Mailman 3 December 2012 - Python-Dev

PEP 1, PEP Purpose and Guidelines
by barry＠zope.com 18 May '21

18 May '21

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

More compact dictionaries with faster iteration
by Raymond Hettinger 03 Jan '15

03 Jan '15

The current memory layout for dictionaries is unnecessarily inefficient. It has a sparse table of 24-byte entries containing the hash value, key pointer, and value pointer. Instead, the 24-byte entries should be stored in a dense table referenced by a sparse table of indices. For example, the dictionary: d = {'timmy': 'red', 'barry': 'green', 'guido': 'blue'} is currently stored as: entries = [['--', '--', '--'], [-8522787127447073495, 'barry', 'green'], ['--', '--', '--'], ['--', '--', '--'], ['--', '--', '--'], [-9092791511155847987, 'timmy', 'red'], ['--', '--', '--'], [-6480567542315338377, 'guido', 'blue']] Instead, the data should be organized as follows: indices = [None, 1, None, None, None, 0, None, 2] entries = [[-9092791511155847987, 'timmy', 'red'], [-8522787127447073495, 'barry', 'green'], [-6480567542315338377, 'guido', 'blue']] Only the data layout needs to change. The hash table algorithms would stay the same. All of the current optimizations would be kept, including key-sharing dicts and custom lookup functions for string-only dicts. There is no change to the hash functions, the table search order, or collision statistics. The memory savings are significant (from 30% to 95% compression depending on the how full the table is). Small dicts (size 0, 1, or 2) get the most benefit. For a sparse table of size t with n entries, the sizes are: curr_size = 24 * t new_size = 24 * n + sizeof(index) * t In the above timmy/barry/guido example, the current size is 192 bytes (eight 24-byte entries) and the new size is 80 bytes (three 24-byte entries plus eight 1-byte indices). That gives 58% compression. Note, the sizeof(index) can be as small as a single byte for small dicts, two bytes for bigger dicts and up to sizeof(Py_ssize_t) for huge dict. In addition to space savings, the new memory layout makes iteration faster. Currently, keys(), values, and items() loop over the sparse table, skipping-over free slots in the hash table. Now, keys/values/items can loop directly over the dense table, using fewer memory accesses. Another benefit is that resizing is faster and touches fewer pieces of memory. Currently, every hash/key/value entry is moved or copied during a resize. In the new layout, only the indices are updated. For the most part, the hash/key/value entries never move (except for an occasional swap to fill a hole left by a deletion). With the reduced memory footprint, we can also expect better cache utilization. For those wanting to experiment with the design, there is a pure Python proof-of-concept here: http://code.activestate.com/recipes/578375 YMMV: Keep in mind that the above size statics assume a build with 64-bit Py_ssize_t and 64-bit pointers. The space savings percentages are a bit different on other builds. Also, note that in many applications, the size of the data dominates the size of the container (i.e. the weight of a bucket of water is mostly the water, not the bucket). Raymond

27 70

Reviving restricted mode?
by Guido van Rossum 14 Aug '14

14 Aug '14

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

compile python 3.3 with bz2 support
by Isml 22 Oct '13

22 Oct '13

hi, everyone: I want to compile python 3.3 with bz2 support on RedHat 5.5 but fail to do that. Here is how I do it: 1、download bzip2 and compile it(make、make -f Makefile_libbz2_so、make install) 2、chang to python 3.3 source directory : ./configure --with-bz2=/usr/local/include 3、make 4、make install after installation complete, I test it： [root@localhost Python-3.3.0]# python3 -c "import bz2" Traceback (most recent call last): File "<string>", line 1, in <module> File "/usr/local/lib/python3.3/bz2.py", line 21, in <module> from _bz2 import BZ2Compressor, BZ2Decompressor ImportError: No module named '_bz2' By the way, RedHat 5.5 has a built-in python 2.4.3. Would it be a problem?

