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<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python - June 2002 Progress Report</title>
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</head>
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<body link="#0000ff" vlink="#800080">
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
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"header">
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<tr>
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<td valign="top" width="300">
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<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
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"C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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</td>
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<td valign="top">
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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
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<h2 align="center">June 2002 Progress Report</h2>
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</td>
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</tr>
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</table>
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<hr>
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<h2>Contents</h2>
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<dl class="index">
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<dt><a href="#intro">Introduction</a></dt>
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<dt><a href="#handle"><code>handle<T></code></a></dt>
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<dt><a href="#object"><code>object</code></a></dt>
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<dl class="index">
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<dt><a href="#operators"><code>object</code> operators</a></dt>
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<dt><a href="#conversions"><code>object</code> conversions</a></dt>
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</dl>
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<dt><a href="#list"><code>list</code></a></dt>
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<dt><a href="#numerics"><code>Numerics</code></a></dt>
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<dt><a href="#community">Community</a></dt>
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<dt><a href="#next">What's Next</a></dt>
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</dl>
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<h2><a name="intro">Introduction</a></h2>
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July was mostly focused on allowing expressive manipulation of
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individual Python objects, or what Ralf Grosse-Kunstleve calls
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"Writing Python in C++". The work began with this <a
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href="http://mail.python.org/pipermail/c++-sig/2002-June/001311.html">posting</a>,
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which outlines the issues and intention.
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<h2><a name="handle"><code>handle<T></code></a></h2>
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The most basic element needed was a replacement for the
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<code>reference<></code> class template and the
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<code>ref</code> typedef from Boost.Python v1, a simple smart
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pointer to a Python object. The old v1 typedef
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"<code>ref</code>" (for
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<code>reference<PyObject></code>) had to be retired because I
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thought it would be too confusing given the importance of <code><a
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href="../../../bind/ref.html">boost::ref</a>()</code> to this
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library. I began a <a
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href="http://mail.python.org/pipermail/c++-sig/2002-June/001311.html">discussion</a>of
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possible names, and it was eventually <a
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href="http://mail.python.org/pipermail/c++-sig/2002-June/001337.html">decided</a>
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to rename <code>reference</code> to <code>handle</code> and supply a
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default argument so that <code>ref</code> could be spelled
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<code>handle<></code> without an additional typedef. There
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were also some interface changes to make it safer and more-efficient
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to interface with the raw
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<code>PyObject*</code>s forced on us by Python's 'C' API. A
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discussion of those protocols can be found <a
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href="http://mail.python.org/pipermail/c++-sig/2002-June/001401.html">here</a>.
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<h2><a name="handle"><code>object</code></a></h2>
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It is intended that users will seldom need or want to work with
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<code>handle<></code>; its major distinguishing features are
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that it gives direct access to the underlying object representation
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through <code>operator*</code> and <code>operator-></code>, and
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that can be <code>NULL</code>, both sources of danger. Instead the
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library provides a class called <code>object</code>, which
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encapsulates a valid Python object and provides a similar interface to
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Python's.
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<h3><a name="operators"><code>object</code> operators</a></h3>
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The first challenge was to provide support for object manipulations
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using a Python-like syntax, mostly in the form of operator overloads:
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<table border="1">
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<tr><th>Python <th>C++
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<tr>
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<td><code>y = x.foo</code> <td><code>y = x.attr("foo");
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<tr>
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<td><code>x.foo = 1</code> <td><code>x.attr("foo") = 1;
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<tr>
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<td><code>y = x[z]</code> <td><code>y = x[z];
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<tr>
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<td><code>x[z] = 1</code> <td><code>x[z] = 1;
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<tr>
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<td><code>y = x[3:-1]</code> <td><code>y = x.slice(3,-1);
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<tr>
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<td><code>y = x[3:]</code> <td><code>y = x.slice(3,_);
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<tr>
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<td><code>y = x[:-2]</code> <td><code>y = x.slice(_,-2);
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<tr>
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<td><code>z = x(1, y)</code> <td><code>z = x(1, y);
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<tr>
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<td><code>z = x.f(1, y)</code> <td><code>z = x.attr("f")(1, y);
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<tr>
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<td><code>not x</code> <td><code>!x
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<tr>
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<td><code>x and y</code> <td><code>x and y
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</table>
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I'm still a unsatisfied with the interface for attribute access. There
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original proposal used a syntax like this one:
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<pre>
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y = x._("foo");
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x._("foo") = 1;
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</pre>
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which was only marginally better than what we've got. Niki Spahiev
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then <a
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href="http://mail.python.org/pipermail/c++-sig/2002-June/001447.html">pointed
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out</a> a potential conflict with the macro which GNU Gettext <a
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href="http://www.gnu.org/manual/gettext/html_mono/gettext.html#SEC6">suggests</a>
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people define. This unfortunate state of affairs forced us into using
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<code>attr</code> instead. I'd still like to find a better interface,
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but the lack of overloadable C++ operators which aren't already used
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in Python is an obstacle. The comma operator is still a possibility,
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but it has the wrong precedence:
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<pre>
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y = x,"foo" // error
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x,"foo" = 1; // error
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y = (x,"foo"); // ok
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(x,"foo") = 1; // ok
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</pre>
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Well, I guess we could consider adding that to the interface without
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removing <code>attr()</code>, to see how it plays out...
