Mailman 3 June 2011 - Python-Dev

PEP 1, PEP Purpose and Guidelines
by barry＠zope.com May 18, 2021

May 18, 2021

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 http://www.python.org/peps/pep-0001.html Enjoy, -Barry -------------------- snip snip -------------------- PEP: 1 Title: PEP Purpose and Guidelines Version: $Revision: 1.36 $ Last-Modified: $Date: 2002/07/29 18:34:59 $ Author: Barry A. Warsaw, Jeremy Hylton Status: Active Type: Informational Created: 13-Jun-2000 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 the feature. We intend PEPs to be the primary mechanisms for proposing new features, for collecting community input on an issue, and for documenting the design decisions that have gone into Python. The PEP author is responsible for building consensus within the community and documenting dissenting opinions. Because the PEPs are maintained as plain text files under CVS control, their revision history is the historical record of the feature proposal[1]. 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[2] or feature request tracker[3]. 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 re-submission. The author of the PEP is then responsible for posting the PEP to the community forums, and marshaling community support for it. As updates are necessary, the PEP author can check in new versions if they have CVS commit permissions, or can email new PEP versions to the PEP editor for committing. Standards track PEPs 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 Final. PEP authors are responsible for collecting community feedback on a PEP before submitting it for review. A PEP that has not been discussed on python-list(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 this fact. PEPs can also be replaced by a different PEP, rendering the original obsolete. This is intended for Informational PEPs, where version 2 of an API can replace version 1. PEP work flow is as follows: Draft -> Accepted -> Final -> Replaced ^ +----> Rejected v Deferred 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]. 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 during discussion. 7. Backwards Compatibility -- All PEPs that introduce backwards incompatibilities must include a section describing these incompatibilities and their severity. The PEP must explain how the author proposes to deal with these incompatibilities. PEP submissions without a sufficient backwards compatibility treatise may be rejected outright. 8. Reference Implementation -- The reference implementation must be completed before any PEP is given status 'Final,' but it need not be completed before the PEP is accepted. It is better to finish the specification and rationale first and reach consensus on it before writing code. The final implementation must include test code and documentation appropriate for either the Python language reference or the standard library reference. PEP Template 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[5]. PEP 9 contains a boilerplate[7] 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 required. PEP: <pep number> Title: <pep title> Version: <cvs version string> Last-Modified: <cvs date string> Author: <list of authors' real names and optionally, email addrs> * Discussions-To: <email address> Status: <Draft | Active | Accepted | Deferred | 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 [1] for more details. ... [1] http://www.python.org 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 [7] for more information about PEP style ... References [7] PEP 1, PEP Purpose and Guidelines, Warsaw, Hylton http://www.python.org/peps/pep-0001.html If you decide to provide an explicit URL for a PEP, please use this as the URL template: http://www.python.org/peps/pep-xxxx.html 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 never padded. 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[6] or better yet, the SourceForge patch manager[2] 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 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 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 for details. Transferring PEP Ownership It occasionally becomes necessary to transfer ownership of PEPs to a new champion. In general, we'd like to retain the original author as a co-author of the transferred PEP, but that's really up to the original author. A good reason to transfer ownership is because the original author no longer has the time or interest in updating it or following through with the PEP process, or has fallen off the face of the 'net (i.e. is unreachable or not responding to email). A bad reason to transfer ownership is because you don't agree with the direction of the PEP. We try to build consensus around a PEP, but if that's not possible, you can always submit a competing PEP. If you are interested 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 reversed. :). References and Footnotes [1] 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 http://cvs.sourceforge.net/cgi-bin/cvsweb.cgi/python/nondist/peps/?cvsroot=… [2] http://sourceforge.net/tracker/?group_id=5470&atid=305470 [3] http://sourceforge.net/tracker/?atid=355470&group_id=5470&func=browse [4] http://www.opencontent.org/openpub/ [5] 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 http://www.python.org/peps/ [6] http://sourceforge.net/tracker/?group_id=5470&atid=305470 [7] PEP 9, Sample PEP Template http://www.python.org/peps/pep-0009.html Copyright This document has been placed in the public domain. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 End:

