<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Object Interface</title> <link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="Chapter 1. python 2.0"> <link rel="up" href="../index.html" title="Chapter 1. python 2.0"> <link rel="prev" href="functions.html" title="Functions"> <link rel="next" href="embedding.html" title="Embedding"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> <td align="center"><a href="../../../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="functions.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="embedding.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="python.object"></a> Object Interface</h2></div></div></div> <div class="toc"><dl> <dt><span class="section"><a href="object.html#python.basic_interface">Basic Interface</a></span></dt> <dt><span class="section"><a href="object.html#python.derived_object_types">Derived Object types</a></span></dt> <dt><span class="section"><a href="object.html#python.extracting_c___objects">Extracting C++ objects</a></span></dt> <dt><span class="section"><a href="object.html#python.enums">Enums</a></span></dt> </dl></div> <p> Python is dynamically typed, unlike C++ which is statically typed. Python variables may hold an integer, a float, list, dict, tuple, str, long etc., among other things. In the viewpoint of Boost.Python and C++, these Pythonic variables are just instances of class <code class="literal">object</code>. We will see in this chapter how to deal with Python objects. </p> <p> As mentioned, one of the goals of Boost.Python is to provide a bidirectional mapping between C++ and Python while maintaining the Python feel. Boost.Python C++ <code class="literal">object</code>s are as close as possible to Python. This should minimize the learning curve significantly. </p> <p> <span class="inlinemediaobject"><img src="../images/python.png" alt="python"></span> </p> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="python.basic_interface"></a>Basic Interface</h3></div></div></div> <p> Class <code class="literal">object</code> wraps <code class="literal">PyObject*</code>. All the intricacies of dealing with <code class="literal">PyObject</code>s such as managing reference counting are handled by the <code class="literal">object</code> class. C++ object interoperability is seamless. Boost.Python C++ <code class="literal">object</code>s can in fact be explicitly constructed from any C++ object. </p> <p> To illustrate, this Python code snippet: </p> <p> </p> <pre class="programlisting"><span class="keyword">def</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">y</span><span class="special">):</span> <span class="keyword">if</span> <span class="special">(</span><span class="identifier">y</span> <span class="special">==</span> <span class="string">'foo'</span><span class="special">):</span> <span class="identifier">x</span><span class="special">[</span><span class="number">3</span><span class="special">:</span><span class="number">7</span><span class="special">]</span> <span class="special">=</span> <span class="string">'bar'</span> <span class="keyword">else</span><span class="special">:</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">items</span> <span class="special">+=</span> <span class="identifier">y</span><span class="special">(</span><span class="number">3</span><span class="special">,</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">x</span> <span class="keyword">def</span> <span class="identifier">getfunc</span><span class="special">():</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">;</span> </pre> <p> Can be rewritten in C++ using Boost.Python facilities this way: </p> <p> </p> <pre class="programlisting"><span class="identifier">object</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">object</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">if</span> <span class="special">(</span><span class="identifier">y</span> <span class="special">==</span> <span class="string">"foo"</span><span class="special">)</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">slice</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="number">7</span><span class="special">)</span> <span class="special">=</span> <span class="string">"bar"</span><span class="special">;</span> <span class="keyword">else</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"items"</span><span class="special">)</span> <span class="special">+=</span> <span class="identifier">y</span><span class="special">(</span><span class="number">3</span><span class="special">,</span> <span class="identifier">x</span><span class="special">);</span> <span class="keyword">return</span> <span class="identifier">x</span><span class="special">;</span> <span class="special">}</span> <span class="identifier">object</span> <span class="identifier">getfunc</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">object</span><span class="special">(</span><span class="identifier">f</span><span class="special">);</span> <span class="special">}</span> </pre> <p> Apart from cosmetic differences due to the fact that we are writing the code in C++, the look and feel should be immediately apparent to the Python