pep-0001.txt

PEP: 1
Title: PEP Purpose and Guidelines
Author: Barry Warsaw, Jeremy Hylton, David Goodger, Nick Coghlan
Status: Active
Type: Process
Content-Type: text/x-rst
Created: 13-Jun-2000
Post-History: 21-Mar-2001, 29-Jul-2002, 03-May-2003, 05-May-2012,
              07-Apr-2013


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 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
============


Python's BDFL
-------------

There are several references 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
programming language.


PEP Editors
-----------

The PEP editors are individuals responsible for managing the administrative
and editorial aspects of the PEP workflow (e.g. assigning PEP numbers and
changing their status).  See `PEP Editor Responsibilities & Workflow`_ for
details.  The current editors are:

* Chris Angelico
* Anthony Baxter
* Georg Brandl
* Brett Cannon
* David Goodger
* \R. David Murray
* Jesse Noller
* Berker Peksag
* Guido van Rossum
* Barry Warsaw

PEP editorship is by invitation of the current editors, and they can be
contacted via the address <peps@python.org>, but you may only need to use this
to contact the editors semi-privately.  All of the PEP workflow can be
conducted via the GitHub `PEP repository`_ issues and pull requests.


Start with an idea for Python
-----------------------------

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 workflow with a
patch submission to the Python `issue tracker`_. The more focused the
PEP, the more successful it tends to be.  The PEP editors reserve 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. python-list@python.org mailing list) or the python-ideas@python.org
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.


Submitting a PEP
----------------

Following a discussion on python-ideas, the proposal should be submitted as a
draft PEP via a `GitHub pull request`_.  The draft must be written in PEP
style as described below, else it will fail review immediately (although minor
errors may be corrected by the editors).

The standard PEP workflow is:

* You, the PEP author, fork the `PEP repository`_, and create a file named
  ``pep-9999.rst`` that contains your new PEP.  Use "9999" as your draft PEP
  number.
* Push this to your GitHub fork and submit a pull request.
* The PEP editors review your PR for structure, formatting, and other errors.
* Once approved, they will assign your PEP a number, and label it as Standards
  Track, Informational, or Process, and give it the status "Draft"

Once the review process is complete, and the PEP editors approve it (note that
this is *not* the same as accepting your PEP!), they will squash commit your
pull request onto master.

The PEP editors 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 PEP-ability.

Developers with git 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 via pull request.
When doing so, let the PEP editors know you have git 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
(or a collaborating developer) have git push privileges.

After a PEP number has been assigned, a draft PEP may be discussed further on
python-ideas (getting a PEP number assigned early can be useful for ease of
reference, especially when multiple draft PEPs are being considered at the
same time). Eventually, all Standards Track PEPs must  be sent to the
`python-dev list <mailto:python-dev@python.org>`__ for review as described
in the next section.

Standards Track PEPs 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 PEP, as ideas that sound
good in principle sometimes turn out to be impractical when subjected to the
test of implementation.

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.

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 this case, the "Discussions-To"
heading in the PEP will identify the appropriate alternative list where
discussion, review and pronouncement on the PEP will occur.

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 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:

.. image:: pep-0001-process_flow.png
   :alt: PEP process flow diagram

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).


PEP Maintenance
---------------

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
behavior.

If changes based on implementation experience and user feedback are made to
Standards track PEPs while in the Accepted or Provisional State, those changes
should be noted in the PEP, such that the PEP accurately describes the state of
the implementation at the point where it is marked Final.

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
   being addressed.

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 at least the current major Python
   platforms (CPython, Jython, IronPython, PyPy).

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.  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.

9. 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.


PEP Formats and Templates
=========================

PEPs are UTF-8 encoded text files using the reStructuredText_ format.
ReStructuredText_ 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 [5]_ for easier
`online reading <https://www.python.org/dev/peps/>`__.


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>
  * 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 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.*

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`_ or `GitHub pull request`_ 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`_, 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`_.  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:

* 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.

* 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, an editor will send it back to the author for
revision, with specific instructions.

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.)

* Add the PEP to a local fork of the PEP repository.  For workflow
  instructions, follow `The Python Developers Guide <http://docs.python.org/devguide>`_

  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`_.
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 <http://www.python.org/dev/peps/>`_

* `Following Python's Development
  <http://docs.python.org/devguide/communication.html>`_

* `Python Developer's Guide <http://docs.python.org/devguide/>`_

* `Frequently Asked Questions for Developers
  <http://docs.python.org/devguide/faq.html>`_


References and Footnotes
========================

.. [1] This historical record is available by the normal git commands
   for retrieving older revisions, and can also be browsed via HTTP here:
   https://github.com/python/peps

.. [2] PEP 2, Procedure for Adding New Modules, Faassen
   (http://www.python.org/dev/peps/pep-0002)

.. [3] PEP 9, Sample Plaintext PEP Template, Warsaw
   (http://www.python.org/dev/peps/pep-0009)

.. [4] PEP 12, Sample reStructuredText PEP Template, Goodger, Warsaw
   (http://www.python.org/dev/peps/pep-0012)

.. [5] More details on the PEP rendering and publication process can be found
   in the PEPs repo README at
   https://github.com/python/peps/blob/master/README.rst

.. _issue tracker:
   http://bugs.python.org/

.. _Open Publication License: http://www.opencontent.org/openpub/

.. _reStructuredText: http://docutils.sourceforge.net/rst.html

.. _Docutils: http://docutils.sourceforge.net/

.. _PEP repository: https://github.com/python/peps

.. _`GitHub pull request`: https://github.com/python/peps/pulls

.. _`GitHub issue`: https://github.com/python/peps/issues


Copyright
=========

This document has been placed in the public domain.


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