8 14

Reviving restricted mode?
by Guido van Rossum Aug. 14, 2014

Aug. 14, 2014

I've received some enthusiastic emails from someone who wants to revive restricted mode. He started out with a bunch of patches to the CPython runtime using ctypes, which he attached to an App Engine bug: http://code.google.com/p/googleappengine/issues/detail?id=671 Based on his code (the file secure.py is all you need, included in secure.tar.gz) it seems he believes the only security leaks are __subclasses__, gi_frame and gi_code. (I have since convinced him that if we add "restricted" guards to these attributes, he doesn't need the functions added to sys.) I don't recall the exploits that Samuele once posted that caused the death of rexec.py -- does anyone recall, or have a pointer to the threads? -- --Guido van Rossum (home page: http://www.python.org/~guido/)

19 35

PEP 3145 (With Contents)
by Eric Pruitt Dec. 25, 2012

Dec. 25, 2012

Alright, I will re-submit with the contents pasted. I never use double backquotes as I think them rather ugly; that is the work of an editor or some automated program in the chain. Plus, it also messed up my line formatting and now I have lines with one word on them... Anyway, the contents of PEP 3145: PEP: 3145 Title: Asynchronous I/O For subprocess.Popen Author: (James) Eric Pruitt, Charles R. McCreary, Josiah Carlson Type: Standards Track Content-Type: text/plain Created: 04-Aug-2009 Python-Version: 3.2 Abstract: In its present form, the subprocess.Popen implementation is prone to dead-locking and blocking of the parent Python script while waiting on data from the child process. Motivation: A search for "python asynchronous subprocess" will turn up numerous accounts of people wanting to execute a child process and communicate with it from time to time reading only the data that is available instead of blocking to wait for the program to produce data [1] [2] [3]. The current behavior of the subprocess module is that when a user sends or receives data via the stdin, stderr and stdout file objects, dead locks are common and documented [4] [5]. While communicate can be used to alleviate some of the buffering issues, it will still cause the parent process to block while attempting to read data when none is available to be read from the child process. Rationale: There is a documented need for asynchronous, non-blocking functionality in subprocess.Popen [6] [7] [2] [3]. Inclusion of the code would improve the utility of the Python standard library that can be used on Unix based and Windows builds of Python. Practically every I/O object in Python has a file-like wrapper of some sort. Sockets already act as such and for strings there is StringIO. Popen can be made to act like a file by simply using the methods attached the the subprocess.Popen.stderr, stdout and stdin file-like objects. But when using the read and write methods of those options, you do not have the benefit of asynchronous I/O. In the proposed solution the wrapper wraps the asynchronous methods to mimic a file object. Reference Implementation: I have been maintaining a Google Code repository that contains all of my changes including tests and documentation [9] as well as blog detailing the problems I have come across in the development process [10]. I have been working on implementing non-blocking asynchronous I/O in the subprocess.Popen module as well as a wrapper class for subprocess.Popen that makes it so that an executed process can take the place of a file by duplicating all of the methods and attributes that file objects have. There are two base functions that have been added to the subprocess.Popen class: Popen.send and Popen._recv, each with two separate implementations, one for Windows and one for Unix based systems. The Windows implementation uses ctypes to access the functions needed to control pipes in the kernel 32 DLL in an asynchronous manner. On Unix based systems, the Python interface for file control serves the same purpose. The different implementations of Popen.send and Popen._recv have identical arguments to make code that uses these functions work across multiple platforms. When calling the Popen._recv function, it requires the pipe name be passed as an argument so there exists the Popen.recv function that passes selects stdout as the pipe for Popen._recv by default. Popen.recv_err selects stderr as the pipe by default. "Popen.recv" and "Popen.recv_err" are much easier to read and understand than "Popen._recv('stdout' ..." and "Popen._recv('stderr' ..." respectively. Since the Popen._recv function does not wait on data to be produced before returning a value, it may return empty bytes. Popen.asyncread handles this issue by returning all data read over a given time interval. The ProcessIOWrapper class uses the asyncread and asyncwrite functions to allow a process to act like a file so that there are no blocking issues that can arise from using the stdout and stdin file objects produced from a subprocess.Popen call. References: [1] [ python-Feature Requests-1191964 ] asynchronous Subprocess http://mail.python.org/pipermail/python-bugs-list/2006-December/ 036524.html [2] Daily Life in an Ivory Basement : /feb-07/problems-with-subprocess http://ivory.idyll.org/blog/feb-07/problems-with-subprocess [3] How can I run an external command asynchronously from Python? - Stack Overflow http://stackoverflow.com/questions/636561/how-can-i-run-an-external- command-asynchronously-from-python [4] 18.1. subprocess - Subprocess management - Python v2.6.2 documentation http://docs.python.org/library/subprocess.html#subprocess.Popen.wait [5] 18.1. subprocess - Subprocess management - Python v2.6.2 documentation http://docs.python.org/library/subprocess.html#subprocess.Popen.kill [6] Issue 1191964: asynchronous Subprocess - Python tracker http://bugs.python.org/issue1191964 [7] Module to allow Asynchronous subprocess use on Windows and Posix platforms - ActiveState Code http://code.activestate.com/recipes/440554/ [8] subprocess.rst - subprocdev - Project Hosting on Google Code http://code.google.com/p/subprocdev/source/browse/doc/subprocess.rst?spec=s… [9] subprocdev - Project Hosting on Google Code http://code.google.com/p/subprocdev [10] Python Subprocess Dev http://subdev.blogspot.com/ Copyright: This P.E.P. is licensed under the Open Publication License; http://www.opencontent.org/openpub/. On Tue, Sep 8, 2009 at 22:56, Benjamin Peterson <benjamin(a)python.org> wrote: > 2009/9/7 Eric Pruitt <eric.pruitt(a)gmail.com>: >> Hello all, >> >> I have been working on adding asynchronous I/O to the Python >> subprocess module as part of my Google Summer of Code project. Now >> that I have finished documenting and pruning the code, I present PEP >> 3145 for its inclusion into the Python core code. Any and all feedback >> on the PEP (http://www.python.org/dev/peps/pep-3145/) is appreciated. > > Hi Eric, > One of the reasons you're not getting many response is that you've not > pasted the contents of the PEP in this message. That makes it really > easy for people to comment on various sections. > > BTW, it seems like you were trying to use reST formatting with the > text PEP layout. Double backquotes only mean something in reST. > > > -- > Regards, > Benjamin >

