diff --git a/pep-0363.txt b/pep-0363.txt index 3a98e2fad..a19d7e7c4 100644 --- a/pep-0363.txt +++ b/pep-0363.txt @@ -2,12 +2,12 @@ PEP: 363 Title: Syntax For Dynamic Attribute Access Version: $Revision$ Last-Modified: $Date$ -Author: Ben North -Status: Draft +Author: Ben North +Status: Rejected Type: Standards Track Content-Type: text/plain Created: 29-Jan-2007 -Post-History: +Post-History: 12-Feb-2007 Abstract @@ -28,13 +28,6 @@ Abstract z = getattr(getattr(y, 'foo_%d' % n), 'bar_%s' % s) -Note - - I wrote this patch mostly to advance my own understanding of and - experiment with the python language, but I've written it up in the - style of a PEP in case it might be a useful idea. - - Rationale Dictionary access and indexing both have a friendly invocation @@ -64,18 +57,17 @@ Rationale x = getattr(y, 'foo_%d' % n, 0) This PEP describes a new syntax for dynamic attribute access --- - "x.(expr)" --- with examples given in the Abstract above. The new - syntax also allows the provision of a default value in the "get" - case, as in: + "x.(expr)" --- with examples given in the Abstract above. + + (The new syntax could also allow the provision of a default value in + the "get" case, as in: x = y.('foo_%d' % n, None) - This 2-argument form of dynamic attribute access is not permitted as - the target of an (augmented or normal) assignment. Also, this part - of the new syntax was not as well received [6] in initial - discussions on python-ideas, and I agree that it does not fit so - cleanly. I'm happy to prepare a revised PEP/patch without the - 2-argument form if the consensus is that this would be preferred. + This 2-argument form of dynamic attribute access would not be + permitted as the target of an (augmented or normal) assignment. The + "Discussion" section below includes opinions specifically on the + 2-argument extension.) Finally, the new syntax can be used with the "del" statement, as in @@ -144,80 +136,18 @@ Performance Impact be a performance penalty of around 1% in the pystones score with the patched version. One suggestion is that this is because the longer main loop in ceval.c hurts the cache behaviour, but this has not - been confirmed. (Maybe a tool like valgrind [2] could help here?) + been confirmed. On the other hand, measurements suggest a speed-up of around 40--45% for dynamic attribute access. -Discussion To Date - - Initial posting of this PEP in draft form was to python-ideas on - 20070209 [4], and the response was generally positive: - - I've thought of the same syntax. I think you should submit this - to the PEP editor and argue on Python-dev for its inclusion in - Python 2.6 -- there's no benefit that I see of waiting until - 3.0. --- Guido van Rossum [5] - - Wow! I have to say this is a compelling idea. The syntax is a - bit foreign looking, but [...] I feel like I could learn to like - it anyway. --- Greg Falcon [6] - - I look forward to seeing this in Python 2.6. --- Josiah - Carlson, further down the thread [8] - - with Greg Falcon expressing the reservations about the 2-argument - form already noted above, and Josiah Carlson raising a query about - performance: - - My only concern with your propsed change is your draft - implementation. [...] Specifically, your changes [...] may - negatively affect general Python performance. --- Josiah - Carlson [7] - - Some initial measurements (see above) suggest the performance - penalty is small, and Josiah Carlson said of such cost that it - "isn't really substantial". [8] - - -Questions To Be Resolved - - * Whether to allow the 2-argument form for default arguments. - - * Whether the performance impact is real; whether it is acceptable; - whether alternative implementations might improve this aspect. - - -Alternative Syntax For The New Feature - - Other syntaxes could be used, for example braces are currently - invalid in a "trailer", so could be used here, giving - - x{'foo_%d' % n} += 1 - - My personal preference is for the - - x.('foo_%d' % n) += 1 - - syntax though: the presence of the dot shows there is attribute - access going on; the parentheses have an analogous meaning to the - mathematical "work this out first" meaning. This is also the - syntax used in the language Matlab [1] for dynamic "field" access - (where "field" is the Matlab term analogous to Python's - "attribute"). - - Discussions on python-ideas (see above) made no comment on the brace - alternative, and the .() notation was well-enough received, so the - brace alternative should be considered rejected, I think. - - Error Cases Only strings are permitted as attribute names, so for instance the following error is produced: - >>> x.(99) = 8 + >>> x.(99) = 8 Traceback (most recent call last): File "", line 1, in TypeError: attribute name must be string, not 'int' @@ -233,29 +163,78 @@ Draft Implementation compile.c, and three new opcodes (load/store/del) with accompanying changes to opcode.h and ceval.c. The patch consists of c.180 additional lines in the core code, and c.100 additional - lines of tests. It is available as sourceforge patch #1657573 [3]. + lines of tests. It is available as sourceforge patch #1657573 [1]. + + +Mailing Lists Discussion + + Initial posting of this PEP in draft form was to python-ideas on + 20070209 [2], and the response was generally positive. The PEP was + then posted to python-dev on 20070212 [3], and an interesting + discussion ensued. A brief summary: + + Initially, there was reasonable (but not unanimous) support for the + idea, although the precise choice of syntax had a more mixed + reception. Several people thought the "." would be too easily + overlooked, with the result that the syntax could be confused with a + method/function call. A few alternative syntaxes were suggested: + + obj.(foo) + obj.[foo] + obj.{foo} + obj{foo} + obj.*foo + obj->foo + obj<-foo + obj@[foo] + obj.[[foo]] + + with "obj.[foo]" emerging as the preferred one. In this initial + discussion, the two-argument form was universally disliked, so it + was to be taken out of the PEP. + + Discussion then took a step back to whether this particular feature + provided enough benefit to justify new syntax. As well as requiring + coders to become familiar with the new syntax, there would also be + the problem of backward compatibility --- code using the new syntax + would not run on older pythons. + + Instead of new syntax, a new "wrapper class" was proposed, with the + following specification / conceptual implementation suggested by + Martin von Loewis: + + class attrs: + def __init__(self, obj): + self.obj = obj + def __getitem__(self, name): + return getattr(self.obj, name) + def __setitem__(self, name, value): + return setattr(self.obj, name, value) + def __delitem__(self, name): + return delattr(self, name) + def __contains__(self, name): + return hasattr(self, name) + + This was considered a cleaner and more elegant solution to the + original problem. (Another suggestion was a mixin class providing + dictionary-style access to an object's attributes.) + + The decision was made that the present PEP did not meet the burden + of proof for the introduction of new syntax, a view which had been + put forward by some from the beginning of the discussion. The + wrapper class idea was left open as a possibility for a future PEP. References - [1] Using Dynamic Field Names :: Data Types (MATLAB Programming) - http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_prog/f2-41859.html - - [2] Valgrind: "suite of tools for debugging and profiling Linux programs" - http://www.valgrind.org/ - - [3] Sourceforge patch #1657573 + [1] Sourceforge patch #1657573 http://sourceforge.net/tracker/index.php?func=detail&aid=1657573&group_id=5470&atid=305470 - [4] http://mail.python.org/pipermail/python-ideas/2007-February/000210.html + [2] http://mail.python.org/pipermail/python-ideas/2007-February/000210.html + and following posts - [5] http://mail.python.org/pipermail/python-ideas/2007-February/000211.html - - [6] http://mail.python.org/pipermail/python-ideas/2007-February/000212.html - - [7] http://mail.python.org/pipermail/python-ideas/2007-February/000213.html - - [8] http://mail.python.org/pipermail/python-ideas/2007-February/000227.html + [3] http://mail.python.org/pipermail/python-dev/2007-February/070939.html + and following posts Copyright