Mailman 3 April 2004 - 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:

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RE: [Python-Dev] Those import related syntax errors again...
by Samuele Pedroni Feb. 23, 2005

Feb. 23, 2005

Hi. [Mark Hammond] > The point isn't about my suffering as such. The point is more that > python-dev owns a tiny amount of the code out there, and I don't believe we > should put Python's users through this. > > Sure - I would be happy to "upgrade" all the win32all code, no problem. I > am also happy to live in the bleeding edge and take some pain that will > cause. > > The issue is simply the user base, and giving Python a reputation of not > being able to painlessly upgrade even dot revisions. I agree with all this. [As I imagined explicit syntax did not catch up and would require lot of discussions.] [GvR] > > Another way is to use special rules > > (similar to those for class defs), e.g. having > > > > <frag> > > y=3 > > def f(): > > exec "y=2" > > def g(): > > return y > > return g() > > > > print f() > > </frag> > > > > # print 3. > > > > Is that confusing for users? maybe they will more naturally expect 2 > > as outcome (given nested scopes). > > This seems the best compromise to me. It will lead to the least > broken code, because this is the behavior that we had before nested > scopes! It is also quite easy to implement given the current > implementation, I believe. > > Maybe we could introduce a warning rather than an error for this > situation though, because even if this behavior is clearly documented, > it will still be confusing to some, so it is better if we outlaw it in > some future version. > Yes this can be easy to implement but more confusing situations can arise: <frag> y=3 def f(): y=9 exec "y=2" def g(): return y return y,g() print f() </frag> What should this print? the situation leads not to a canonical solution as class def scopes. or <frag> def f(): from foo import * def g(): return y return g() print f() </frag> [Mark Hammond] > > This probably won't be a very popular suggestion, but how about pulling > > nested scopes (I assume they are at the root of the problem) > > until this can be solved cleanly? > > Agreed. While I think nested scopes are kinda cool, I have lived without > them, and really without missing them, for years. At the moment the cure > appears worse then the symptoms in at least a few cases. If nothing else, > it compromises the elegant simplicity of Python that drew me here in the > first place! > > Assuming that people really _do_ want this feature, IMO the bar should be > raised so there are _zero_ backward compatibility issues. I don't say anything about pulling nested scopes (I don't think my opinion can change things in this respect) but I should insist that without explicit syntax IMO raising the bar has a too high impl cost (both performance and complexity) or creates confusion. [Andrew Kuchling] > >Assuming that people really _do_ want this feature, IMO the bar should be > >raised so there are _zero_ backward compatibility issues. > > Even at the cost of additional implementation complexity? At the cost > of having to learn "scopes are nested, unless you do these two things > in which case they're not"? > > Let's not waffle. If nested scopes are worth doing, they're worth > breaking code. Either leave exec and from..import illegal, or back > out nested scopes, or think of some better solution, but let's not > introduce complicated backward compatibility hacks. IMO breaking code would be ok if we issue warnings today and implement nested scopes issuing errors tomorrow. But this is simply a statement about principles and raised impression. IMO import * in an inner scope should end up being an error, not sure about 'exec's. We will need a final BDFL statement. regards, Samuele Pedroni.

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Can we please have a better dict interpolation syntax?
by Greg Ewing July 13, 2004

July 13, 2004

I have just had the experience of writing a bunch of expressions of the form "create index %(table)s_lid1_idx on %(table)s(%(lid1)s)" % params and found myself getting quite confused by all the parentheses and "s" suffixes. I would *really* like to be able to write this as "create index %{table}_lid1_idx on %{table}(%{lid1})" % params which I find to be much easier on the eyes. Greg Ewing, Computer Science Dept, +--------------------------------------+ University of Canterbury, | A citizen of NewZealandCorp, a | Christchurch, New Zealand | wholly-owned subsidiary of USA Inc. | greg(a)cosc.canterbury.ac.nz +--------------------------------------+

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PEP 292 - simpler string substitutions
by Barry Warsaw July 13, 2004