10 26

Providing a mechanism for PEP 3115 compliant dynamic class creation
by Nick Coghlan April 21, 2012

April 21, 2012

In reviewing a fix for the metaclass calculation in __build_class__ [1], I realised that PEP 3115 poses a potential problem for the common practice of using "type(name, bases, ns)" for dynamic class creation. Specifically, if one of the base classes has a metaclass with a significant __prepare__() method, then the current idiom will do the wrong thing (and most likely fail as a result), since "ns" will probably be an ordinary dictionary instead of whatever __prepare__() would have returned. Initially I was going to suggest making __build_class__ part of the language definition rather than a CPython implementation detail, but then I realised that various CPython specific elements in its signature made that a bad idea. Instead, I'm thinking along the lines of an "operator.prepare(metaclass, bases)" function that does the metaclass calculation dance, invoking __prepare__() and returning the result if it exists, otherwise returning an ordinary dict. Under the hood we would refactor this so that operator.prepare and __build_class__ were using a shared implementation of the functionality at the C level - it may even be advisable to expose that implementation via the C API as PyType_PrepareNamespace(). The correct idiom for dynamic type creation in a PEP 3115 world would then be: from operator import prepare cls = type(name, bases, prepare(type, bases)) Thoughts? Cheers, Nick. [1] http://bugs.python.org/issue1294232 -- Nick Coghlan | ncoghlan(a)gmail.com | Brisbane, Australia