coder. </p> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="python.derived_object_types"></a>Derived Object types</h3></div></div></div> <p> Boost.Python comes with a set of derived <code class="literal">object</code> types corresponding to that of Python's: </p> <div class="itemizedlist"><ul class="itemizedlist" type="disc"> <li class="listitem"> list </li> <li class="listitem"> dict </li> <li class="listitem"> tuple </li> <li class="listitem"> str </li> <li class="listitem"> long_ </li> <li class="listitem"> enum </li> </ul></div> <p> These derived <code class="literal">object</code> types act like real Python types. For instance: </p> <pre class="programlisting"><span class="identifier">str</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">==></span> <span class="string">"1"</span> </pre> <p> Wherever appropriate, a particular derived <code class="literal">object</code> has corresponding Python type's methods. For instance, <code class="literal">dict</code> has a <code class="literal">keys()</code> method: </p> <pre class="programlisting"><span class="identifier">d</span><span class="special">.</span><span class="identifier">keys</span><span class="special">()</span> </pre> <p> <code class="literal">make_tuple</code> is provided for declaring <span class="emphasis"><em>tuple literals</em></span>. Example: </p> <pre class="programlisting"><span class="identifier">make_tuple</span><span class="special">(</span><span class="number">123</span><span class="special">,</span> <span class="char">'D'</span><span class="special">,</span> <span class="string">"Hello, World"</span><span class="special">,</span> <span class="number">0.0</span><span class="special">);</span> </pre> <p> In C++, when Boost.Python <code class="literal">object</code>s are used as arguments to functions, subtype matching is required. For example, when a function <code class="literal">f</code>, as declared below, is wrapped, it will only accept instances of Python's <code class="literal">str</code> type and subtypes. </p> <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">name</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">object</span> <span class="identifier">n2</span> <span class="special">=</span> <span class="identifier">name</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"upper"</span><span class="special">)();</span> <span class="comment">// NAME = name.upper() </span> <span class="identifier">str</span> <span class="identifier">NAME</span> <span class="special">=</span> <span class="identifier">name</span><span class="special">.</span><span class="identifier">upper</span><span class="special">();</span> <span class="comment">// better </span> <span class="identifier">object</span> <span class="identifier">msg</span> <span class="special">=</span> <span class="string">"%s is bigger than %s"</span> <span class="special">%</span> <span class="identifier">make_tuple</span><span class="special">(</span><span class="identifier">NAME</span><span class="special">,</span><span class="identifier">name</span><span class="special">);</span> <span class="special">}</span> </pre> <p> In finer detail: </p> <pre class="programlisting"><span class="identifier">str</span> <span class="identifier">NAME</span> <span class="special">=</span> <span class="identifier">name</span><span class="special">.</span><span class="identifier">upper</span><span class="special">();</span> </pre> <p> Illustrates that we provide versions of the str type's methods as C++ member functions. </p> <pre class="programlisting"><span class="identifier">object</span> <span class="identifier">msg</span> <span class="special">=</span> <span class="string">"%s is bigger than %s"</span> <span class="special">%</span> <span class="identifier">make_tuple</span><span class="special">(</span><span class="identifier">NAME</span><span class="special">,</span><span class="identifier">name</span><span class="special">);</span> </pre> <p> Demonstrates that you can write the C++ equivalent of <code class="literal">"format" % x,y,z</code> in Python, which is useful since there's no easy way to do that in std C++. </p> <div class="sidebar"> <p class="title"><b></b></p> <p> <span class="inlinemediaobject"><img src="../images/alert.png" alt="alert"></span> <span class="bold"><strong>Beware</strong></span> the common pitfall of forgetting that the constructors of most of Python's mutable types make copies, just as in Python. </p> </div> <p> Python: </p> <pre class="programlisting"><span class="special">>>></span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">dict</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">__dict__</span><span class="special">)</span> <span class="comment"># copies x.__dict__ </span><span class="special">>>></span> <span class="identifier">d</span><span class="special">[</span><span class="string">'whatever'</span><span class="special">]</span> <span class="special">=</span> <span class="number">3</span> <span class="comment"># modifies the copy </span></pre> <p> C++: </p> <pre class="programlisting"><span class="identifier">dict</span> <span class="identifier">d</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"__dict__"</span><span class="special">));</span> <span class="comment">// copies x.