Cplusplus-sig

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cplusplus-sig@python.org

May 1999

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Restarting
by Geoffrey Furnish July 11, 1999

July 11, 1999

Hello all, I'd like to revive this sig. I know I've been quiet of late, but hey, so has everyone else. In the interest of restarting the discussion, I guess I'll try to restate what I think we need to accomplish here. But I represent it only as my opinion. I hope others will comment, or suggest wholly different agendas, whatever. Anyway, this post is just intended to kick start renewed discussion of what the goals should be and the means for achieving them. First off, "THE GOAL", in my mind, is to come up with a set of core enabling code that would ultimately be directly absorbed by Guido into the Python distribution. When it's all said and done, I want Python extension programmers to be able to pull down a stock distribution, and using that, write extension modules in C++ with the full conveniences accorded by the C++ language. A lot of the action for Python, in the circles I travel, is in the business of hooking Python up to other code. I frequently find myself writing this "glue code" that we call Python compiled extensions, which talks to Python's C api in a pretty low level C-ish way, and talks to other compiled assets, using fairly upscale C++. I'd like the code which lives in this space, to be able to talk to the Python API in a similarly upscale fashion. I've discussed this with Guido a few times over the last two years. He's a little concerned about the idea of absorbing too much code that he doesn't understand, into the core, because of the support issues. And he wants to see a serious consensus emerge on the SIG. [Guido, please feel free to further clarify here]. Both of these seem like very reasonable issues to me, but I'm optimistic that both can be satisfactorily redressed. Regarding the first point, I figure that much of the C++ support machinery that I envision being developed in this SIG, is pretty lightweight stuff. For example, the CXX_Objects package that Paul has been distributing, is a pretty thin veneer over the Python C API, which happens to really make life a lot easier on the Python extension programmer working in C++, without really representing all that much code. I think that is representative of how most of the work of this SIG can come out. Moreover, I think there's a pretty good set of dividing lines between some of the different subsystems, so that we should be able to advocate some of them to Guido for inclusion into the core Python distribution, without necessarily having to couch it as an all-or-nothing kind of arrangement. As for consensus, well, everybody speak up! In the sig charter, I proposed the following list of example topics for this sig. I'll just quote that list, and insert some comments betwinst. > 1.Autoconf support for enabling C++ support in Python. This must be > managed in a way which does not change the C API, or the behavior of > Python if not configured for C++ support, in any (observable?) way. In > other words, conventional C extension modules must continue to work, > whether Python is configured with C++ support or not. I've recently produced a new patch set for fixing the Python-1.5.2 configure script to support C++. Yell loud if you don't agree, but I'm under the impression that there is a pretty universal awareness of the basic issue that we need main() compiled in C++ in order to support the possibility of the python executable hosting other C++ code buried in extension modules. This much we need, and I anticipate we can all agre on, almost irrespective of exactly /what/ the specific C++ interface we come up with is. Or stated another way, people wanting to write Python extension modules in C++ will need this, no matter exactly what they do for the rest of the C++ binding. I will put the latest version of this patch up on the C++ Sig web site soon. I have a little learning to do about how to administer the www site for this sig, but as soon as I can, I'll post a patch against the 1.5.2 distribution which adds autoconf support for C++ enabled python configurations. > 2.Type safety in the Python API. There's a lot here, but one of the most basic things I was thinking about when I wrote that, was this business of parsing tuples, especially arg tuples. Right now you do this with a format string, and you'd better do it right! What I'd like to see instead, is a system that uses C++'s innate awareness of the types of identifiers, to directly construct those format strings on your behalf. There are probably similar issues involved with extracting elements from the various Python containers. > 3.Introducing C++ classes for the various major Python abstractions, > such as dictionaries, lists, tuples, modules, etc. Paul and I spent some real time talking over this when I was at LLNL, and his CXX_Objects package is a really good incarnation of the basic ideas I think are important here. I've got to get reacquainted with the Python C aPI and see how much coverage we have right now. A pretty basic way of saying what classes I figure we need, is to look at the list of all Python C API entry points. They all look sort of like: Py<Pkg>_<Operation>. To me, this would seem to translate naturally to: namespace Py { class <Pkg> { PyObject *<Operation>( ... ); The idea then is to come up with C++ classes representing the major subdivisions of the Python C API, with member functions corresponding to the various function entry points in this package family in the C API. That's at least a pretty decent first approximation to what should be done. A little additional work to bring [] subscripting operators to Dicts and so forth, seems prudent as well. So anyway, I think Paul's CXX_Objects thing looks to me like a pretty significant step along the path toward "wrapping" the Python C API. > 4.Providing semantics for these classes which are natural both to the > Python programmer, as well as the C++ programmer. For example, the > PyDict class should have an operator[], and should also support STL > style iterators. Many issues of this form. Here I was trying to say