3 8

ctypes: Configurable bitfield allocation strategy
by Vlad Riscutia July 21, 2011

July 21, 2011

I recently started looking at some ctypes issues. I dug a bit into http://bugs.python.org/issue6069 and then I found http://bugs.python.org/issue11920. They both boil down to the fact that bitfield allocation is up to the compiler, which is different in GCC and MSVC. Currently we have hard-coded allocation strategy based on paltform in cfields.c: > if (bitsize /* this is a bitfield request */ > && *pfield_size /* we have a bitfield open */ > #ifdef MS_WIN32 > /* MSVC, GCC with -mms-bitfields */ > && dict->size * 8 == *pfield_size > #else > /* GCC */ > && dict->size * 8 <= *pfield_size > #endif > && (*pbitofs + bitsize) <= *pfield_size) { > /* continue bit field */ > fieldtype = CONT_BITFIELD; > #ifndef MS_WIN32 > } else if (bitsize /* this is a bitfield request */ > && *pfield_size /* we have a bitfield open */ > && dict->size * 8 >= *pfield_size > && (*pbitofs + bitsize) <= dict->size * 8) { > /* expand bit field */ > fieldtype = EXPAND_BITFIELD; > #endif So when creating a bitfield structure, it's size can be different on Linux vs Windows. class MyStructure(ctypes.BigEndianStructure): _pack_ = 1 # aligned to 8 bits, not ctypes default of 32 _fields_ = [ ("Data0", ctypes.c_uint32, 32), ("Data1", ctypes.c_uint8, 3), ("Data2", ctypes.c_uint16, 12), ] sizeof for above structure is 6 on GCC build and 7 on MSVC build. This leads to some confusion and issues, because we can't always interop correctly with code compiled with different compiler than the one Python is compiled with on the platform. Short term solution is to add a warning in the documentation about this. Longer term though, I think it would be better to add a property on the Structure class for configurable allocation strategy, for example Native (default), GCC, MSVC and when allocating the bitfield, use given strategy. Native would behave similar to what happens now, but we would also allow GCC-style allocation on Windows for example and the ability to extend this if we ever run into similar issues with other compilers. This shouldn't be too difficult to implement, will be backwards compatible and it would improve interop. I would like to hear some opinions on this. Thank you, Vlad

4 8

Support the /usr/bin/python2 symlink upstream
by Kerrick Staley July 9, 2011

July 9, 2011

Hello, There is a need for the default Python2 install to place a symlink at /usr/bin/python2 that points to /usr/bin/python, or for the documentation to recommend that packagers ensure that python2 is defined. Also, all documentation should be changed to recommend that "#!/usr/bin/env python2" be used as the shebang for Python 2 scripts. This is needed because some distributions (Arch Linux, in particular), point /usr/bin/python to /usr/bin/python3, while others (including Slackware, Debian, and the BSDs, probably more) do not even define the python2 command. This means that a script has no way of achieving cross-platform compatibility. The point at which many distributions begin to alias /usr/bin/python to /usr/bin/python3 is due soon, and for the next couple of years, it would be best to use a python2 or python3 shebang in all scripts, making no assumptions about plain python, which should only be invoked interactively. This email from about 3 years ago seems relevant: : http://mail.python.org/pipermail/python-3000/2008-March/012421.html Again, this issue needs to be addressed by the Python developers themselves so that different *nix distributions will handle it consistently, allowing Python scripts to continue to be cross-platform. Thanks, Kerrick Staley