July 13, 2004

PEP 292 describes simpler string substitutions, i.e. $-strings. PEP 320 says that this is going into Python 2.4. I've had some code laying around that implemented $-strings as a subclass of unicode instead of what PEP 292 describes as a new string method. The advantage to this approach is that the feature is provided with no language changes, or changes to the existing string classes. I've cleaned up my code and posted a patch to SF. I've also checked in an updated PEP 292 that fits my implementation more closely. Please review the PEP and the patch: http://www.python.org/peps/pep-0292.html The reference implementation in the patch adds a couple of extra things that aren't part of the PEP, but were discussed at Monday's sprint. Specifically: - a top level package called 'stringlib' is added to the standard library. - the dstring class implemented PEP 292 is available through stringlib.dstring - some handy dict subclasses that work with the dstring class are also provided. The idea was that some of the constants and functions we want to keep from the string module would go in the stringlib package. That's not essential to the PEP of course. If this stuff is approved for Python 2.4, I'll add test cases and documentation. -Barry

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Re: [Python-Dev] Python startup time: String objects
by Martin v. L�wis June 27, 2004

June 27, 2004

> Even with cPickle? AFAIR: Yes. It was not orders of magnitude slower anymore, but significantly slower. > I'm still worried about your patch, because it changes all uses of > marchal, not just how code objects are marshalled. The marshal module > *is* used by other apps... The change is backwards-compatible in the sense that existing files can be unmarshalled just fine. Problems will only arise if new marshal output is unmarshalled by old versions, which could be solved with an option during marshalling. Regards, Martin

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slots, properties, descriptors, and pydoc
by John Belmonte May 31, 2004

May 31, 2004

Hello, I'd like to make some observations and proposals about pydoc regarding slots, properties, and descriptors in general. Given this program: class CountingDescriptor(object): def __init__(self, doc=None): self.val = 0 self.__doc__ = doc def __get__(self, obj, objtype): self.val += 1 return self.val class Foo(object): __slots__ = {'bar': 'slot doc test'} counter = CountingDescriptor(doc='descriptor doc test') def _set_counter_multiple(self, val): print 'I hear you want to set me to %s?' % val counter_multiple = property( lambda self: self.counter*2, _set_counter_multiple, doc='property doc test' ) print help(Foo) The output is: Help on class Foo in module __main__: class Foo(__builtin__.object) | Properties defined here: | | counter_multiple | property doc test | | lambdaself | | <set> = _set_counter_multiple(self, val) | | ------------------------------------------------------------ | Data and other attributes defined here: | | __slots__ = {'bar': 'slot doc test'} | | bar = <member 'bar' of 'Foo' objects> | | counter = 2 Observations (in order from least to most arguable :-): 1) pydoc's property display doesn't handle lambda well 2) pydoc doesn't show the doc string of general descriptors 3) Using dictionary values as a way to set the doc string of slots, as suggested in Guido's "Unifying types and classes", has not been implemented. Currently there is no way to set a doc string on a descriptor generated by __slots__. Even if you could, pydoc would not display it, according to #2. 4) pydoc is displaying implementation details of properties (set and get method names, etc.). Such details should probably not be displayed, similar to underbar-prefixed method and data attributes. Proposals: 1) remove property-specific handling from pydoc 2) extend pydoc with descriptor-specific handling. In a section called "Descriptors defined here", list each attribute name, with associated doc string if it exists. 3) extend the __slots__ handler to set the descriptor's doc string to dictionary values if the slot's value is a dictionary. This is the only Python change being proposed. 4) have pydoc make a special case for the display of __slots__. Alternatives: A) don't display __slots__ if the value is a dictionary. The display of __slots__ is redundant, because all items are displayed according to #2. B) if for some reason distinguishing slots from other descriptors is essential, then always display __slots__, but normalize it to a list of attribute names (show only dictionary keys, etc.) With these changes, the new output of the example would be: Help on class Foo in module __main__: class Foo(__builtin__.object) | Descriptors defined here: | | bar | slot doc test | | counter | descriptor doc test | | counter_multiple | property doc test I'd be willing to work on a PEP. If these issues are covered in or conflict with an existing PEP, please let me know. Thanks, -John -- http:// if ile.org/

