <Standards Track | Informational | Process>
* Content-Type: <text/x-rst | text/plain>
* Requires: <pep numbers>
  Created: <date created on, in dd-mmm-yyyy format>
* Python-Version: <version number>
  Post-History: <dates of postings to python-ideas and/or python-dev>
* Replaces: <pep number>
* Superseded-By: <pep number>
* Resolution: <url>

The Author header lists the names, and optionally the email addresses of all the authors/owners of the PEP. 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. For historical reasons the format "address@dom.ain (Random J. User)" may appear in a PEP, however new PEPs must use the mandated format above, and it is acceptable to change to this format when PEPs are updated.

If there are multiple authors, each should be on a separate line following RFC 2822 continuation line conventions. Note that personal email addresses in PEPs will be obscured as a defense against spam harvesters.

The Sponsor field records which developer (core, or otherwise approved by the Steering Council) is sponsoring the PEP. If one of the authors of the PEP is a core developer then no sponsor is necessary and thus this field should be left out.

The BDFL-Delegate field is used to record the individual appointed by the Steering Council to make the final decision on whether or not to approve or reject a PEP. (The delegate's email address is currently omitted due to a limitation in the email address masking for reStructuredText PEPs)

Note: The Resolution header is required for Standards Track PEPs only. It contains a URL that should point to an email message or other web resource where the pronouncement about the PEP is made.

For a PEP where final pronouncement will be made on a list other than python-dev, a Discussions-To header will indicate the mailing list or URL where the pronouncement will occur. A temporary Discussions-To header may also be used when a draft PEP is being discussed prior to submission for pronouncement. No Discussions-To header is necessary if the PEP is being discussed privately with the author, or on the python-list, python-ideas or python-dev mailing lists. Note that email addresses in the Discussions-To header will not be obscured.

The Type header specifies the type of PEP: Standards Track, Informational, or Process.

The format of a PEP is specified with a Content-Type header. The acceptable values are "text/plain" for plaintext PEPs (see PEP 9 [3]) and "text/x-rst" for reStructuredText PEPs (see PEP 12 [4]). reStructuredText is strongly preferred, but for backwards compatibility plain text is currently still the default if no Content-Type header is present.

The Created header records the date that the PEP was assigned a number, while Post-History is used to record the dates of when new versions of the PEP are posted to python-ideas and/or python-dev. Both headers should be in dd-mmm-yyyy format, e.g. 14-Aug-2001.

Standards Track PEPs will typically have a Python-Version header which indicates the version of Python that the feature will be released with. Standards Track PEPs without a Python-Version header indicate interoperability standards that will initially be supported through external libraries and tools, and then potentially supplemented by a later PEP to add support to the standard library. Informational and Process PEPs do not need a Python-Version header.

PEPs may have a Requires header, indicating the PEP numbers that this PEP depends on.

PEPs may also have a Superseded-By header indicating that a PEP has been rendered obsolete by a later document; the value is the number of the PEP that replaces the current document. The newer PEP must have a Replaces header containing the number of the PEP that it rendered obsolete.

Auxiliary Files

PEPs may include auxiliary files such as diagrams. Such files should be named pep-XXXX-Y.ext, where "XXXX" is the PEP number, "Y" is a serial number (starting at 1), and "ext" is replaced by the actual file extension (e.g. "png").

Alternatively, all support files may be placed in a subdirectory called pep-XXXX, where "XXXX" is the PEP number. When using a subdirectory, there are no constraints on the names used in files.

Reporting PEP Bugs, or Submitting PEP Updates

How you report a bug, or submit a PEP update depends on several factors, such as the maturity of the PEP, the preferences of the PEP author, and the nature of your comments. For the early draft stages of the PEP, it's probably best to send your comments and changes directly to the PEP author. For more mature, or finished PEPs you may want to submit corrections as a GitHub issue [12] or GitHub pull request [11] so that your changes don't get lost.

When in doubt about where to send your changes, please check first with the PEP author and/or a PEP editor.

