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 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 under CVS control, their revision history is the historical record of the feature proposal [1].

PEP Types

There are three kinds of PEP:

1. A Standards Track PEP describes a new feature or implementation for Python.
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 implementors 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 release schedules, procedures, guidelines, changes to the decision-making process, and changes to the tools or environment used in Python development.

PEP Work Flow

The PEP editors assign PEP numbers and change their status. The current PEP editors are David Goodger and Barry Warsaw. Please send all PEP-related email to <peps@python.org>.

The PEP process begins with a new idea for Python. It is highly recommended that a single PEP contain a single key proposal or new idea. The more focussed the PEP, the more successful it tends to be. The PEP editor reserves the right to reject PEP proposals if they appear too unfocussed or too broad. If in doubt, split your PEP into several well-focussed ones.

Each PEP must have a champion -- someone who writes the PEP using the style and format described below, shepherds the discussions in the appropriate forums, and attempts to build community consensus around the idea. The PEP champion (a.k.a. Author) should first attempt to ascertain whether the idea is PEP-able. Posting to the comp.lang.python newsgroup (a.k.a. python-list@python.org mailing list) is recommended. Small enhancements or patches often don't need a PEP and can be injected into the Python development work flow with a patch submission to the SourceForge patch manager [6] or feature request tracker [7].

The PEP champion then emails the PEP editor <peps@python.org> with a proposed title and a rough, but fleshed out, draft of the PEP. This draft must be written in PEP style as described below.

If the PEP editor approves, he will assign the PEP a number, label it as Standards Track, Informational, or Process, give it status "Draft", and create and check-in the initial draft of the PEP. The PEP editor will not unreasonably deny a PEP. Reasons for denying PEP status include duplication of effort, being technically unsound, not providing proper motivation or addressing backwards compatibility, or not in keeping with the Python philosophy. The BDFL (Benevolent Dictator for Life, Guido van Rossum) can be consulted during the approval phase, and is the final arbiter of the draft's PEP-ability.

If a pre-PEP is rejected, the author may elect to take the pre-PEP to the comp.lang.python newsgroup (a.k.a. python-list@python.org mailing list) to help flesh it out, gain feedback and consensus from the community at large, and improve the PEP for re-submission.

The author of the PEP is then responsible for posting the PEP to the community forums, and marshaling community support for it. As updates are necessary, the PEP author can check in new versions if they have CVS commit permissions, or can email new PEP versions to the PEP editor for committing.

Standards Track PEPs consist of two parts, a design document and a reference implementation. The PEP should be reviewed and accepted before a reference implementation is begun, unless a reference implementation will aid people in studying the PEP. Standards Track PEPs must include an implementation -- in the form of code, a patch, or a URL to same -- before it can be considered Final.

PEP authors are responsible for collecting community feedback on a PEP before submitting it for review. A PEP that has not been discussed on python-list@python.org and/or python-dev@python.org will not be accepted. However, wherever possible, long open-ended discussions on public mailing lists should be avoided. Strategies to keep the discussions efficient include: setting up a separate SIG mailing list for the topic, having the PEP author accept private comments in the early design phases, setting up a wiki page, etc. PEP authors should use their discretion here.

Once the authors have completed a PEP, they must inform the PEP editor that it is ready for review. PEPs are reviewed by the BDFL and his chosen consultants, who may accept or reject a PEP or send it back to the author(s) for revision. For a PEP that is pre-determined to be acceptable (e.g., it is an obvious win as-is and/or its implementation has already been checked in) the BDFL may also initiate a PEP review, first notifying the PEP author(s) and giving them a chance to make revisions.

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 BDFL. (However, "pythonic" is an imprecise term; it may be defined as whatever is acceptable to the BDFL. 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 accepted by the BDFL, the status will be changed to "Final".

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

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

PEP work flow is as follows:

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:

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. Copyright/public domain -- Each PEP must either be explicitly labelled as placed in the public domain (see this PEP as an example) or licensed under the Open Publication License [8].

4. 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 any of the current Python platforms (CPython, Jython, Python .NET).

5. Motivation -- The motivation is critical for PEPs that want to change the Python language. 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.

6. Rationale -- The rationale fleshes out the specification by describing what motivated the design and 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.

7. 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.

8. 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. It is better to finish the specification and rationale first and reach consensus on it before writing code.

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

PEP Formats and Templates

There are two PEP formats available to authors: plaintext and reStructuredText [9].

Plaintext PEPs are written in plain ASCII text, contain minimal structural markup, and should adhere to a rigid style. PEP 9 contains a boilerplate template [3] you can use to get started writing your plaintext PEP.

ReStructuredText [9] PEPs allow for rich markup that is still quite easy to read, but results in much better-looking and more functional HTML. PEP 12 contains a boilerplate template [4] for use with reStructuredText PEPs.

There is a Python script that converts both styles of PEPs to HTML for viewing on the web [5]. Parsing and conversion of plaintext PEPs is self-contained within the script. reStructuredText PEPs are parsed and converted by Docutils [10] code called from the script.

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>
  Version: <cvs version string>
  Last-Modified: <cvs date string>
  Author: <list of authors' real names and optionally, email addrs>
* Discussions-To: <email address>
  Status: <Draft | Active | Accepted | Deferred | Rejected |
           Final | Replaced>
  Type: <Standards Track | Informational | Process>
* Content-Type: <text/plain | text/x-rst>
* Requires: <pep numbers>
  Created: <date created on, in dd-mmm-yyyy format>
* Python-Version: <version number>
  Post-History: <dates of postings to python-list and python-dev>
* Replaces: <pep number>
* Replaced-By: <pep number>

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.

While a PEP is in private discussions (usually during the initial Draft phase), a Discussions-To header will indicate the mailing list or URL where the PEP is being discussed. No Discussions-To header is necessary if the PEP is being discussed privately with the author, or on the python-list or python-dev email 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]). Plaintext ("text/plain") is 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-list and/or python-dev. Both headers should be in dd-mmm-yyyy format, e.g. 14-Aug-2001.

Standards Track PEPs must have a Python-Version header which indicates the version of Python that the feature will be released with. 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 Replaced-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.

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 to the SourceForge bug manager [11] or better yet, the SourceForge patch manager [6] so that your changes don't get lost. If the PEP author is a SourceForge developer, assign the bug/patch to him, otherwise assign it to the PEP editor.

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

PEP authors who are also Python/SourceForge committers can update the PEPs themselves by using "cvs commit" to commit their changes. PEP authors with python.org access should also remember to push the formatted PEP text out to the web by doing the following:

% python pep2html.py -i NUM

where NUM is the number of the PEP you want to push out. See

% python pep2html.py --help

for details.

Transferring PEP Ownership

It occasionally becomes necessary to transfer ownership of PEPs to a new champion. In general, we'd like 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 you don't agree with the direction of the PEP. We try to build consensus around a PEP, but if that's not possible, you can always submit a competing PEP.

If you are interested in assuming ownership of a PEP, send a message asking to take over, addressed to both the original author and the PEP editor <peps@python.org>. If the original author doesn't respond to email in a timely manner, the PEP editor will make a unilateral decision (it's not like such decisions can't be reversed :).

References and Footnotes

Copyright

This document has been placed in the public domain.