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<h1 class="c1"><a href="../index.html">Boost.Python</a></h1>
<h2 class="c2">February 2002 Progress Report</h2>
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<hr>
<h2>Contents</h2>
<dl class="index">
<dt><a href="#Python10">Python10 Conference Report</a>
<dt><a href="#progress">Boost.Python v2 Progress</a>
<dd>
<dl class="index">
<dt><a href="#documentation">Documentation</a>
<dt><a href="#conversion">Overhaul of
<code>to_python</code>/<code>from_python</code>
conversion mechanism</a>
<dt><a href="#miscellaneous">Miscellaneous</a>
</dl>
</dl>
<h2><a name="Python10">Python10 Conference Report</a></h2>
I spent the first week of February at the Python10 conference
in Alexandria, VA. I'm including this experience report
for two reasons: firstly, it documents where my time was
used. Secondly, a public presence for Boost.Python and
interaction between the Python and C++ communities is
important to the future of Boost.Python, which in turn is
important to the Kull Project.
<p>Andy Koenig, of all people, was the keynote speaker of
this year's opening plenary session. He presented his
"impressions of a polyglot outsider", which
studiously avoided any mention of C++ until the end of his
talk, when he was asked about standardization. I was
surprised to learn that the C++ community at large wanted a
few more years before beginning but when ANSI accepted
HP's request for a standard, the process was forced to
start: it was a matter of participating or having
standardization proceed without one's input. Andy managed
to highlight very effectively the balance of strengths in
Python, one of the most important being its support for
extension via libraries. In many ways that makes Python a
good analogue for C++ in the interpreted world
<p>There were several kind mentions of the Boost.Python
library from people who found it indispensable. I was
particularly happy that Karl MacMillan, Michael Droettboom,
and Ichiro Fujinaga from Johns Hopkins is using it to do OCR
on a vast library of music notation, since in a previous life
I was an author of music notation software. These guys are
also drawing on Ullrich Koethe's VIGRA library for image
manipulation (Ullrich has been a major contributor to
Boost.Python). They also have a system for writing the
Boost.Python wrapper code in C++ comments, which allows them
to keep all of the code in one place. I've asked them to
send me some information on that.
<p>The development of Swig has been gaining momentum again
(the basic description at
www.boost.org/libs/python/doc/comparisons.html still
applies). The talk given about it by David Beazly was very
well-attended, and they appear to have quite a few users.
Swig's strengths (coverage of many langauages) and
weaknesses (incomplete C++ language support) haven't
changed, although the C++ support seems to have improved
considerably - they now claim to have a complete model of the
C++ type system. It seems to be mostly geared at wrapping
what Walter Landry calls "C-Tran": C++ code which
traffics in built-in types with little use of abstraction.
I'm not knocking that, either: I'm sure a lot of that
code exists, so it's a valuable service. One feature Swig
has which I'd like to steal is the ability to unwrap a
single Python argument into multiple C++ arguments, for
example, by converting a Python string into a pointer and
length. When his talk was over, David approached me about a
possible joint workshop on language binding, which sounds
like a fun idea to me.
<p>I spent some considerable time talking with Steven Knight,
the leader of the Scons build tool effort. We had a lot to
share with one another, and I gained a much better
appreciation for many of the Scons design decisions. Scons
seems to be concentrating on being the ultimate build system
substrate, and Steve seemed to think that we were on the
right track with our high-level design. We both hope that the
Boost.Build V2 high-level architecture can eventually be
ported to run on top of Scons.
<p>They also have a highly-refined and successful development
procedure which I'd like to emulate for Boost.Build V2.
Among many other things they do, their source-control system
automatically ensures that when you check in a new test, it
is automatically run on the currently checked-in state of the
code, and is expected to fail -- a relatively obvious good
idea which I've never heard before.
<p>Guido Van Rossum's "State of the Python
Union" address was full of questions for the community
about what should be done next, but the one idea Guido seemed
to stress was that core language stability and continuing
library development would be a good idea (sound familiar?) I
mentioned the Boost model as a counterpoint to the idea of
something like CPAN (the massive Perl library archives), and
it seemed to generate some significant interest. I've
offered to work with anyone from the Python community who
wants to set up something like Boost.
<p>There was some discussion of "string
interpolation" (variable substitution in strings), and
Guido mentioned that he had some thoughts about the
strengths/weaknesses of Python's formatting interface. It
might be useful for those working on formatting for boost to
contact him and find out what he has to say.
<p>Ka-Ping Yee demoed a Mailman discussion thread weaver.
This tool weaves the various messages in a discussion thread
into a single document so you can follow the entire
conversation. Since we're looking very seriously at
moving Boost to Mailman, this could be a really useful thing
for us to have. If we do this, we'll move the yahoogroups
discussions into the mailman archive so old discussions can
be easily accessed in the same fashion.
<p>And, just because it's cool, though perhaps not
relevant: http://homepages.ulb.ac.be/~arigo/psyco/ is a
promising effort to accelerate the execution of Python code
to speeds approaching those of compiled languages. It
reminded me a lot of Todd Veldhuizen's research into
moving parts of C++ template compilation to runtime, only
coming from the opposite end of things.
