Title: PEP Purpose and Guidelines
Author: Barry Warsaw, Jeremy Hylton, David Goodger, Nick Coghlan
Post-History: 21-Mar-2001, 29-Jul-2002, 03-May-2003, 05-May-2012
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 _.
There are three kinds of PEP:
1. A **Standards Track** PEP describes a new feature or implementation
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 Work Flow
There are several reference in this PEP to the "BDFL". This acronym stands
for "Benevolent Dictator for Life" and refers to Guido van Rossum, the
original creator of, and the final design authority for, the Python
Submitting a PEP
The PEP editors assign PEP numbers and change their status. Please send
all PEP-related email to <email@example.com> (no cross-posting please).
Also see `PEP Editor Responsibilities & Workflow`_ below.
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. 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 Python `issue tracker`_. The more focused the
PEP, the more successful it tends to be. The PEP editor reserves the
right to reject PEP proposals if they appear too unfocused or too
broad. If in doubt, split your PEP into several well-focused 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. firstname.lastname@example.org mailing
list) or the python-ideas mailing list is the best way to go about this.
Vetting an idea publicly before going as far as writing a PEP is meant
to save the potential author time. Many ideas have been brought
forward for changing Python that have been rejected for various
reasons. Asking the Python community first if an idea is original
helps prevent too much time being spent on something that is
guaranteed to be rejected based on prior discussions (searching
the internet does not always do the trick). It also helps to make sure
the idea is applicable to the entire community and not just the author.
Just because an idea sounds good to the author does not
mean it will work for most people in most areas where Python is used.
Once the champion has asked the Python community as to whether an
idea has any chance of acceptance, a draft PEP should be presented to
python-ideas. This gives the author a chance to flesh out the draft
PEP to make properly formatted, of high quality, and to address
initial concerns about the proposal.
Following a discussion on python-ideas, the proposal should be sent to
the `python-dev list <mailto:email@example.com>`__ with the draft
PEP and the PEP editors <firstname.lastname@example.org>. This
draft must be written in PEP style as described below, else it will be
sent back without further regard until proper formatting rules are
If the PEP editor approves, they 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 can be consulted
during the approval phase, and is the final arbiter of the draft's
Developers with hg push privileges for the `PEP repository`_ may claim
PEP numbers directly by creating and committing a new PEP. When doing so,
the developer must handle the tasks that would normally be taken care of by
the PEP editors (see `PEP Editor Responsibilities & Workflow`_). This
includes ensuring the initial version meets the expected standards for
submitting a PEP. Alternately, even developers may choose to submit PEPs
through the PEP editors. When doing so, let the PEP editors know you have
hg push privileges and they can guide you through the process of updating
the PEP repository directly.
As updates are necessary, the PEP author can check in new versions if
they have hg push privileges, or can email new PEP versions to
the PEP editors for publication.
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. 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.
PEP Review & Resolution
Once the authors have completed a PEP, they may request a review for
style and consistency from the PEP editors. However, the content and
final acceptance of the PEP must be requested of the BDFL, usually via
an email to the python-dev mailing list. 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
predetermined 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.
The final authority for PEP approval is the BDFL. 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's delegate (or "PEP czar") for that PEP. If their self-nomination
is accepted by the other core developers and the BDFL, then they will have
the authority to approve (or reject) that PEP. This process happens most
frequently with PEPs where the BDFL has granted in principle approval for
*something* to be done, but there are details that need to be worked out
before the PEP can be accepted.
If the final decision on a PEP is to be made by a delegate rather than
directly by the BDFL, this will be recorded by including the
"BDFL-Delegate" header in the PEP.
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 _ 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".
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. 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:
.. image:: pep-0001-1.png
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).
In general, Standards track PEPs are no longer modified after they have
reached the Final state. Once a PEP has been completed, the Language and
Standard Library References become the formal documentation of the expected
Informational and Process PEPs 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 PEP being updated.
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
3. Copyright/public domain -- Each PEP must either be explicitly
labeled as placed in the public domain (see this PEP as an
example) or licensed under the `Open Publication License`_.