40 184

PEP 3101 implementation vs. documentation
by Ben Wolfson July 6, 2011

July 6, 2011

Hello, I'm writing because discussion in a bug report I submitted (<http://bugs.python.org/issue12014>) has suggested that, insofar as at least part of the issue revolves around the interpretation of PEP 3101, that aspect belonged on python-dev. In particular, I was told that the PEP, not the documentation, is authoritative. Since I'm the one who thinks something is wrong, it seems appropriate for me to be the one to bring it up. Basically, the issue is that the current behavior of str.format is at variance at least with the documentation <http://docs.python.org/library/string.html#formatstrings>, is almost certainly at variance with PEP3101 in one respect, and is in my opinion at variance with PEP3101 in another respect as well, regarding what characters can be present in what the grammar given in the documentation calls an element_index, that is, the bit between the square brackets in "{0.attr[idx]}". Both discovering the current behavior and interpreting the documentation are pretty straightforward; interpreting what the PEP actually calls for is more vexed. I'll do the first two things first. TOC for the remainder: 1. What does the current implementation do? 2. What does the documentation say? 3. What does the PEP say? [this part is long, but the PEP is not clear, and I wanted to be thorough] 4. Who cares? 1. What does the current implementation do? Suppose you have this dictionary: d = {"@": 0, "!": 1, ":": 2, "^": 3, "}": 4, "{": {"}": 5}, } Then the following expressions have the following results: (a) "{0[@]}".format(d) --> '0' (b) "{0[!]}".format(d) --> ValueError: Missing ']' in format string (c) "{0[:]}".format(d) --> ValueError: Missing ']' in format string (d) "{0[^]}".format(d) --> '3' (e) "{0[}]}".format(d) --> ValueError: Missing ']' in format string (f) "{0[{]}".format(d) --> ValueError: unmatched '{' in format (g) "{0[{][}]}".format(d) --> '5' Given (e) and (f), I think (g) should be a little surprising, though you can probably guess what's going on and it's not hard to see why it happens if you look at the source: (e) and (f) fail because MarkupIterator_next (in Objects/stringlib/string_format.h) scans through the string looking for curly braces, because it treats them as semantically significant no matter what context they occur in. So, according to MarkupIterator_next, the *first* right curly brace in (e) indicates the end of the replacement field, giving "{0[}". In (f), the second left curly brace indicates (to MarkupIterator_next) the start of a *new* replacement field, and since there's only one right curly brace, it complains. In (g), MarkupIterator_next treats the second left curly brace as starting a new replacement field and the first right curly brace as closing it. However, actually, those braces don't define new replacement fields, as indicated by the fact that the whole expression treats the element_index fields as just plain old strings. (So the current implementation is somewhat schizophrenic, acting at one point as if the braces have to be balanced because '{[]}' is a replacement field and at another point treating the braces as insignificant.) The explanation for (b) and (c) is that parse_field (same source file) treats ':' and '!' as indicating the end of the field_name section of the replacement field, regardless of whether those characters occur within square brackets or not. So, that's what the current implementation does, in those cases. 2. What does the documentation say? The documentation gives a grammar for replacement fields: """ replacement_field ::= "{" [field_name] ["!" conversion] [":" format_spec] "}" field_name ::= arg_name ("." attribute_name | "[" element_index "]")* arg_name ::= [identifier | integer] attribute_name ::= identifier element_index ::= integer | index_string index_string ::= <any source character except "]"> + conversion ::= "r" | "s" format_spec ::= <described in the next section> """ Given this definition of index_string, all of (a)--(g) should be legal, and the results should be the strings '0', '1', '2', '3', "{'}': 5}", and '5'. There is no need to exclude ':', '!', '}', or '{' from the index_string field; allowing them creates no ambiguity, because the field is delimited by square brackets. Tangent: the definition of attribute_name here also does not describe the current behavior ("{0. ;}".format(x) works fine and will call getattr(x, " ;")) and the PEP does not require the attribute_name to be an identifier; in fact it explicitly states that the attribute_name doesn't need to be a valid Python identifier. attribute_name should read (to reflect something like actual behavior, anyway) "<any source character except '[', '.', ':', '!', '{', or '}'> +". The same goes for arg_name (with the integer alternation). Observe: >>> x = lambda: None >>> setattr(x, ']]', 3) >>> "{].]]}".format(**{"]":x}) # (h) '3' One can also presently do this (hence "something like actual behavior"): >>> setattr(x, 'f}', 4) >>> "{a{s.f}}".format(**{"a{s":x}) '4' But not this: >>> "{a{s.func_name}".format(**{"a{s":x}) as it raises a ValueError, for the same reason as explains (g) above. 