3 13

PEP 289 - Generator Expressions - Let's Move Forward
by Guido van Rossum May 6, 2004

May 6, 2004

I'd like to get generator expressions checked into CVS. Regarding the early-or-late binding issue, here's what I'd like to see happen: I'd like the late-binding (i.e. non-capture) version checked in and released with Python 2.4a1 and a2. If we find there are real problems with these semantics, we can switch to early-binding semantics in 2.4b1; this would be almost completely backwards compatible. But if it works in the alpha release, we'll stick with late binding. My reasons for (still!) preferring late binding are: (a) I find it hard to defend automatic variable capture given Python's general late-binding semantics (b) I believe that use cases needing early binding are uncommon and strained: they all involve creating a list of generator expressions, which IMO is a pretty unusual thing to do I cannot volunteer to do the final code review and checkin, and I believe Raymond is still on vacation (he's asked me to relieve him from the responsibility of shepherding this patch) so we need someone to volunteer. The patch is here: http://sf.net/tracker/?func=detail&aid=872326&group_id=5470&atid=305470 --Guido van Rossum (home page: http://www.python.org/~guido/)

11 29

Small additions to optparser
by Gustavo Niemeyer May 4, 2004

May 4, 2004

Hi Folks, I have two small customizations I've used in most projects I've plugged optparse into. Since they're simple and generic, I'd like to integrate them back in optparser. These features are: - Add a help formatter which uses capitalized headers. - Add a "help" keyword in OptionParser, allowing straightforward addition of a custom help messages. Comments? Also, how are we currently handling changes in optparser? Do we have an "official" maintainer (Greg?). IOW, should I: - Send a proposed patch to SourceForge - Contact the original project - Commit the patch FWIW, here is the proposed functionality: class CapitalizedHelpFormatter(optparse.IndentedHelpFormatter): def format_usage(self, usage): return optparse.IndentedHelpFormatter \ .format_usage(self, usage).capitalize() def format_heading(self, heading): return optparse.IndentedHelpFormatter \ .format_heading(self, heading).capitalize() class OptionParser(optparse.OptionParser): def __init__(self, *args, **kwargs): optparse.OptionParser.__init__(self, *args, **kwargs) self._override_help = kwargs.get("help") def format_help(self, formatter=None): if self._override_help: return self._override_help else: return optparse.OptionParser.format_help(self, formatter) -- Gustavo Niemeyer http://niemeyer.net

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RE: [Python-Dev] PEP 289 - Generator Expressions - Let's Move Forward
by Michael Chermside May 3, 2004

May 3, 2004

> I'd like to get generator expressions checked into CVS. > > Regarding the early-or-late binding issue, here's what I'd like to see > happen: I'd like the late-binding (i.e. non-capture) version checked > in and released with Python 2.4a1 and a2. If we find there are real > problems with these semantics, we can switch to early-binding > semantics in 2.4b1 Sounds like a good plan, but I have one question: Given the extent to which alpha releases are tested (and subtracting out those who use them only to check old code for compatibility and don't try out the new features) are we going to be able to tell if there are problems with the semantics? My best idea is that those who currently favor the early-binding approach (and I'm sure you've heard from plenty of them) should be encouraged to try it out in the alpha and report any issues, along with actual use cases. Someone collects these responses, then you (Guido) look over the results (with a grain of salt since it's mostly being tested by folks who object) and decide if it's a problem. Will that work? Anyone have a better idea? How do we go about asking people to try it out (is a mention by Guido on comp.lang.python sufficient)? -- Michael Chermside

8 8

Accessing base class C struct of subclass instance
by Nick Bastin May 3, 2004

May 3, 2004

Ok, strange subject, but I have what seems like a simple question that the documentation doesn't seem to cover (and a quick perusal of the code doesn't answer, either). If I define a new style class in C, and subclass it in python, how can I get the base class instance object from a subclass instance in C? -- Nick

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