Python-Dev August 2004

python-dev@python.org

170 participants
282 discussions

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:

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RE: [Python-Dev] Those import related syntax errors again...
by Samuele Pedroni 23 Feb '05

23 Feb '05

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.

18 78

mimetypes and _winreg
by Mike Brown 02 Feb '05

02 Feb '05

I thought it would be nice to try to improve the mimetypes module by having it, on Windows, query the Registry to get the mapping of filename extensions to media types, since the mimetypes code currently just blindly checks posix-specific paths for httpd-style mapping files. However, it seems that the way to get mappings from the Windows registry is excessively slow in Python. I'm told that the reason has to do with the limited subset of APIs that are exposed in the _winreg module. I think it is that EnumKey(key, index) is querying for the entire list of subkeys for the given key every time you call it. Or something. Whatever the situation is, the code I tried below is way slower than I think it ought to be. Does anyone have any suggestions (besides "write it in C")? Could _winreg possibly be improved to provide an iterator or better interface to get the subkeys? (or certain ones? There are a lot of keys under HKEY_CLASSES_ROOT, and I only need the ones that start with a period). Should I file this as a feature request? Thanks -Mike from _winreg import HKEY_CLASSES_ROOT, OpenKey, EnumKey, QueryValueEx i = 0 typemap = {} try: while 1: subkeyname = EnumKey(HKEY_CLASSES_ROOT, i) try: subkey = OpenKey(HKEY_CLASSES_ROOT, subkeyname) if subkeyname[:1] == '.': data = QueryValueEx(subkey, 'Content Type')[0] print subkeyname, '=', data typemap[subkeyname] = data # data will be unicode except EnvironmentError, WindowsError: pass i += 1 except WindowsError: pass

3 4

Another test_compiler mystery
by Tim Peters 30 Sep '04

30 Sep '04

I've noticed several times now, in both debug and release builds, that if I run regrtest.py with -uall, *sometimes* it just stops after running test_compiler: $ python_d regrtest.py -uall test_grammar test_opcodes ... test_compare test_compile test_compiler $ There's no indication of error, it just ends. It's not consistent. Happened once when I was running with -v, and test_compiler's output ended here: ... compiling C:\Code\python\lib\test\test_operator.py compiling C:\Code\python\lib\test\test_optparse.py compiling C:\Code\python\lib\test\test_os.py compiling C:\Code\python\lib\test\test_ossaudiodev.py compiling C:\Code\python\lib\test\test_parser.py In particular, there's no Ran M tests in Ns output, so it doesn't look like unittest (let alone regrtest) ever got control back. Hmm. os.listdir() is in sorted order on NTFS, so test_compiler should be chewing over a lot more files after test_parser.py. *This* I could blame on a blown C stack -- although I'd expect a much nastier symptom then than just premature termination. Anyone else?

23 50

Alternative Implementation for PEP 292: Simple String Substitutions
by Raymond Hettinger 14 Sep '04

14 Sep '04

Before it's too late and the API gets frozen, I would like to propose an alternate implementation for PEP292 that exposes two functions instead of two classes. Current way: print Template('Turn $direction') % dict(direction='right') Proposed: print dollarsub('Turn $direction', dict(direction='right')) or: print dollarsub('Turn $direction', direction='right') My main issue with the current implementation is that we get no leverage from using a class instead of a function. Though the API is fairly simple either way, it is easier to learn and document functions than classes. We gain nothing from instantiation -- the underlying data remains immutable and no additional state is attached. The only new behavior is the ability to apply the mod operator. Why not do it in one step. I had thought a possible advantage of classes was that they could be usefully subclassed. However, a little dickering around showed that to do anything remotely interesting (beyond changing the pattern alphabet) you have to rewrite everything by overriding both the method and the pattern. Subclassing gained you nothing, but added a little bit of complexity. A couple of simple exercises show this clearly: write a subclass using a different escape character or one using dotted identifiers for attribute lookup in the local namespace -- either way subclasses provide no help and only get in the way. One negative effect of the class implementation is that it inherits from unicode and always returns a unicode result even if all of the inputs (mapping values and template) are regular strings. With a function implementation, that can be avoided (returning unicode only if one of the inputs is unicode). The function approach also makes it possible to have keyword arguments (see the example above) as well as a mapping. This isn't a big win, but it is nice to have and reads well in code that is looping over multiple substitutions (mailmerge style): for girl in littleblackbook: print dollarsub(loveletter, name=girl[0].title(), favoritesong=girl[3]) Another minor advantage for a function is that it is easier to lookup in the reference. If a reader sees the % operator being applied and looks it up in the reference, it is going to steer them in the wrong direction. This is doubly true if the Template instantiation is remote from the operator application. Summary for functions: * is more appropriate when there is no state * no unnecessary instantiation * can be applied in a single step * a little easier to learn/use/document * doesn't force result to unicode * allows keyword arguments * easy to find in the docs Raymond ----------- Sample Implementation ------------- def dollarsub(template, mapping=None, **kwds): """A function for supporting $-substitutions.""" typ = type(template) if mapping is None: mapping = kwds def convert(mo): escaped, named, braced, bogus = mo.groups() if escaped is not None: return '$' if bogus is not None: raise ValueError('Invalid placeholder at index %d' % mo.start('bogus')) val = mapping[named or braced] return typ(val) return _pattern.sub(convert, template) def safedollarsub(template, mapping=None, **kwds): """A function for $-substitutions. This function is 'safe' in the sense that you will never get KeyErrors if there are placeholders missing from the interpolation dictionary. In that case, you will get the original placeholder in the value string. """ typ = type(template) if mapping is None: mapping = kwds def convert(mo): escaped, named, braced, bogus = mo.groups() if escaped is not None: return '$' if bogus is not None: raise ValueError('Invalid placeholder at index %d' % mo.start('bogus')) if named is not None: try: return typ(mapping[named]) except KeyError: return '$' + named try: return typ(mapping[braced]) except KeyError: return '${' + braced + '}' return _pattern.sub(convert, template)

