It has been a while since I posted a copy of PEP 1 to the mailing
lists and newsgroups. I've recently done some updating of a few
sections, so in the interest of gaining wider community participation
in the Python development process, I'm posting the latest revision of
PEP 1 here. A version of the PEP is always available on-line at
-------------------- snip snip --------------------
Title: PEP Purpose and Guidelines
Version: $Revision: 1.36 $
Last-Modified: $Date: 2002/07/29 18:34:59 $
Author: Barry A. Warsaw, Jeremy Hylton
Post-History: 21-Mar-2001, 29-Jul-2002
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. The PEP should provide a
concise technical specification of the feature and a rationale for
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 plain text files under CVS
control, their revision history is the historical record of the
Kinds of PEPs
There are two kinds of PEPs. A standards track PEP describes a
new feature or implementation for Python. 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.
PEP Work Flow
The PEP editor, Barry Warsaw <peps(a)python.org>, assigns numbers
for each PEP and changes its status.
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 successfully 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. 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 or feature request tracker.
The PEP champion then emails the PEP editor <peps(a)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 or informational, 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 arbitrator
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(a)python.org
mailing list) to help flesh it out, gain feedback and consensus
from the community at large, and improve the PEP for
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 consists 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, patch, or URL to same - before it can be considered
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(a)python.org and/or python-dev(a)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, 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.
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
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:
Draft -> Accepted -> Final -> Replaced
Some informational PEPs may also have a status of `Active' if they
are never meant to be completed. E.g. PEP 1.
What belongs in a successful PEP?
Each PEP should have the following parts:
1. Preamble -- RFC822 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.
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, JPython, 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.
PEPs are written in plain ASCII text, and should adhere to a
rigid style. There is a Python script that parses this style and
converts the plain text PEP to HTML for viewing on the web.
PEP 9 contains a boilerplate template you can use to get
started writing your PEP.
Each PEP must begin with an RFC822 style header preamble. The
headers must appear in the following order. Headers marked with
`*' are optional and are described below. All other headers are
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 | Final | Replaced>
Type: <Informational | Standards Track>
* 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 entry should be
address(a)dom.ain (Random J. User)
if the email address is included, and just
Random J. User
if the address is not given. If there are multiple authors, each
should be on a separate line following RFC 822 continuation line
conventions. Note that personal email addresses in PEPs will be
obscured as a defense against spam harvesters.
Standards track PEPs must have a Python-Version: header which
indicates the version of Python that the feature will be released
with. Informational PEPs do not need a Python-Version: header.
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.
Created: 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.
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.
PEP Formatting Requirements
PEP headings must begin in column zero and the initial letter of
each word must be capitalized as in book titles. Acronyms should
be in all capitals. The body of each section must be indented 4
spaces. Code samples inside body sections should be indented a
further 4 spaces, and other indentation can be used as required to
make the text readable. You must use two blank lines between the
last line of a section's body and the next section heading.
You must adhere to the Emacs convention of adding two spaces at
the end of every sentence. You should fill your paragraphs to
column 70, but under no circumstances should your lines extend
past column 79. If your code samples spill over column 79, you
should rewrite them.
Tab characters must never appear in the document at all. A PEP
should include the standard Emacs stanza included by example at
the bottom of this PEP.
A PEP must contain a Copyright section, and it is strongly
recommended to put the PEP in the public domain.
When referencing an external web page in the body of a PEP, you
should include the title of the page in the text, with a
footnote reference to the URL. Do not include the URL in the body
text of the PEP. E.g.
Refer to the Python Language web site  for more details.
When referring to another PEP, include the PEP number in the body
text, such as "PEP 1". The title may optionally appear. Add a
footnote reference that includes the PEP's title and author. It
may optionally include the explicit URL on a separate line, but
only in the References section. Note that the pep2html.py script
will calculate URLs automatically, e.g.:
Refer to PEP 1  for more information about PEP style
 PEP 1, PEP Purpose and Guidelines, Warsaw, Hylton
If you decide to provide an explicit URL for a PEP, please use
this as the URL template:
PEP numbers in URLs must be padded with zeros from the left, so as
to be exactly 4 characters wide, however PEP numbers in text are
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 or better yet, the SourceForge patch manager so that
your changes don't get lost. If the PEP author is a SF developer,
assign the bug/patch to him, otherwise assign it to the PEP
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 SF committers, can update the PEPs
themselves by using "cvs commit" to commit their changes.
Remember to also 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
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 assuming ownership of a PEP, send a message
asking to take over, addressed to both the original author and the
PEP editor <peps(a)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 be
References and Footnotes
 This historical record is available by the normal CVS commands
for retrieving older revisions. For those without direct access
to the CVS tree, you can browse the current and past PEP revisions
via the SourceForge web site at
 The script referred to here is pep2html.py, which lives in
the same directory in the CVS tree as the PEPs themselves.
Try "pep2html.py --help" for details.
The URL for viewing PEPs on the web is
 PEP 9, Sample PEP Template
This document has been placed in the public domain.
