[PYTHON META-SIG] New SIG

[Geoffrey Furnish]

Hello and Happy New Year to all!

I would like to propose a new SIG, one for discussing improvements to
Python to support writing extensions in C++.

At the last Python workshop, I presented a talk in which I said (among
other things), that the Python API, as seen from a C++ perspective,
could be more convenient.  I am starting now to write significant
amounts of C++ Python extension code, so I am now ready to tackle this
problem.  In the (perhaps vain) hope that the results of this effort
might be useful to others, I am thinking it might be useful to carry
out some of the design discussions in a pseudo-public forum, and a new
SIG seems ideally suited.

There are several issues with writing extensions in C++ that need to
be addressed.

1) Python main() should be compiled in C++.

2) The Python API should be divided into classes, according to what
   sorts of PyObjects they work on.  In other words, instead of
   calling PyDict_XXX, you ought to be able to do PyDict *d; d->XXX();

3) Error handling should use C++ exceptions instead of return values.
   This would free the C++ extension writer from having to check for
   NULL results all the time, and allow a more intentional coding
   style.  User functions could then throw exceptions to register
   errors, and for that matter, the C++ized Python API functions could
   do the same.

4) To support 3), method invocation should be wrapped in a try block.
   Thus any exception thrown by code in 3), would be caught without
   trashing the interpretter.  This will require that more things
   besides just main() be compiled in C++.

5) Method invocation should be made more convenient to the extension
   writer.  Currently if you have an extension facility packaged as a
   C++ class, you must cobble together a bunch of global functions
   (the one regarded as the "methods" by the Python extension
   facility), which merely invoke the true (C++) methods of your
   extension object.  It would sure be nice if that artificial level
   of indirection could be avoided (or, at a minimum, automated).

6) Various portions of the Python API could be made typesafe.
   Py_BuildValue, for instance, could be made to dispense with the
   format string, and could take arguments by reference, relieving the
   extension writer from the need to count &'s and *'s.

I have already started on this project list.  I have hacked up some
autoconf support for compiling main with C++ if requested (user says
./configure --with-cxx).  (If not so requested, Python is built just
as it was before).  I have also done some preliminary work on
constructing a PyDict class, and have done some studying of the Python
core to see how some of these other items might be accomplished.  In
short, my work in this area is already under way, and I hope to be
able to cut some patch sets in the reasonably near term future, which
could be distributed to others who wish to write Python extension
modules in C++.  It would seem then, that to maximize the possibility
for positive effect, that it would be useful to set up a SIG for
discussion of these goals and the implementations thereof.

I suggest the sig be named:

cxx-sig@python.org

or, even (preferably, if possible) c++-sig@python.org (does anyone
know if majordomo can handle list names like this yet?  I set up a C++
related majordomo list a couple years back and read in the
documentation to avoid special chars, I wonder if this has been
rectified yet?).

Anyway, assuming this sig is authorized and created, and whatever name
it is given, my proposed short description would be:

c++-sig: A SIG for discussing improvements to Python to support
extension modules coded in C++.

My proposed long description would be:
-------------------------------------------------
C++-SIG, a SIG for discussing development of a C++ interface to Python

This list exists in order to discuss the design and development of 
a C++ binding for the compiled interface to Python, and associated 
classes and tools for the construction of Python extension modules 
in C++.

Major issues to be discussed/resolved, include, but are not limited
to:

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. 

2) Typesafety in the Python API.  

3) Introducing C++ classes for the various major Python abstractions,
   such as dictionaries, lists, tuples, modules, whatever.

4) Providing semantics for these classes which are natural both to a
   Python affecinado, and to a C++ affecinado.  For example, the
   PyDict class should have an operator[], and should also support STL
   style iterators.  Many issues of this form.

5) Method invocation.  How to make the invocation of C++ member
   functions as natural and/or painless as possible.

6) Error handling.  How to irradicate NULL checking in favor of C++
   style exceptions.

-------------------------------------------------

I welcome any and all discussion of the merits of the creation of such
a SIG, of the possible impact on the Python community, or of any other
concerns that people may have.  My personal view is that this is work
that I will do whether it gains popular support or not, but if it can
be done in a way which is genuinely useful to others, so much the
better.  One could hope that at some point in the indefinite future,
these enhancements might wander into the official Python core
distribution.  I am certainly committed to providing the C++ support
in a strictly add-on fashion, that is, not taking anything away from
C, while adding value to C++.  And finally, I am of course, willing to
serve as owner of the list, if that is a necessary aspect of getting
it created.  BTW, if it matters, LLNL is an institutional PSA member.

Cheers to all, and looking forward to your reactions,

-- 
Geoffrey Furnish		email: furnish@llnl.gov
LLNL X/ICF			phone: 510-424-4227	fax: 510-423-0925

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