16 70

Python 3.0 list of goals
by A.M. Kuchling 07 Sep '04

07 Sep '04

The thread about bytes() is about a Python 3.0 feature. Guido's presentations have mentioned various changes he'd like to make in 3.0, but there's no master list of things that would change. I think it would be useful to have such a list, because it would focus our language development effort on ones that are steps to 3.0, and maybe people can invent ways to smooth the transition. I've started a list in the Wiki at http://www.python.org/moin/Python3.0 , but should it be a PEP? (PEP 3000, perhaps?) --amk

9 11

Alternative placeholder delimiters for PEP 292
by Andrew Durdin 04 Sep '04

04 Sep '04

A Yet Simpler Proposal, modifying that of PEP 292 I propose that the Template module not use $ to set off placeholders; instead, placeholders are delimited by braces {}. The following rules for {}-placeholders apply: 1. {{ and }} are escapes; they are replaced with a single { or } respectively. 2. {identifier} names a substitution placeholder matching a mapping key of "identifier". By default, "identifier" must spell a Python identifier as defined in Identifiers and Keywords[1]. No other characters have special meaning. If the left-brace { is unmatched, appears at the end of the string, or is followed by any non-identifier character, a ValueError will be raised at interpolation time[2]. If a single, unmatched right-brace } occurs in the string, a ValueError will be raised at interpolation time. This avoids ambiguity: did the user want a single right-brace, or did they inadvertently omit the left-brace? This will also cause a probably erroneous "{foo}}" or "{{foo}" to raise a ValueError. Rationale There are several reasons for preferring the paired delimiters {} to a single prefixed $: 1. The placeholder name stands out more clearly from its surroundings, due to the presence of a closing delimiter, and also to the fact that the braces bear less resemblance to any alphabetic characters than the dollar sign: "Hello, {name}, how are you?" vs "Hello, $name, how are you?" 2. Only two characters have special meanings in the string, as opposed to three. Additionally, dollar signs are expected to be more often used in templated strings (e.g. for currency values) than braces: "The {item} costs ${price}." vs "The $item costs $$$price." 3. The placeholder syntax is consistent, and does not change even when valid identifier characters follow the placeholder but are not part of the placeholder: "Look at the {plural}s!" vs "Look at the ${plural}s!" 4. The template substitution could be changed in future to support dotted names without breaking existing code. The example below would break if the $-template were changed to allow dotted names: "Evaluate {obj}.__doc__" vs "Evaluate $obj.__doc__" There are two drawbacks to the proposal when compared with $-templates: 1. An extra character must be typed for each placeholder in the common case: "{name}, welcome to {site}!" vs "$name, welcome to $site!" 2. Templated strings containing braces become more complicated: "dict = {{'{key}': '{value}'}}" vs "dict = {'$key': '$value'}" The first is not a real issue; the extra closing braces needed for the placeholder when compared with the number of other characters in the templated string will usually be insignificant. Furthermore, the {}-placeholders require fewer characters to be typed in the less common case when valid identifier characters follow the placeholder but are not part of it. The need for braces in a templated string is not expected to occur frequently. Because of this, the second drawback is considered of minor importance. Reference Implementation If the general nature of feedback on this proposal is positive, or expressive of interest in an implementation, then a reference implementation will be created forthwith. References and Notes [1] Identifiers and Keywords http://www.python.org/doc/current/ref/identifiers.html [2] The requirement for interpolation-time error raising is the same as in PEP 292. Although not a part of this proposal, I suggest that it would be better if the error occured when the Template instance is created. It is worth noting that the PEP 292 reference implementation (in python/nondist/sandbox/string/string/template.py) does not fully conform to the PEP as regards raising errors for invalid template strings: >>> t = Template("This should cause an error: $*") >>> t % {} u'This should cause an error: $*' >>> t = Template("This also should cause an error: $") >>> t % {} u'This also should cause an error: $'

