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.
In Python 2.5 `0or` was accepted by the Python parser. It became an
error in 2.6 because "0o" became recognizing as an incomplete octal
number. `1or` still is accepted.
On other hand, `1if 2else 3` is accepted despites the fact that "2e" can
be recognized as an incomplete floating point number. In this case the
tokenizer pushes "e" back and returns "2".
Shouldn't it do the same with "0o"? It is possible to make `0or` be
parseable again. Python implementation is able to tokenize this example:
$ echo '0or' | ./python -m tokenize
1,0-1,1: NUMBER '0'
1,1-1,3: NAME 'or'
1,3-1,4: OP '['
1,4-1,5: OP ']'
1,5-1,6: NEWLINE '\n'
2,0-2,0: ENDMARKER ''
On other hand, all these examples look weird. There is an assymmetry:
`1or 2` is a valid syntax, but `1 or2` is not. It is hard to recognize
visually the boundary between a number and the following identifier or
keyword, especially if numbers can contain letters ("b", "e", "j", "o",
"x") and underscores, and identifiers can contain digits. On both sides
of the boundary can be letters, digits, and underscores.
I propose to change the Python syntax by adding a requirement that there
should be a whitespace or delimiter between a numeric literal and the
Pathlib's symlink_to() and link_to() methods have different argument
a.symlink_to(b) # Creates a symlink from A to B
a.link_to(b) # Creates a hard link from B to A
I don't think link_to() was intended to be implemented this way, as the
docs say "Create a hard link pointing to a path named target.". It's also
inconsistent with everything else in pathlib, most obviously symlink_to().
Bug report here: https://bugs.python.org/issue39291
This /really/ irks me. Apparently it's too late to fix link_to(), so I'd
like to suggest we add a new hardlink_to() method that matches the
symlink_to() argument order. link_to() then becomes deprecated/undocumented.
It's finally time to schedule the last releases in Python 2's life. There will be two more releases of Python 2.7: Python 2.7.17 and Python 2.7.18.
Python 2.7.17 release candidate 1 will happen on October 5th followed by the final release on October 19th.
I'm going to time Python 2.7.18 to coincide with PyCon 2020 in April, so attendees can enjoy some collective catharsis. We'll still say January 1st is the official EOL date.
Thanks to Sumana Harihareswara, there's now a FAQ about the Python 2 sunset on the website: https://www.python.org/doc/sunset-python-2/
Browser Link: https://www.python.org/dev/peps/pep-0616/
Title: String methods to remove prefixes and suffixes
Author: Dennis Sweeney <sweeney.dennis650(a)gmail.com>
Sponsor: Eric V. Smith <eric(a)trueblade.com>
Type: Standards Track
This is a proposal to add two new methods, ``cutprefix`` and
``cutsuffix``, to the APIs of Python's various string objects. In
particular, the methods would be added to Unicode ``str`` objects,
binary ``bytes`` and ``bytearray`` objects, and
If ``s`` is one these objects, and ``s`` has ``pre`` as a prefix, then
``s.cutprefix(pre)`` returns a copy of ``s`` in which that prefix has
been removed. If ``s`` does not have ``pre`` as a prefix, an
unchanged copy of ``s`` is returned. In summary, ``s.cutprefix(pre)``
is roughly equivalent to ``s[len(pre):] if s.startswith(pre) else s``.
The behavior of ``cutsuffix`` is analogous: ``s.cutsuffix(suf)`` is
roughly equivalent to
``s[:-len(suf)] if suf and s.endswith(suf) else s``.
There have been repeated issues [#confusion]_ on the Bug Tracker
and StackOverflow related to user confusion about the existing
``str.lstrip`` and ``str.rstrip`` methods. These users are typically
expecting the behavior of ``cutprefix`` and ``cutsuffix``, but they
are surprised that the parameter for ``lstrip`` is interpreted as a
set of characters, not a substring. This repeated issue is evidence
that these methods are useful, and the new methods allow a cleaner
redirection of users to the desired behavior.