3 2

Make extension module initialisation more like Python module initialisation
by Stefan Behnel 06 Aug '13

06 Aug '13

Hi, I suspect that this will be put into a proper PEP at some point, but I'd like to bring this up for discussion first. This came out of issues 13429 and 16392. http://bugs.python.org/issue13429 http://bugs.python.org/issue16392 Stefan The problem =========== Python modules and extension modules are not being set up in the same way. For Python modules, the module is created and set up first, then the module code is being executed. For extensions, i.e. shared libraries, the module init function is executed straight away and does both the creation and initialisation. This means that it knows neither the __file__ it is being loaded from nor its package (i.e. its FQMN). This hinders relative imports and resource loading. In Py3, it's also not being added to sys.modules, which means that a (potentially transitive) re-import of the module will really try to reimport it and thus run into an infinite loop when it executes the module init function again. And without the FQMN, it's not trivial to correctly add the module to sys.modules either. We specifically run into this for Cython generated modules, for which it's not uncommon that the module init code has the same level of complexity as that of any 'regular' Python module. Also, the lack of a FQMN and correct file path hinders the compilation of __init__.py modules, i.e. packages, especially when relative imports are being used at module init time. The proposal ============ I propose to split the extension module initialisation into two steps in Python 3.4, in a backwards compatible way. Step 1: The current module init function can be reduced to just creating the module instance and returning it (and potentially doing some simple C level setup). Optionally, after creating the module (and this is the new part), the module init code can register a C callback function that will be called after setting up the module. Step 2: The shared library importer receives the module instance from the module init function, adds __file__, __path__, __package__ and friends to the module dict, and then checks for the callback. If non-NULL, it calls it to continue the module initialisation by user code. The callback ============ The callback is defined as follows:: int (*PyModule_init_callback)(PyObject* the_module, PyModuleInitContext* context) "PyModuleInitContext" is a struct that is meant mostly for making the callback more future proof by allowing additional parameters to be passed in. For now, I can see a use case for the following fields:: struct PyModuleInitContext { char* module_name; char* qualified_module_name; } Both names are encoded in UTF-8. As for the file path, I consider it best to retrieve it from the module's __file__ attribute as a Python string object to reduce filename encoding problems. Note that this struct argument is not strictly required, but given that this proposal would have been much simpler if the module init function had accepted such an argument in the first place, I consider it a good idea not to let this chance pass by again. The registration of the callback uses a new C-API function: int PyModule_SetInitFunction(PyObject* module, PyModule_init_callback callback) The function name uses "Set" instead of "Register" to make it clear that there is only one such function per module. An alternative would be a new module creation function "PyModule_Create3()" that takes the callback as third argument, in addition to what "PyModule_Create2()" accepts. This would require users to explicitly pass in the (second) version argument, which might be considered only a minor issue. Implementation ============== The implementation requires local changes to the extension module importer and a new C-API function. In order to store the callback, it should use a new field in the module object struct. Open questions ============== It is not clear how extensions should be handled that register more than one module in their module init function, e.g. compiled packages. One possibility would be to leave the setup to the user, who would have to know all FQMNs anyway in this case, although not the import file path. Alternatively, the import machinery could use a stack to remember for which modules a callback was registered during the last init function call, set up all of them and then call their callbacks. It's not clear if this meets the intention of the user. Alternatives ============ 1) It would be possible to make extension modules optionally export another symbol, e.g. "PyInit2_modulename", that the shared library loader would call in addition to the required function "PyInit_modulename". This would remove the need for a new API that registers the above callback. The drawback is that it also makes it easier to write broken code because a Python version or implementation that does not support this second symbol would simply not call it, without error. The new C-API function would let the build fail instead if it is not supported. 2) The callback could be made available as a Python function in the module dict, thus also removing the need for an explicit registration API. However, this approach would add overhead to both sides, the importer code and the user provided module init code, as it would require additional dictionary handling and the implementation of a one-time Python function in user code. It would also suffer from the problem that missing support in the runtime would pass silently. 3) The callback could be registered statically in the PyModuleDef struct by adding a new field. This is not trivial to do in a backwards compatible way because the struct would grow longer without explicit initialisation by existing user code. Extending PyModuleDef_HEAD_INIT might be possible but would still break at least binary compatibility. 4) Pass a new context argument into the module init function that contains all information necessary to properly and completely set up the module at creation time. This would provide a much simpler and cleaner solution than the proposed solution. However, it will not be possible before Python 4 as it breaks backwards compatibility with all existing extension modules at both the source and binary level.