PEP authors with git push privileges for the PEP repository can update the PEPs themselves by using "git push" or the GitHub PR interface to submit their changes.

Transferring PEP Ownership

It occasionally becomes necessary to transfer ownership of PEPs to a new champion. In general, it is preferable to retain the original author as a co-author of the transferred PEP, but that's really up to the original author. A good reason to transfer ownership is because the original author no longer has the time or interest in updating it or following through with the PEP process, or has fallen off the face of the 'net (i.e. is unreachable or not responding to email). A bad reason to transfer ownership is because the author doesn't agree with the direction of the PEP. One aim of the PEP process is to try to build consensus around a PEP, but if that's not possible, an author can always submit a competing PEP.

If you are interested in assuming ownership of a PEP, you can also do this via pull request. Fork the PEP repository [10], make your ownership modification, and submit a pull request. You should also send a message asking to take over, addressed to both the original author and the PEP editors <peps@python.org>. If the original author doesn't respond to email in a timely manner, the PEP editors will make a unilateral decision (it's not like such decisions can't be reversed :).

PEP Editor Responsibilities & Workflow

A PEP editor must subscribe to the <peps@python.org> list and must watch the PEP repository [10]. Most correspondence regarding PEP administration can be handled through GitHub issues and pull requests, but you may also use <peps@python.org> for semi-private discussions. Please do not cross-post!

For each new PEP that comes in an editor does the following:

• Make sure that the PEP is either co-authored by a core developer, has a core developer as a sponsor, or has a sponsor specifically approved for this PEP by the Steering Council.
• Read the PEP to check if it is ready: sound and complete. The ideas must make technical sense, even if they don't seem likely to be accepted.
• The title should accurately describe the content.
• The file name extension is correct (i.e. .rst).
• Skim the PEP for obvious defects in language (spelling, grammar, sentence structure, etc.), and code style (examples should conform to PEP 8 & PEP 7). Editors may correct problems themselves, but are not required to do so. (Text format is checked by Travis CI.)

If the PEP isn't ready, an editor will send it back to the author for revision, with specific instructions. If reST formatting is a problem, ask the author(s) to use PEP 12 as a template and resubmit.

Once the PEP is ready for the repository, a PEP editor will:

• Assign a PEP number (almost always just the next available number, but sometimes it's a special/joke number, like 666 or 3141). (Clarification: For Python 3, numbers in the 3000s were used for Py3k-specific proposals. But now that all new features go into Python 3 only, the process is back to using numbers in the 100s again. Remember that numbers below 100 are meta-PEPs.)

• Check that the author has correctly labeled the PEP's type ("Standards Track", "Informational", or "Process"), and marked its status as "Draft".

• Add the PEP to a local fork of the PEP repository. For workflow instructions, follow The Python Developers Guide

  The git repo for the peps is:

  https://github.com/python/peps
  

• Run ./genpepindex.py and ./pep2html.py <PEP Number> to ensure they are generated without errors. If either triggers errors, then the web site will not be updated to reflect the PEP changes.

• Commit and push the new (or updated) PEP

• Monitor python.org to make sure the PEP gets added to the site properly. If it fails to appear, running make will build all of the current PEPs. If any of these are triggering errors, they must be corrected before any PEP will update on the site.

• Send email back to the PEP author with next steps (post to python-list & -dev).

Updates to existing PEPs should be submitted as a GitHub pull request [11]. Questions may of course still be sent to <peps@python.org>.

Many PEPs are written and maintained by developers with write access to the Python codebase. The PEP editors monitor the python-checkins list for PEP changes, and correct any structure, grammar, spelling, or markup mistakes they see.

PEP editors don't pass judgment on PEPs. They merely do the administrative & editorial part (which is generally a low volume task).

Resources:

• Index of Python Enhancement Proposals
• Following Python's Development
• Python Developer's Guide
• Frequently Asked Questions for Developers

References and Footnotes

Copyright

This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.