As another testimonial for the usefulness of these methods, several
users on Python-Ideas [#pyid]_ reported frequently including similar
functions in their own code for productivity. The implementation
often contained subtle mistakes regarding the handling of the empty
string (see `Specification`_).
The builtin ``str`` class will gain two new methods with roughly the
def cutprefix(self: str, pre: str, /) -> str:
def cutsuffix(self: str, suf: str, /) -> str:
if suf and self.endswith(suf):
The only difference between the real implementation and the above is
that, as with other string methods like ``replace``, the
methods will raise a ``TypeError`` if any of ``self``, ``pre`` or
``suf`` is not an instace of ``str``, and will cast subclasses of
``str`` to builtin ``str`` objects.
Note that without the check for the truthyness of ``suf``,
``s.cutsuffix('')`` would be mishandled and always return the empty
string due to the unintended evaluation of ``self[:-0]``.
Methods with the corresponding semantics will be added to the builtin
``bytes`` and ``bytearray`` objects. If ``b`` is either a ``bytes``
or ``bytearray`` object, then ``b.cutsuffix()`` and ``b.cutprefix()``
will accept any bytes-like object as an argument.
Note that the ``bytearray`` methods return a copy of ``self``; they do
not operate in place.
The following behavior is considered a CPython implementation detail,
but is not guaranteed by this specification::
>>> x = 'foobar' * 10**6
>>> x.cutprefix('baz') is x is x.cutsuffix('baz')
>>> x.cutprefix('') is x is x.cutsuffix('')
That is, for CPython's immutable ``str`` and ``bytes`` objects, the
methods return the original object when the affix is not found or if
the affix is empty. Because these types test for equality using
shortcuts for identity and length, the following equivalent
expressions are evaluated at approximately the same speed, for any
``str`` objects (or ``bytes`` objects) ``x`` and ``y``::
>>> (True, x[len(y):]) if x.startswith(y) else (False, x)
>>> (True, z) if x != (z := x.cutprefix(y)) else (False, x)
The two methods will also be added to ``collections.UserString``,
where they rely on the implementation of the new ``str`` methods.
Motivating examples from the Python standard library
The examples below demonstrate how the proposed methods can make code
one or more of the following:
The code will not depend on the user to count the length of a
The code does not require a call to the Python built-in
The methods give a higher-level API for code readability, as
opposed to the traditional method of string slicing.
fix_name = fix_name[len(self.FILE_PREFIX):]
fix_name = fix_name.cutprefix(self.FILE_PREFIX)
name = name[2:]
name = name.cutprefix("c.")
This is an interesting case because the author chose to use the
``str.replace`` method in a situation where only a prefix was
intended to be removed.
self.funcname = funcname.replace("context.", "")
self.contextfunc = True
self.funcname = funcname
self.contextfunc = False
self.funcname = funcname.cutprefix("context.")
self.contextfunc = True
self.funcname = funcname
self.contextfunc = False
- Arguably further improved::
self.contextfunc = funcname.startswith("context.")
self.funcname = funcname.cutprefix("context.")
creationDate = creationDate[:-len('\\n')]
creationDate = creationDate.cutsuffix('\\n')
reported_name = self._name
if _USE_POSIX and self._prepend_leading_slash:
reported_name = self._name[1:]
if _USE_POSIX and self._prepend_leading_slash:
retval = os.path.basename(archiveName[:-7])
if ((retval.startswith('tcl') or retval.startswith('tk'))
retval = retval[:-4]
retval = os.path.basename(archiveName[:-7])
if retval.startswith(('tcl', 'tk')):
retval = retval.cutsuffix('-src')
Depending on personal style, ``archiveName[:-7]`` could also be
changed to ``archiveName.cutsuffix('.tar.gz')``.
output = output[:-1]
output = output.cutsuffix("\n")
text = text[1:]
text = text[:-1]
if line.endswith("\n"): line = line[:-1]
line = line.cutsuffix("\n")
In the following example, the meaning of the code changes slightly,
but in context, it behaves the same.