5 15

PEP 423 : naming conventions and recipes related to packaging
by Benoît Bryon 05 Jul '13

05 Jul '13

Hi, Here is an informational PEP proposal: http://hg.python.org/peps/file/52767ab7e140/pep-0423.txt Could you review it for style, consistency and content? Additional notes: * Original discussion posted to distutils-sig(a)python.org * started on May 2012 at http://mail.python.org/pipermail/distutils-sig/2012-May/018551.html * continues in June 2012 at http://mail.python.org/pipermail/distutils-sig/2012-June/018641.html * that's why I set "Discussion-To:<distutils-sig(a)python.org>" header. * Original document was edited as a contrib to cpython documentation: * http://bugs.python.org/issue14899 * file history at https://bitbucket.org/benoitbryon/cpython/history/Doc/packaging/packagename… * but it looked like a PEP, so posted to peps(a)python.org... Regards, Benoit (benoitbb on irc.freenode.net)

11 20

Bad python 2.5 build on OSX 10.8 mountain lion
by Guido van Rossum 15 Mar '13

15 Mar '13

As discussed here, the python 2.5 binary distributed by Apple on mountain lion is broken. Could someone file an official complaint? This is really bad... --Guido -- --Guido van Rossum (python.org/~guido)

3 10

Cut/Copy/Paste items in IDLE right click context menu
by Andrew Svetlov 17 Feb '13

17 Feb '13

Hi. There are issue for subject: http://bugs.python.org/issue1207589 It has patches, implemented well and committed. I've pushed it to versions 2.7, 3.2, 3.3 and 3.4. My thoughts are: the issue is not brand new subject but improvement for current IDLE functionality. Added code cannot break any existing program/library I hope, it's related to humans who use IDLE only. It is desirable feature and probably IDLE users will be ok with new items in context menu even they are still 2.7 users. There are another opinion: it is new feature, not a bug, and the patch should be applied to 3.4 only. If you look on discussion for issue (http://bugs.python.org/issue1207589) you can see we cannot make decision, votes are split. I want to raise the question on this mailing list (as Terry Reedy suggested) to ask for community opinion. Thanks, Andrew Svetlov

7 13

Point of building without threads?
by Antoine Pitrou 16 Feb '13

16 Feb '13

Hello, I guess a long time ago, threading support in operating systems wasn't very widespread, but these days all our supported platforms have it. Is it still useful for production purposes to configure --without-threads? Do people use this option for something else than curiosity of mind? Regards Antoine.

10 22

Proposing "Argument Clinic", a new way of specifying arguments to builtins for CPython
by Larry Hastings 16 Feb '13