that the wrapper classes implemented above, should be good STL citizens. However, that was only loosely defined at the time that I wrote it. Now I would point to the recently published "Generic Programming and the STL" by Matt Austern, which contains comprehensive documentation of the services exported by the STL components. We should try to make our classes that wrap the Python C API, support the services of similarly named STL classes as much as possible. There may be some issues here of just how far we can go with this. Python is very dynamically typed, and C++ is statically typed. For instance, a PyDict holds PyObject *'s, not int's. So there may be some issues about just how "normal" we can make the Python C++ wrapper classes look, in comparison to their STL counterparts, but we should do what we can. In particular, I think it ought to be possible to to at least implement enough services to support a wide sampling of the STL algorithms, operating over the elements of the Python containers. For example, maybe we need to write a functor library so you can write something like: Py::Dict a; // Fill a with elements, either from C++ or from Python script // language. sort( a.begin(), a.end(), PyLexicographicalLess<Py::String>() ); Or whatever. Anyway, hopefully you can understand the basic idea I'm trying to convey. > 5.Method invocation. How to make the invocation of C++ member > functions as natural and/or painless as possible. I have code that makes it possible to invoke methods of an object in the Python language, and have it result in invoking methods of an underlying C++ object. I'll dust that code off and try to post it soon. There were some comments on this code about a year ago, and I tried to fold in some of the constructive criticisms that I received, when last I rewrote that. Anyway, I'll get that cleaned up, and post it to the www site so poeople can look it over. > 6.Error handling. How to irradicate NULL checking in favor of C++ > style exceptions. I really want to expunge this business of NULL checking of the results of Python C API calls. The wrapper classes envisioned above, should do that, and throw appropriate exceptions when something goes gonzo in the C aPI functions they call. Then in the C++ code which uses these wrapper classes, you could replace all blecherous NULL checking, with a conventional C++ try block. Or even just let it go unhandled, if you have been suitably rigorous in the use of ctor/dtor pairs. For this to work, there needs to be a try/catch block around this methodobject invoker thingie in the Python core. Again, I have code that implements an "exception safe" method invocation semantic. I'll repackage that for 1.5.2, and post it for comment. In any event, I hope people will comment on this, and voice any other views or raise any other issues that ought to be heard and considered by all. Also, if you have code which you think should be considered by the larger community in the context of this effort to codify a C++ interface for Python extension programming, then by all means, send it to me, and I'll try to post it on the SIG www page. We should get all the ideas and competing propositions out on the table where every one can see, review, comment, contribute. One last thing for this initial attempt at revitalizing this SIG. In the past, there has been a lot of talk about the dearth of compilers at the level anticipated by the code that I and Paul have posted. There are some developments on this front that I think are worthing bringing to people's attention: 1) Sun C++ 5.0 is out. Although still a far cry short of "ANSI C++", it is a huge step up over their previous 4.2 product. They left out partial specialization and member templates, which is extremely dissapointing, but still, the overall product is a huge step up over its direct predecessor. 2) MS VC++ 6.0 is reputed to have support for "inline member templates" and most of the rest of the modern template machinery. Evidently it still lacks out of line member templtes, and partial specialization, but--so I am told--on the whole it is a big step forward. 3) For those who may not have heard yet, EGCS has been named the official successor to the GCC throne. This is extremely good news for the C++ community. I've been working with developer snapshots of egcs for several months now. 10/5/98 was the snapshot that marked the viability of egcs in my book. I'm pretty sure that current egcs snapshots will successfully handle code at the level of what Paul and I have put up on this list before. I'll be running precise evaluations of this over the next few days, but I'm pretty sure late model egcs snapshots are "there". Expect a GCC 3.0 shortly, which will be bascially a shaken down release of the current egcs developers snapshots. In other words, we are probably just days or weeks away from having an extremely high quality freeware virtually-ISO C++ compiler. Cheers to all. Please express your views.. -- Geoffrey Furnish Actel Corporation furnish(a)actel.com Senior Staff Engineer 955 East Arques Ave voice: 408-522-7528 Placement & Routing Sunnyvale, CA 94086-4533 fax: 408-328-2303 "... because only those who write the code truly control the project." -- Jamie Zawinski

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www site updated
by Geoffrey Furnish May 25, 1999

May 25, 1999

Just letting everyone know that the C++ www site has been updated ever so slightly, to include the patch I spoke about previously which introduces --with-cxx to Python's configure script. Please try it out and let me know what you think about this patch. In case anyone has forgotten, the sig url is: http://www.python.org/sigs/c++-sig/ I will be working up another patch with more of the support that has been talked about before. Also, I will put up submissions by others, just send me whatever you want others to be able to reach. -- Geoffrey Furnish Actel Corporation furnish(a)actel.com Senior Staff Engineer 955 East Arques Ave voice: 408-522-7528 Placement & Routing Sunnyvale, CA 94086-4533 fax: 408-328-2303 "... because only those who write the code truly control the project." -- Jamie Zawinski

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