3. What does the PEP say? Well... It's actually hard to tell! Summary: The PEP does not contain a grammar for replacement fields, and is surprisingly nonspecific about what can appear where, at least when talking about the part of the replacement field before the format specifier. The most reasonable interpretation of the parts of the PEP that seem to be relevant favors the documentation, rather than the implementation. This can be separated into two sub-questions. A. What does the PEP say about ':' and '!'? It says two things that pertain to element_index fields. The first is this: """ The rules for parsing an item key are very simple. If it starts with a digit, then it is treated as a number, otherwise it is used as a string. Because keys are not quote-delimited, it is not possible to specify arbitrary dictionary keys (e.g., the strings "10" or ":-]") from within a format string. """ So it notes that some things can't be used: - Because anything composed entirely of digits is treated as a number, you can't get a string composed entirely of digits. Clear enough. - What's the explanation for the second example, though? Well, you can't have a right square bracket in the index_string, so that would already mean that you can't do this: "{0[:-]]}".format(...) regardless of the whether colons are legal or not. So, although the PEP gives an example of a string that can't in the element_index part of a replacement field, and that string contains a colon, that string would have been disallowed for other reasons anyway. The second is this: """ The str.format() function will have a minimalist parser which only attempts to figure out when it is "done" with an identifier (by finding a '.' or a ']', or '}', etc.). """ This requires some interpretation. For one thing, the contents of an element_index aren't identifiers. For another, it's not true that you're done with an identifier (or whatever) whenever you see *any* of those; it depends on context. When parsing this: "{0[a.b]}" the parser should not stop at the '.'; it should keep going until it reaches the ']', and that will give the element_index. And when parsing this: "{0.f]oo[bar].baz}", it's done with the identifier "foo" when it reaches the '[', not when it reaches the second '.', and not when it reaches the ']', either (recall (h)). The "minimalist parser" is, I take it, one that works like this: when parsing an arg_name you're done when you reach a '[', a ':', a '!', a '.', '{', or a '}'; the same rules apply when parsing a attribute_name; when parsing an element_index you're done when you see a ']'. Now as regards the curly braces there are some other parts of the PEP that perhaps should be taken into account (see below), but I can find no justification for excluding ':' and '!' from the element_index field; the bit quoted above about having a minimalist parser isn't a good justification for that, and it's the only part of the entire PEP that comes close to addressing the question. B. What does it say about '}' and '{'? It still doesn't say much explicitly. There's the quotation I just gave, and then these passages: """ Brace characters ('curly braces') are used to indicate a replacement field within the string: [...] Braces can be escaped by doubling: """ Taken by itself, this would suggest that wherever there's an unescaped brace, there's a replacement field. That would mean that the current implementation's behavior is correct in (e) and (f) but incorrect in (g). However, I think this is a bad interpretation; unescaped braces can indicate the presence of a replacement field without that meaning that *within* a replacement field braces have that meaning, no matter where within the replacement field they occur. Later in the PEP, talking about this example: "{0:{1}}".format(a, b) We have this: """ These 'internal' replacement fields can only occur in the format specifier part of the replacement field. Internal replacement fields cannot themselves have format specifiers. This implies also that replacement fields cannot be nested to arbitrary levels. Note that the doubled '}' at the end, which would normally be escaped, is not escaped in this case. The reason is because the '{{' and '}}' syntax for escapes is only applied when used *outside* of a format field. Within a format field, the brace characters always have their normal meaning. """ The claim "within a format field, the brace characters always have their normal meaning" might be taken to mean that within a *replacement* field, brace characters always indicate the start (or end) of a replacement field. But the PEP at this point is clearly talking about the formatting section of a replacement field---the part that follows the ':', if present. ("Format field" is nowhere defined in the PEP, but it seems reasonable to take it to mean "the format specifier of a replacement field".) However, it seems most reasonable to me to take "normal meaning" to mean "just a brace character". Note that the present implementation only kinda sorta conforms to the PEP in this respect: >>> import datetime >>> format(datetime.datetime.now(), "{{%Y") '{{2011' >>> "{0:{{%{1}}".format(datetime.datetime.now(), 'Y') # (i) Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: unmatched '{' in format >>> "{0:{{%{1}}}}".format(datetime.datetime.now(), 'Y') # (j) '{2011}' Here the brace characters in (i) and (j) are treated, again in MarkupIterator_next, as indicating the start of a replacement field. In (i), this leads the function to throw an exception; since they're balanced in (j), processing proceeds further, and the doubled braces aren't treated as indicating the start or end of a replacement field---because they're escaped. Given that