16 Feb '13

Say there, the Python core development community! Have I got a question for you! *ahem* Which of the following four options do you dislike least? ;-) 1) CPython continues to provide no "function signature" objects (PEP 362) or inspect.getfullargspec() information for any function implemented in C. 2) We add new hand-coded data structures representing the metadata necessary for function signatures for builtins. Which means that, when defining arguments to functions in C, we'd need to repeat ourselves *even more* than we already do. 3) Builtin function arguments are defined using some seriously uncomfortable and impenetrable C preprocessor macros, which produce all the various types of output we need (argument processing code, function signature metadata, possibly the docstrings too). 4) Builtin function arguments are defined in a small DSL; these are expanded to code and data using a custom compile-time preprocessor step. All the core devs I've asked said "given all that, I'd prefer the hairy preprocessor macros". But by the end of the conversation they'd changed their minds to prefer the custom DSL. Maybe I'll make a believer out of you too--read on! I've named this DSL preprocessor "Argument Clinic", or Clinic for short**. Clinic works similarly to Ned Batchelder's brilliant "Cog" tool: http://nedbatchelder.com/code/cog/ You embed the input to Clinic in a comment in your C file, and the output is written out immediately after that comment. The output's overwritten every time the preprocessor is run. In short it looks something like this: /*[clinic] input to the DSL [clinic]*/ ... output from the DSL, overwritten every time ... /*[clinic end:<checksum>]*/ The input to the DSL includes all the metadata about the function that we need for the function signature: * the name of the function, * the return annotation (if any), * each parameter to the function, including * its name, * its type (in C), * its default value, * and a per-parameter docstring; * and the docstring for the function as a whole. The resulting output contains: * the docstring for the function, * declarations for all your parameters, * C code handling all argument processing for you, * and a #define'd methoddef structure for adding the function to the module. I discussed this with Mark "HotPy" Shannon, and he suggested we break our existing C functions into two. We put the argument processing into its own function, generated entirely by Clinic, and have the implementation in a second function called from the first. I like this approach simply because it makes the code cleaner. (Note that this approach should not cause any overhead with a modern compiler, as both functions will be "static".) But it also provides an optimization opportunity for HotPy: it could read the metadata, and when generating the JIT'd code it could skip building the PyObjects and argument tuple (and possibly keyword argument dict), and the subsequent unpacking/decoding, and just call the implementation function directly, giving it a likely-measurable speed boost. And we can go further! If we add a new extension type API allowing you to register both functions, and external modules start using it, sophisticated Python implementations like PyPy might be able to skip building the tuple for extension type function calls--speeding those up too! Another plausible benefit: alternate implementations of Python could read the metadata--or parse the input to Clinic themselves--to ensure their reimplementations of the Python standard library conform to the same API! Clinic can also run general-purpose Python code ("/*[python]"). All output from "print" is redirected into the output section after the Python code. As you've no doubt already guessed, I've made a prototype of Argument Clinic. You can see it--and some sample conversions of builtins using it for argument processing--at this BitBucket repo: https://bitbucket.org/larry/python-clinic I don't claim that it's fabulous, production-ready code. But it's a definite start! To save you a little time, here's a preview of using Clinic for dbm.open(). The stuff at the same indent as a declaration are options; see the "clinic.txt" in the repo above for full documentation. /*[clinic] dbm.open -> mapping basename=dbmopen const char *filename; The filename to open. const char *flags="r"; How to open the file. "r" for reading, "w" for writing, etc. int mode=0666; default=0o666 If creating a new file, the mode bits for the new file (e.g. os.O_RDWR). Returns a database object. [clinic]*/ PyDoc_STRVAR(dbmopen__doc__, "dbm.open(filename[, flags=\'r\'[, mode=0o666]]) -> mapping\n" "\n" " filename\n" " The filename to open.\n" "\n" " flags\n" " How to open the file. \"r\" for reading, \"w\" for writing, etc.\n" "\n" " mode\n" " If creating a new file, the mode bits for the new file\n" " (e.g. os.O_RDWR).\n" "\n" "Returns a database object.\n" "\n"); #define DBMOPEN_METHODDEF \ {"open", (PyCFunction)dbmopen, METH_VARARGS | METH_KEYWORDS, dbmopen__doc__} static PyObject * dbmopen_impl(PyObject *self, const char *filename, const char *flags, int mode); static PyObject * dbmopen(PyObject *self, PyObject *args, PyObject *kwargs) { const char *filename; const char *flags = "r"; int mode = 0666; static char *_keywords[] = {"filename", "flags", "mode", NULL}; if (!PyArg_ParseTupleAndKeywords(args, kwargs, "s|si", _keywords, &filename, &flags, &mode)) return NULL; return dbmopen_impl(self, filename, flags, mode); } static PyObject * dbmopen_impl(PyObject *self, const char *filename, const char *flags, int mode) /*[clinic end:eddc886e542945d959b44b483258bf038acf8872]*/ As of this writing, I also have sample conversions in the following files available for your perusal: Modules/_cursesmodule.c Modules/_dbmmodule.c Modules/posixmodule.c Modules/zlibmodule.c Just search in C files for '[clinic]' and you'll find everything soon enough. As you can see, Clinic has already survived some contact with the enemy. I've already converted some tricky functions--for example, os.stat() and curses.window.addch(). The latter required adding a new positional-only processing mode for functions using a legacy argument processing approach. (See "clinic.txt" for more.) If you can suggest additional tricky functions to support, please do! Big unresolved questions: * How would we convert all the builtins to use Clinic? I fear any solution will involve some work by hand. Even if we can automate big chunks of it, fully automating it would require parsing arbitrary C. This seems like overkill for a one-shot conversion. (Mark Shannon says he has some ideas.) * How do we create the Signature objects? My current favorite idea: Clinic also generates a new, TBD C structure defining all the information necessary for the signature, which is also passed in to the new registration API (you remember, the one that takes both the argument-processing function and the implementation function). This is secreted away in some new part of the C function object. At runtime this is converted on-demand into a Signature object. Default values for arguments are represented in C as strings; the conversion process attempts eval() on the string, and if that works it uses the result, otherwise it simply passes through the string. * Right now Clinic paves over the PyArg_ParseTuple API for you. If we convert CPython to use Clinic everywhere, theoretically we could replace the parsing API with something cleaner and/or faster. Does anyone have good ideas (and time, and energy) here? * There's actually a fifth option, proposed by Brett Cannon. We constrain the format of docstrings for builtin functions to make them machine-readable, then generate the function signature objects from that. But consider: generating *everything* in the signature object may get a bit tricky (e.g. Parameter.POSITIONAL_ONLY), and this might gunk up the docstring. But the biggest unresolved question... is this all actually a terrible idea? //arry/ ** "Is this the right room for an argument?" "I've told you once...!"

17 33

Python-Dev December 2012