if name.endswith(('Mixin', 'Tests')):
value = value[:-1]
value = value.cutsuffix(os.pathsep)
self.assertTrue(r.startswith(clsname + '('), r)
inner = r[len(clsname) + 1 : -1]
self.assertTrue(r.startswith(clsname + '('), r)
inner = r.cutprefix(clsname + '(').cutsuffix(')')
Expand the lstrip and rstrip APIs
Because ``lstrip`` takes a string as its argument, it could be viewed
as taking an iterable of length-1 strings. The API could therefore be
generalized to accept any iterable of strings, which would be
successively removed as prefixes. While this behavior would be
consistent, it would not be obvious for users to have to call
``'foobar'.cutprefix(('foo,))`` for the common use case of a
Allow multiple prefixes
Some users discussed the desire to be able to remove multiple
prefixes, calling, for example, ``s.cutprefix('From: ', 'CC: ')``.
However, this adds ambiguity about the order in which the prefixes are
removed, especially in cases like ``s.cutprefix('Foo', 'FooBar')``.
After this proposal, this can be spelled explicitly as
Remove multiple copies of a prefix
This is the behavior that would be consistent with the aforementioned
expansion of the ``lstrip/rstrip`` API -- repeatedly applying the
function until the argument is unchanged. This behavior is attainable
from the proposed behavior via the following::
>>> s = 'foo' * 100 + 'bar'
>>> while s != (s := s.cutprefix("foo")): pass
The above can be modififed by chaining multiple ``cutprefix`` calls
together to achieve the full behavior of the ``lstrip``/``rstrip``
generalization, while being explicit in the order of removal.
While the proposed API could later be extended to include some of
these use cases, to do so before any observation of how these methods
are used in practice would be premature and may lead to choosing the
Raising an exception when not found
There was a suggestion that ``s.cutprefix(pre)`` should raise an
exception if ``not s.startswith(pre)``. However, this does not match
with the behavior and feel of other string methods. There could be
``required=False`` keyword added, but this violates the KISS
Alternative Method Names
Several alternatives method names have been proposed. Some are listed
below, along with commentary for why they should be rejected in favor
of ``cutprefix`` (the same arguments hold for ``cutsuffix``)
PHP) what ``strip`` methods do in Python.
This would avoid adding a new method, but for different
behavior, it's better to have two different methods than one
method with a keyword argument that select the behavior.
All of the other methods of the string API, e.g.
``str.startswith()``, use ``lowercase`` rather than
``cutleft``, ``leftcut``, or ``lcut``
The explicitness of "prefix" is preferred.
``removeprefix``, ``deleteprefix``, ``withoutprefix``, etc.
All of these might have been acceptable, but they have more
characters than ``cut``. Some suggested that the verb "cut"
implies mutability, but the string API already contains verbs
like "replace", "strip", "split", and "swapcase".
Users may benefit from the mnemonic that "strip" means working
with sets of characters, while other methods work with
substrings, so re-using "strip" here should be avoided.
See the pull request on GitHub [#pr]_.
.. [#pr] GitHub pull request with implementation
.. [#pyid] Discussion on Python-Ideas
.. [#confusion] Comment listing Bug Tracker and StackOverflow issues
This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.
As an experiment, I thought I would try moving the thread state (what
you get from _PyThreadState_GET() ) to TLS.
It works, passing all the tests, and seems sound.
It is a small patch (< 50 lines) and doesn't increase the overall code size.
My branch is GCC/Clang only, so will need a bit of extra code for
Windows. It should only need a few more lines; I haven't done it as I
don't have a Windows machine to test it on.
This is a *much* cleaner approach to removing the global variable than
adding lots of extra parameters all over the place.
The steering council wants to remind folks that if you have witnessed or
experienced any conduct that you think may go against the PSF Code of
Conduct to please report those incidents to conduct(a)python.org. This
includes reporting micro-aggressions like feeling dismissed so that any
pattern of such behaviour can be detected and handled as a larger issue. If
you are on the fence of reporting something we encourage you to report the
incident and let the Conduct WG make the decision as to how to handle the