__dict__ </span><span class="identifier">d</span><span class="special">[</span><span class="char">'whatever'</span><span class="special">]</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span> <span class="comment">// modifies the copy </span></pre> <a name="derived_object_types.class__lt_t_gt__as_objects"></a><h3> <a name="id771905"></a> class_<T> as objects </h3> <p> Due to the dynamic nature of Boost.Python objects, any <code class="literal">class_<T></code> may also be one of these types! The following code snippet wraps the class (type) object. </p> <p> We can use this to create wrapped instances. Example: </p> <pre class="programlisting"><span class="identifier">object</span> <span class="identifier">vec345</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">class_</span><span class="special"><</span><span class="identifier">Vec2</span><span class="special">>(</span><span class="string">"Vec2"</span><span class="special">,</span> <span class="identifier">init</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>())</span> <span class="special">.</span><span class="identifier">def_readonly</span><span class="special">(</span><span class="string">"length"</span><span class="special">,</span> <span class="special">&</span><span class="identifier">Point</span><span class="special">::</span><span class="identifier">length</span><span class="special">)</span> <span class="special">.</span><span class="identifier">def_readonly</span><span class="special">(</span><span class="string">"angle"</span><span class="special">,</span> <span class="special">&</span><span class="identifier">Point</span><span class="special">::</span><span class="identifier">angle</span><span class="special">)</span> <span class="special">)(</span><span class="number">3.0</span><span class="special">,</span> <span class="number">4.0</span><span class="special">);</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">vec345</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"length"</span><span class="special">)</span> <span class="special">==</span> <span class="number">5.0</span><span class="special">);</span> </pre> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="python.extracting_c___objects"></a>Extracting C++ objects</h3></div></div></div> <p> At some point, we will need to get C++ values out of object instances. This can be achieved with the <code class="literal">extract<T></code> function. Consider the following: </p> <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">o</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"length"</span><span class="special">);</span> <span class="comment">// compile error </span></pre> <p> In the code above, we got a compiler error because Boost.Python <code class="literal">object</code> can't be implicitly converted to <code class="literal">double</code>s. Instead, what we wanted to do above can be achieved by writing: </p> <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(</span><span class="identifier">o</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"length"</span><span class="special">));</span> <span class="identifier">Vec2</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special"><</span><span class="identifier">Vec2</span><span class="special">&>(</span><span class="identifier">o</span><span class="special">);</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span> <span class="special">==</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">length</span><span class="special">());</span> </pre> <p> The first line attempts to extract the "length" attribute of the Boost.Python <code class="literal">object</code>. The second line attempts to <span class="emphasis"><em>extract</em></span> the <code class="literal">Vec2</code> object from held by the Boost.Python <code class="literal">object</code>. </p> <p> Take note that we said "attempt to" above. What if the Boost.Python <code class="literal">object</code> does not really hold a <code class="literal">Vec2</code> type? This is certainly a possibility considering the dynamic nature of Python <code class="literal">object</code>s. To be on the safe side, if the C++ type can't be extracted, an appropriate exception is thrown. To avoid an exception, we need to test for extractibility: </p> <pre class="programlisting"><span class="identifier">extract</span><span class="special"><</span><span class="identifier">Vec2</span><span class="special">&></span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">o</span><span class="special">);</span> <span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">check</span><span class="special">())</span> <span class="special">{</span> <span class="identifier">Vec2</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">();</span> <span class="special">...