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<h3><a name="operators"><code>object</code> conversions</a></h3>
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The <code>object</code> class also provided an opportunity to replace
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Boost.Python v1's <code>to_python()</code> as a user-level
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interface. Instead, <code>object</code> has a templated constructor
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which can be used to convert any C++ object to Python using the same
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underlying mechanisms used for the arguments to <code><a
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href="call.html">call</a><></code>.
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<p>Incidentally, the implementation of operator and conversion support
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for object uncovered an inordinate number of compiler bugs in our
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targeted platforms. It was a lot more "interesting" than it
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should have been.
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<h2><a name="list"><code>list</code></a></h2>
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With <code>object</code> implemented, it was time to begin replacing
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the ad-hoc implementations of <code>list</code>, <code>string</code>,
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and <code>dictionary</code> supplied by Boost.Python v1 with something
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more robust. I started with <code>list</code> as an example. Because
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<code>object</code> already provides all of the requisite operators,
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publicly deriving <code>list</code> from object seemed like a good
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choice. The remaining issues were what do do about the one-argument
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list constructor (which in Python attempts to convert its argument to
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a list), and how to deal converting with <code>list</code> arguments
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to wrapped functions. Some of the issues are laid out in <a
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href="http://mail.python.org/pipermail/c++-sig/2002-June/001551.html">this
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thread</a>. Ultimately, it was decided that <code>list(x)</code>
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should do the same thing in C++ as in Python (conversion), while
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<code>list</code> arguments should only match Python
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<code>list</code>s (and <code>list</code> subclasses). The
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implementation worked well, and provided a <a
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href="http://mail.python.org/pipermail/c++-sig/2002-June/001586.html">roadmap</a>
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for the protocol to be used for implementation of the other built-in
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types.
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<h2><a name="numerics">Numerics</a></h2>
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Support for C++ <code>long long</code> and <code>unsigned long
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long</code>
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(and <code>__int64</code> on MSVC) to/from python conversions was
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added this month. We also improved handling of numeric overflows when
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converting, e.g., a Python int to a type with a more limited range of
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representation.
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<h2><a name="community">Community</a></h2>
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<ul>
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<li>Ralf W. Grosse-Kunstleve and Nick Sauter have implemented
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<a href="http://cci.lbl.gov/boost/">multiplatform nightly
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build-and-test</a> runs for Boost.Python V2 at LBL.
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<li>Dave Hawkes has made significant progress on generating the
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Python <a
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href="http://mail.python.org/pipermail/c++-sig/2002-June/001503.html">built-in
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function and API wrappers</a>
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<li>Achim Domma has agreed to take up the job of implementing the
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<code>str</code>, <code>dict</code>, and <code>tuple</code> classes.
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</ul>
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Deep thanks to all the Boost.Python contributors! This project
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wouldn't be possible without your participation.
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<h2><a name="next">What's Next</a></h2>
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As I write this we are already well into the month of July, so I
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suggest you consult the <a
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href="http://mail.python.org/pipermail/c++-sig/2002-July/">Mailing
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List Archive</a> if you want to know what's been happening. Otherwise
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you'll just have to wait till next month (hopefully the beginning).
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
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13 November, 2002
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<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
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</p>
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<p><i>© Copyright <a href="http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a>
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2002. </i></p>
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</body>
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</html>
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