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
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_. Both are UTF-8-encoded text files.
Plaintext PEPs are written with minimal structural markup that adheres
to a rigid style. PEP 9 contains a instructions and a template _
you can use to get started writing your plaintext PEP.
ReStructuredText_ 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 instructions and a template _ for reStructuredText
There is a Python script that converts both styles of PEPs to HTML for
viewing on the web _. Parsing and conversion of plaintext PEPs is
self-contained within the script. reStructuredText PEPs are parsed
and converted by Docutils_ 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: <version string>
Last-Modified: <date string>
Author: <list of authors' real names and optionally, email addrs>
* BDFL-Delegate: <PEP czar's real name>
* Discussions-To: <email address>
Status: <Draft | Active | Accepted | Deferred | Rejected |
Withdrawn | Final | Superseded>
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>
* 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 <email@example.com>
if the email address is included, and just
Random J. User
if the address is not given. For historical reasons the format
"firstname.lastname@example.org (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
The BDFL-Delegate field is used to record cases where the final decision to
approve or reject a PEP rests with someone other than the BDFL. (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.*
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, python-ideas 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 _)
and "text/x-rst" for reStructuredText PEPs (see PEP 12 _).
Plaintext ("text/plain") is the default if no Content-Type header is
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 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
PEPs may include auxiliary files such as diagrams. Such files must 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").
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 Python `issue tracker`_ so that your
changes don't get lost. If the PEP author is a Python 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 committers can update the
PEPs themselves by using "hg push" to submit their changes.
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 <email@example.com>. 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 :).
PEP Editor Responsibilities & Workflow
A PEP editor must subscribe to the <firstname.lastname@example.org> list. All
PEP-related correspondence should be sent (or CC'd) to
<email@example.com> (but please do not cross-post!).
For each new PEP that comes in an editor does the following:
* 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
* The title should accurately describe the content.
* Edit the PEP for language (spelling, grammar, sentence structure,
etc.), markup (for reST PEPs), code style (examples should match PEP
8 & 7).
If the PEP isn't ready, the editor will send it back to the author for
revision, with specific instructions.
Once the PEP is ready for the repository, the 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, we used numbers in the 3000s for
Py3k-specific proposals. But now that all new features go into
Python 3 only, we're back to using numbers in the 100s again.
Remember that numbers below 100 are meta-PEPs.)
* Add the PEP to a local clone of the PEP repository. For mercurial work
flow instructions, follow `The Python Developers Guide <http://docs.python.org/devguide>`_
The mercurial repo for the peps is::
* 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
* Send email back to the PEP author with next steps (post to
python-list & -dev).
Updates to existing PEPs also come in to firstname.lastname@example.org. Many PEP
authors are not Python committers yet, so PEP editors do the commits for them.
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 we see.
The editors don't pass judgment on PEPs. We merely do the
administrative & editorial part. Except for times like this, there's
relatively low volume.
* `Index of Python Enhancement Proposals <http://www.python.org/dev/peps/>`_
* `Following Python's Development
* `Python Developer's Guide <http://docs.python.org/devguide/>`_
* `Frequently Asked Questions for Developers
References and Footnotes
..  This historical record is available by the normal hg commands
for retrieving older revisions, and can also be browsed via HTTP here:
..  PEP 2, Procedure for Adding New Modules, Faassen
..  PEP 9, Sample Plaintext PEP Template, Warsaw
..  PEP 12, Sample reStructuredText PEP Template, Goodger, Warsaw
..  The script referred to here is pep2pyramid.py, the successor to
pep2html.py, both of which live in the same directory in the hg
repo as the PEPs themselves. Try ``pep2html.py --help`` for
details. The URL for viewing PEPs on the web is
.. _issue tracker:
.. _Open Publication License: http://www.opencontent.org/openpub/
.. _reStructuredText: http://docutils.sourceforge.net/rst.html
.. _Docutils: http://docutils.sourceforge.net/
.. _PEP repository: http://hg.python.org/peps
This document has been placed in the public domain.