the format spec part of a replacement field can contain further replacement fields, this is, I think, correct behavior, but it's not easy to see how it comports with the PEP, whose language is not very exact. The call to the built-in format() bypasses the mechanism that leads to these results. The PEP is very, very nonspecific about the parts of the replacement field that precede the format specifier. I don't know what kind of discussion surrounded the drawing up of the grammar that appears in the documentation, but I think that it, and not the implementation, should be followed. The implementation only works the way it does because of parsing shortcuts: it raises ValueErrors for (b) and (c) because it generalizes something true of the attribute_name field (encountering a ':' or '!' means one has moved on to the format_spec or conversion part of the replacement field) to the element_index field. And it raises an error for (e) and (f), but lets (g) through, for the reasons already mentioned. It is, in that respect, inconsistent; it treats the curly brace as having one semantic significance at one point and an entirely different significance at another point, so that it does the right thing in the case of (g) entirely by accident. There is, I think, no way to construe the PEP so that it is reasonable to do what the present implementation does in all three cases (if "{" indicates the start of a replacement field in (f), it should do so in (g) as well); I think it's actually pretty difficult to construe the PEP in any way that makes what it does in the case of (e) and (f) correct. 4. Who cares? Well, I do. (Obviously.) I even have a use case: I want to be able to put arbitrary (or as close to arbitrary as possible) strings in the element_index field, because I've got some objects that (should!) enable me to do this: p.say("I'm warning you, {e.red.underline[don't touch that!]}") and have this written ("e" for "effects") to p.out: I'm warning you, \x1b[31m\x1b[4mdon't touch that!\x1b[0m I have a way around the square bracket problem, but it would be quite burdensome to have to deal with all of !:{} as well; enough that I would fall back on something like this: "I'm warning you, {0}".format(e.red.underline("don't touch that!")) or some similar interpolation-based strategy, which I think would be a shame, because of the way it chops up the string. But I also think the present behavior is extremely counterintuitive, unnecessarily complex, and illogical (even unpythonic!). Isn't it bizarre that (g) should work, given what (e) and (f) do? Isn't it strange that (b) and (c) should fail, given that there's no real reason for them to do so---no ambiguity that has to be avoided? And something's gotta give; the documentation and the implementation do not correspond. Beyond the counterintuitiveness of the present implementation, it is also, I think, self-inconsistent. (e) and (f) fail because the interior brace is treated as starting or ending a replacement field, even though interior replacement fields aren't allowed in that part of a replacement field. (g) succeeds because the braces are balanced: they are treated at one point as if they were part of a replacement field, and at another (correctly) as if they are not. But this makes the failure of (e) and (f) unaccountable. It would not have been impossible for the PEP to allow interior replacement fields anywhere, and not just in the format spec, in which case we might have had this: (g') "{0[{][}]}".format(range(10), **{'][':4}) --> '3' or this: (g'') "{0[{][}]}".format({'4':3}, **{'][':4}) --> '3' or something with that general flavor. As far as I can tell, the only way to consistently maintain that (e) and (f) should fail requires that one take (g') or (g'') to be correct: either the interior braces signal replacement fields (hence must be balanced) or they don't (or they're escaped). Regarding the documentation, it could of course be rendered correct by changing *it*, rather than the implementation. But wouldn't it be tremendously weird to have to explain that, in the part of the replacement field preceding the conversion, you can't employ any curly braces, unless they're balanced, and you can't employ ':' or '!' at all, even though they have no semantic significance? So these are out: {0[{].foo} {0[}{}]} {0[a:b]} But these are in: {0[{}{}]} {0[{{].foo.}}} (k) ((k) does work, if you give it an object with the right structure, though it probably should not.) And, moreover, someone would then have to actually change the documentation, whereas there's a patch already, attached to the bug report linked way up at the top of this email, that makes (a)--(g) all work, leaves (i) and (j) as they are, and has the welcome side-effect of making (k) *not* work (if any code anywhere relies on (k) or things like it working, I will be very surprised: anyway the fact that (k) works is, technically, undocumented). It is also quite simple. It doesn't effect the built-in format(), because the built-in format() is concerned only with the format-specifier part of the replacement field and not all the stuff that comes before that, telling str.format *what* object to format. Thanks for reading, -- Ben Wolfson "Human kind has used its intelligence to vary the flavour of drinks, which may be sweet, aromatic, fermented or spirit-based. ... Family and social life also offer numerous other occasions to consume drinks for pleasure." [Larousse, "Drink" entry]