</span> </pre> <p> <span class="inlinemediaobject"><img src="../images/tip.png" alt="tip"></span> The astute reader might have noticed that the <code class="literal">extract<T></code> facility in fact solves the mutable copying problem: </p> <pre class="programlisting"><span class="identifier">dict</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special"><</span><span class="identifier">dict</span><span class="special">>(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"__dict__"</span><span class="special">));</span> <span class="identifier">d</span><span class="special">[</span><span class="string">"whatever"</span><span class="special">]</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span> <span class="comment">// modifies x.__dict__ ! </span></pre> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="python.enums"></a>Enums</h3></div></div></div> <p> Boost.Python has a nifty facility to capture and wrap C++ enums. While Python has no <code class="literal">enum</code> type, we'll often want to expose our C++ enums to Python as an <code class="literal">int</code>. Boost.Python's enum facility makes this easy while taking care of the proper conversions from Python's dynamic typing to C++'s strong static typing (in C++, ints cannot be implicitly converted to enums). To illustrate, given a C++ enum: </p> <pre class="programlisting"><span class="keyword">enum</span> <span class="identifier">choice</span> <span class="special">{</span> <span class="identifier">red</span><span class="special">,</span> <span class="identifier">blue</span> <span class="special">};</span> </pre> <p> the construct: </p> <pre class="programlisting"><span class="identifier">enum_</span><span class="special"><</span><span class="identifier">choice</span><span class="special">>(</span><span class="string">"choice"</span><span class="special">)</span> <span class="special">.</span><span class="identifier">value</span><span class="special">(</span><span class="string">"red"</span><span class="special">,</span> <span class="identifier">red</span><span class="special">)</span> <span class="special">.</span><span class="identifier">value</span><span class="special">(</span><span class="string">"blue"</span><span class="special">,</span> <span class="identifier">blue</span><span class="special">)</span> <span class="special">;</span> </pre> <p> can be used to expose to Python. The new enum type is created in the current <code class="literal">scope()</code>, which is usually the current module. The snippet above creates a Python class derived from Python's <code class="literal">int</code> type which is associated with the C++ type passed as its first parameter. </p> <div class="note"><table border="0" summary="Note"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td> <th align="left">Note</th> </tr> <tr><td align="left" valign="top"> <p> <span class="bold"><strong>what is a scope?</strong></span> </p> <p> The scope is a class that has an associated global Python object which controls the Python namespace in which new extension classes and wrapped functions will be defined as attributes. Details can be found <a href="../../../../v2/scope.html" target="_top">here</a>. </p> </td></tr> </table></div> <p> You can access those values in Python as </p> <p> </p> <pre class="programlisting"><span class="special">>>></span> <span class="identifier">my_module</span><span class="special">.</span><span class="identifier">choice</span><span class="special">.</span><span class="identifier">red</span> <span class="identifier">my_module</span><span class="special">.</span><span class="identifier">choice</span><span class="special">.</span><span class="identifier">red</span> </pre> <p> where my_module is the module where the enum is declared. You can also create a new scope around a class: </p> <p> </p> <pre class="programlisting"><span class="identifier">scope</span> <span class="identifier">in_X</span> <span class="special">=</span> <span class="identifier">class_</span><span class="special"><</span><span class="identifier">X</span><span class="special">>(</span><span class="string">"X"</span><span class="special">)</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span> <span class="special">...</span> <span class="special">)</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span> <span class="special">...</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Expose X::nested as X.nested </span><span class="identifier">enum_</span><span class="special"><</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">nested</span><span class="special">>(</span><span class="string">"nested"</span><span class="special">)</span> <span class="special">.</span><span class="identifier">value</span><span class="special">(</span><span class="string">"red"</span><span class="special">,</span> <span class="identifier">red</span><span class="special">)</span> <span class="special">.</span><span class="identifier">value</span><span class="special">(</span><span class="string">"blue"</span><span class="special">,</span> <span class="identifier">blue</span><span class="special">)</span> <span class="special">;</span> </pre> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2002-2005 Joel de Guzman, David Abrahams<p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top"> http://www.boost.org/LICENSE_1_0.txt </a>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="functions.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="embedding.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>