<h2><a name="progress">Boost.Python v2 Progress</a></h2>
Here's what actually got accomplished.
<h3><a name="documentation">Documentation</a></h3>
<p>My first priority upon returning from Python10 was to get
some documentation in place. After wasting an unfortunate
amount of time looking at automatic documentation tools which
don't quite work, I settled down to use Bill Kempf's
HTML templates designed to be a boost standard. While they
are working well, it is highly labor-intensive.
<p>I decided to begin with the high-level reference material,
as opposed to tutorial, narrative, or nitty-gritty details of
the framework. It seemed more important to have a precise
description of the way the commonly-used components work than
to have examples in HTML (since we already have some test
modules), and since the low-level details are much
less-frequently needed by users it made sense for me to
simply respond to support requests for the time being.
<p>After completing approximately 60% of the high-level docs
(currently checked in to libs/python/doc/v2), I found myself
ready to start documenting the mechanisms for creating
to-/from-python converters. This caused a dilemma: I had
realized during the previous week that a much simpler,
more-efficient, and easier-to-use implementation was
possible, but I hadn't planned on implementing it right
away, since what was already in place worked adequately. I
had also received my first query on the C++-sig about how to
write such a converter
<p>Given the labor-intensive nature of documentation writing,
I decided it would be a bad idea to document the conversion
mechanism if I was just going to rewrite it. Often the best
impetus for simplifying a design is the realization that
understandably documenting its current state would be too
difficult, and this was no exception.
<h3><a name="conversion">Overhaul of
<code>to_python</code>/<code>from_python</code> conversion
mechanism</a></h3>
<p>There were two basic realizations involved here:
<ol>
<li><code>to_python</code> conversion could be a one-step
process, once an appropriate conversion function is found.
This allows elimination of the separate indirect
convertibility check
<li>There are basically two categories of from_python
conversions: those which lvalues stored within or held by
the Python object (essentially extractions), like what
happens when an instance of a C++ class exposed with class_
is used as the target of a wrapped member function), and
those in which a new rvalue gets created, as when a Python
Float is converted to a C++
<code>complex<double></code> or a Python tuple is
converted to a C++ <code>std::vector<></code>. From
the client side, there are two corresponding categories of
conversion: those which demand an lvalue conversion and
those which can accept an lvalue or an rvalue conversion.
</ol>
The latter realization allowed the following collapse, which
considerably simplified things:
<blockquote>
<table border="1" summary="Conversion protocol">
<tr>
<th>Target Type
<th>Eligible Converters
<tr>
<td><code>T</code>
<td rowspan="5"><code>T</code> rvalue or lvalue
<tr>
<td><code>T const</code>
<tr>
<td><code>T volatile</code>
<tr>
<td><code>T const volatile</code>
<tr>
<td><code>T const&</code>
<tr>
<td><code>T const*</code>
<td rowspan="9"><code>T</code> lvalue
<tr>
<td><code>T volatile*</code>
<tr>
<td><code>T const volatile*</code>
<tr>
<td><code>T&</code>
<tr>
<td><code>T volatile&</code>
<tr>
<td><code>T const volatile&</code>
<tr>
<td><code>T* const&</code>
<tr>
<td><code>T const* const&</code>
<tr>
<td><code>T volatile*const&</code>
<tr>
<td><code>T const volatile*const&</code>
</table>
</blockquote>
This job included the following additional enhancements:
<ul>
<li>Elimination of virtual functions, which cause object
code bloat
<li>Registration of a single converter function for all
lvalue conversions, two for all rvalue conversions
<li>Killed lots of unneeded code
<li>Increased opacity of registry interface
<li>Eliminated all need for decorated runtime type
identifiers
<li>Updated test modules to reflect new interface
<li>Eliminated the need for users to worry about converter
lifetime issues Additional Builtin Conversion Enhancements
<li>Support for complex<float>,
complex<double>, and complex<long double>
conversions
<li>Support for bool conversions
<li>NULL pointers representable by None in Python
<li>Support for conversion of Python classic classes to
numeric types
</ul>
<h3><a name="miscellaneous">Miscellaneous</a></h3>
These don't fit easily under a large heading:
<ul>
<li>Support CallPolicies for class member functions
<li>from_python_data.hpp: revamped type alignment
metaprogram so that it's fast enough for KCC
<li>classfwd.hpp header forward-declares class_<T>
<li>indirect_traits.hpp:
<li>added is_pointer_to_reference
<li>fixed bugs
<li>Reduced recompilation dependencies
<li>msvc_typeinfo works around broken MS/Intel typeid()
implementation
<li>Many fixes and improvements to the type_traits library
in order to work around compiler bugs and suppress warnings
<li>Eliminated the need for explicit acquisition of
converter registrations
<li>Expanded constructor support to 6 arguments
<li>Implemented generalized pointer lifetime support
<li>Updated code generation for returning.hpp
<li>Tracked down and fixed cycle GC bugs
<li>Added comprehensive unit tests for destroy_reference,
pointer_type_id, select_from_python, complex<T>,
bool, and classic class instance conversions
</ul>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
<p class="c3">© Copyright <a href=
"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002. Distributed
under the Boost Software License, Version 1.0. (See accompanying file
LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</p>