> The point isn't about my suffering as such. The point is more that
> python-dev owns a tiny amount of the code out there, and I don't believe we
> should put Python's users through this.
> Sure - I would be happy to "upgrade" all the win32all code, no problem. I
> am also happy to live in the bleeding edge and take some pain that will
> The issue is simply the user base, and giving Python a reputation of not
> being able to painlessly upgrade even dot revisions.
I agree with all this.
[As I imagined explicit syntax did not catch up and would require
lot of discussions.]
> > Another way is to use special rules
> > (similar to those for class defs), e.g. having
> > <frag>
> > y=3
> > def f():
> > exec "y=2"
> > def g():
> > return y
> > return g()
> > print f()
> > </frag>
> > # print 3.
> > Is that confusing for users? maybe they will more naturally expect 2
> > as outcome (given nested scopes).
> This seems the best compromise to me. It will lead to the least
> broken code, because this is the behavior that we had before nested
> scopes! It is also quite easy to implement given the current
> implementation, I believe.
> Maybe we could introduce a warning rather than an error for this
> situation though, because even if this behavior is clearly documented,
> it will still be confusing to some, so it is better if we outlaw it in
> some future version.
Yes this can be easy to implement but more confusing situations can arise:
What should this print? the situation leads not to a canonical solution
as class def scopes.
from foo import *
> > This probably won't be a very popular suggestion, but how about pulling
> > nested scopes (I assume they are at the root of the problem)
> > until this can be solved cleanly?
> Agreed. While I think nested scopes are kinda cool, I have lived without
> them, and really without missing them, for years. At the moment the cure
> appears worse then the symptoms in at least a few cases. If nothing else,
> it compromises the elegant simplicity of Python that drew me here in the
> first place!
> Assuming that people really _do_ want this feature, IMO the bar should be
> raised so there are _zero_ backward compatibility issues.
I don't say anything about pulling nested scopes (I don't think my opinion
can change things in this respect)
but I should insist that without explicit syntax IMO raising the bar
has a too high impl cost (both performance and complexity) or creates
> >Assuming that people really _do_ want this feature, IMO the bar should be
> >raised so there are _zero_ backward compatibility issues.
> Even at the cost of additional implementation complexity? At the cost
> of having to learn "scopes are nested, unless you do these two things
> in which case they're not"?
> Let's not waffle. If nested scopes are worth doing, they're worth
> breaking code. Either leave exec and from..import illegal, or back
> out nested scopes, or think of some better solution, but let's not
> introduce complicated backward compatibility hacks.
IMO breaking code would be ok if we issue warnings today and implement
nested scopes issuing errors tomorrow. But this is simply a statement
about principles and raised impression.
IMO import * in an inner scope should end up being an error,
not sure about 'exec's.
We will need a final BDFL statement.
regards, Samuele Pedroni.
From: Samuele Pedroni [mailto:email@example.com]
> the first candidate would be a generalization of 'class'
> (although that make it redundant with 'class' and meta-classes)
> so that
> KEYW-TO-BE kind name [ '(' expr,... ')' ] [ maybe  extended syntax ]:
> would be equivalent to
> name = kind(name-as-string,(expr,...),dict-populated-executing-suite)
[fixed up to exclude the docstring, as per the followup message]
I like this - it's completely general, and easy to understand. Then again,
I always like constructs defined in terms of code equivalence, it seems to
be a good way to make the semantics completely explicit.
The nice thing, to me, is that it solves the immediate problem (modulo a
suitable "kind" to work for properties), as well as being extensible to
allow it to be used in more general contexts.
The downside may be that it's *too* general - I've no feel for how it would
look if overused - it might feel like people end up defining their own
> the remaining problem would be to pick a suitable KEYW-TO-BE
Someone, I believe, suggested reusing "def" - this might be nice, but IIRC
it won't work because of the grammar's strict lookahead limits. (If it does
work, then "def" looks good to me).
If def won't work, how about "define"? The construct is sort of an extended
form of def. Or is that too cute?
By the way, can I just say that I am +1 on Michael Hudson's original patch
for [...] on definitions. Even though it doesn't solve the issue of
properties, I think it's a nice solution for classmethod and staticmethod,
and again I like the generality.
I tried adding a variety of new instructions to the PVM, initially with a
code compression goal for the bytecodes, and later with a performance goal.
accesses to locals with an index<16 using a one byte instruction (no
accesses to consts with an index<16 using a one byte instruction (no
accesses to locals with an index<16 using a one byte instruction (no
compare ops using a one byte instruction (no oparg)
PyStone score for best of 10 runs.
umodified 2.3a2 22200
using enum, 22200 (compacting the opcode numeric space using an enum instead
USING_LOAD_FAST_N, USING_LOAD_CONST_N 22350
USING_LOAD_FAST_N, USING_STORE_FAST_N, 22000
USING_LOAD_FAST_N, USING_LOAD_CONST_N, USING_STORE_FAST_N 22200
USING_LOAD_FAST_N, USING_LOAD_CONST_N, USING_STORE_FAST_N, USING_SHORT_CMP
While reducing the size of compiled bytecodes by about 1%, the proposed
modifications at best increase performance by 2%, and at worst reduce
performance by 3%.