3 3

Alternative Implementation for PEP 292: Simple String Substitutions
by Raymond Hettinger 03 Sep '04

03 Sep '04

> > * doesn't force result to unicode [Aahz] > This is the main reason I'm +0, pending further arguments. Employing class logic for a function problem is my main reason. Any function can be wrapped in class logic and use an overloaded operator (string.template for example) -- it's invariably the wrong thing to do for reasons of complexity, separation of instantiation and application, and the issues that always arise when an operator is overloaded (a good technique that should only be applied sparingly). [Aahz] > OTOH, I also > like using %, so you'd have to come up with more points to move me > beyond +0. The reasons center around the remoteness of instantiation from application, the differences from current uses of %, and an issue with the % operator itself. When application is remote from instantiation (for instance, when a template argument is supplied to a function), we get several problems. Given a line like "r = t % m", it is hard to verify that the code is correct. Should there be KeyError trapping? Can m be a tuple? Is t a %template, a safetemplate, or dollartemplate? Is the result a str or Unicode object? Should the decision of safe vs regular substitution be made at the point of instantiation or application? The temptation will be to reuse existing code that uses the % operator which unfortunately is not the same (especially with respect to applying tuples, return types, and auto-stringizing). The % operator is hard to search for in the docs and has precedence issues arising from its primary use for modulo arithemetic. Also, using a function makes it possible to use both % formatting and $ formatting (I do have use a case for this). Further, the % operator mnemonically only makes sense with % identifier tags -- it makes less sense with $ tags. Whatever answer is chosen, it should be forward looking and consider that others will want to add functionality (local namespace lookups for example) and that some poor bastards are going to waste time figuring out how to subclass the current implementation. In time, there will likely be more than two of these -- do you want more classes or more functions? Raymond

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RE: [Python-Dev] PEP 309 updated slightly
by Moore, Paul 02 Sep '04

02 Sep '04

From: Aahz >On Mon, Aug 30, 2004, Peter Harris wrote: >> >> I have updated PEP309 a little, [...] > >> I hope there will still be time to squeeze it in before 2.4 beta 1, and >> if there is any reason why not (apart from there has to be a cut-off >> point somewhere, which I accept), please someone let me know what work >> is still needed to make it ready to go in. > What's PEP309? PEP 309 is the "Partial Function Application" class. (Used to be referred to as "Currying", but that was agreed not to be an accurate name). Paul __________________________________________________________________________ This e-mail and the documents attached are confidential and intended solely for the addressee; it may also be privileged. If you receive this e-mail in error, please notify the sender immediately and destroy it. As its integrity cannot be secured on the Internet, the Atos Origin group liability cannot be triggered for the message content. Although the sender endeavours to maintain a computer virus-free network, the sender does not warrant that this transmission is virus-free and will not be liable for any damages resulting from any virus transmitted. __________________________________________________________________________

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[Python-Dev] FW: [Python-checkins] python/nondist/peps pep-0318.txt, 1.25, 1.26
by Raymond Hettinger 02 Sep '04

02 Sep '04

> List some possible reasons why arriving at consensus about > decorators has been so hard (or impossible) to achieve Thanks. I think that was an important contribution to the discussion. At this point, looking at the meta-discussion is likely to best way to help cut through the current morass. +There is no one clear reason why this should be so, but a +few problems seem to be most problematic. A possible social issue is that decorators can be used in a tremendous variety of ways, each of which is only needed or appreciated by small, disjoint groups of users. For instance, applications to ctypes have unique needs that not understood or shared by others. Some users want to use decorators for metaclass magic that scares the socks off of the rest. > + Almost everyone agrees that decorating/transforming a function at > + the end of its definition is suboptimal. One other point of agreement is that no one like having to write the function name three times: def f(): ... f = deco(f) > +* Syntactic constraints. There is an existing (though clumsy) syntax. Most of the proposals are less flexible but more elegant. This trade-off has created much more consternation than if there were no existing ways to apply decorations. +* Overall unfamiliarity with the concept. For people who have a + passing acquaintance with algebra (or even basic arithmetic) or have + used at least one other programming language, much of Python is + intuitive. Very few people will have had any experience with the + decorator concept before encountering it in Python. There's just no + strong preexisting meme that captures the concept. My take on this one is that there are some existing memes from C# and Java and that in the future they will be more widely known. However, there are not good existing, thought-out concepts of what exactly decorators should do in terms of preserving docstrings, attributes, using parameters, etc. Most have only a single application in mind and the full implications (and possible applications) of a given syntax are shrouded in the mists of the future. Like metaclasses, everyone knew they were powerful when they were put in, but no one knew how they would be used or whether they were necessary. In fact, two versions later, we still don't know those answers. Raymond

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Python-Dev August 2004