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PEP 397 (Python launcher for Windows) reference implementation
by Vinay Sajip July 5, 2011

July 5, 2011

PEP 397 (Python launcher for Windows) has a reference implementation in Python. Does anyone know of a C implementation, or is planning/working on one? I realise this is the final objective, so such implementation might be premature, but perhaps someone has been experimenting ... Regards, Vinay Sajip

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Issue10403 - using 'attributes' instead of members in documentation
by Senthil Kumaran July 4, 2011

July 4, 2011

Hello! http://bugs.python.org/issue10403 is a documentation bug which talks about using the term 'attribute' instead of the term 'member' when it denotes the class attributes. Agreed. But the discussion goes on to mention that, "Members and methods" should just be "attributes". I find this bit odd. If the two terms are used together, then replacing it with attributes is fine. But the term 'methods' cannot be replaced with 'attributes' as it changes the meaning. Take this case, :class:`BZ2File` provides all of the methods specified by the :class:`io.BufferedIOBase`, except for :meth:`detach` and :meth:`truncate`. Iteration and the :keyword:`with` statement are supported. is correct, whereas replacing "methods with attributes" would make it as :class:`BZ2File` provides all of the attributes specified by the :class:`io.BufferedIOBase`, except for :meth:`detach` and :meth:`truncate`. Iteration and the :keyword:`with` statement are supported. It does not seem correct. My stance is, "It is attributes instead of term members and the term method when it denotes methods can be left as such." Can I still hold on to that and modify the patch which Adam has submitted or the 'attribute' substitution everywhere makes sense? Thanks, Senthil

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time.sleep(-1) behaviour
by Ulrich Eckhardt July 1, 2011

July 1, 2011

Hi! This is a request for clarification for the thread "how to call a function for evry 10 seconds" from the user mailinglist/newsgroup. The gist is this: 1. On Linux/Python 2.6, time.sleep(-1.0) raises an IOError. 2. On MS Windows/Python 2.5 or 2.7 this sleeps forever. It seems that converting this to a 32-bit integer for Sleep() causes an underflow. Questions: 1. Is the IOError somehow explainable? 2. Is the IOError acceptable or a bug? 3. Is the behaviour under MS Windows acceptable or a bug? 4. Since time.sleep() is documented to possibly sleep a bit longer than specified, someone suggested that the overall sleep time could also be truncated to a minimum of zero, i.e. treating negative values as zero. Greetings! Uli ************************************************************************************** Domino Laser GmbH, Fangdieckstra�e 75a, 22547 Hamburg, Deutschland Gesch�ftsf�hrer: Thorsten F�cking, Amtsgericht Hamburg HR B62 932 ************************************************************************************** Visit our website at http://www.dominolaser.com ************************************************************************************** Diese E-Mail einschlie�lich s�mtlicher Anh�nge ist nur f�r den Adressaten bestimmt und kann vertrauliche Informationen enthalten. Bitte benachrichtigen Sie den Absender umgehend, falls Sie nicht der beabsichtigte Empf�nger sein sollten. Die E-Mail ist in diesem Fall zu l�schen und darf weder gelesen, weitergeleitet, ver�ffentlicht oder anderweitig benutzt werden. E-Mails k�nnen durch Dritte gelesen werden und Viren sowie nichtautorisierte �nderungen enthalten. Domino Laser GmbH ist f�r diese Folgen nicht verantwortlich. **************************************************************************************

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