Enabling all of the proposed opcodes results in a 1% performance loss.
In general, it would seem that adding opcodes in bulk, even if many opcodes
switch to the same labels, results in a minor performance loss.
Running PyStone under windows results in a fairly large variation in
results. A zip file containing the source files I modified can be found at
If someone would like to try this code on their systems, I would be grateful
to know what kind of results they achieve.
The various proposed opcodes are controlled by a set of #defines in the file
The results of my static analysis indicate that the indices used on
LOAD_FAST, LOAD_CONST, STORE_FAST are almost always small. There may be some
benefit to optimising these instructions to use single byte opargs.
The results of my static and dynamic analysis indicate that the (COMPARE_OP,
JUMP_IF_FALSE, POP_TOP) pattern is highly used. Im looking at what changes
would need to be made to the compiler to remove the need for this sequence
On Tuesday, Feb 25, 2003, at 15:04 US/Eastern,
> python23.zip is good for end users of programs written in Python, but
> not so good for Python programmers: AFAIK it won't show source lines
> in tracebacks for modules loaded from the zip file.
Speaking entirely from a point of ignorance, why are the source line #s
not shown for frames that are implemented in modules loaded from
Assuming the ZIP archive could be exactly identical to what one might
find in /usr/lib/python2.3/, the zip could contain all the py + pyc as
found in the normal library?
As such, it would be trivial for the developer to unzip the zip into--
for example-- /tmp/ for reference purposes. Assuming the developer
has a copy of the 2.3 source lying around and has the zip with just the
PYC, the lines numbers are still very useful.
All things considered, I would think it would be highly desirable for
the developer's Python development environment to be as much like a
stock deployment environment as possible. Java made a grave mistake
in this regard -- the whole class loader mechanism can cause massive
problems-- very annoying and hard to debug problems-- when moving code
from a development environment into deployment if the class loader that
loads a particular class changes between the two environments.
I got a problem report for Stackless today, that
it seems to leak with tracebacks.
After trying other Python versions, I found out
that this is a "feature" of Python and not related
to Stackless. The problem becomes only more visible,
since people are keeping thousands of threads alive.
Here the problem:
When an exception has been raised in a frame, and
it already is handled in an except clause, the
exception is not cleared out from tstate and also
stays alive in the frame object.
Only when the frame is left, eval_frame calls
which clear all these, breaking cycles.
Does this need to be so, and for what reason?
Would it be equivalent if I cleared error info
in the context of a finally: ?
If not, please give me advice how to solve this
problem. It exists in all long-running frames
which have seen exceptions.
Thanks a lot - chris
Christian Tismer :^) <mailto:firstname.lastname@example.org>
Mission Impossible 5oftware : Have a break! Take a ride on Python's
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whom do you want to sponsor today? http://www.stackless.com/
I'm not a python-dev regular, so sorry if this is a FAQ. What's the status
of defining a syntax for function attributes (PEP 232)? I'm using __doc__
to carry metadata about methods right now, but would very much like to use
function attributes. However, without a specialized syntax, I'm stuck
doing things like
VeryLongMethodName.MetadataName = "foo"
which is fine if it's a one-off, but I'd like others to use the code, and
this isn't exactly a friendly mechanism. The proposals in the PEP would be
fine; I was thinking something like
"""this is the docstring"""
.this_is_a_function_attribute = 1
but that's just off the top of my head.
I'm happy to do some work writing a PEP if there's some consensus about
what syntax would be preferable.
>>>The last point is probably compiler dependent. GCC has the tendency
>>>to use the same layout for the assembler code as you use in the
>>>C source code, so placing often used code close to the top
>>>results in better locality (at least on my machines).
>> My experience with gcc (on x86) is that it uses a lookup table
>> for contiguous switch statements rather than a long chain of
>> compares/branches. A quick look at the assembler output from ceval.c
>> suggests it's using a lookup table.
>Right, but the code for the case implementations itself is
>ordered (more or less) in the order you use in the C file. At
>least that was the case at the time (which must have been GCC
>2.95.x or even earlier).
Yeah - I think I must have been reading too fast - on second reading,
you clearly said "locality".
Andrew McNamara, Senior Developer, Object Craft
>The general problem with the ceval switch statement is that it
>is too big. Adding new opcodes will only make it bigger, so I doubt
>that much can be gained in general by trying to come up with new
>The last point is probably compiler dependent. GCC has the tendency
>to use the same layout for the assembler code as you use in the
>C source code, so placing often used code close to the top
>results in better locality (at least on my machines).
My experience with gcc (on x86) is that it uses a lookup table
for contiguous switch statements rather than a long chain of
compares/branches. A quick look at the assembler output from ceval.c
suggests it's using a lookup table. What architecture did you observe
Andrew McNamara, Senior Developer, Object Craft