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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.embedding"></a>Embedding</h2></div></div></div>
<div class="toc"><dl><dt><span class="section"><a href="embedding.html#python.using_the_interpreter">Using the interpreter</a></span></dt></dl></div>
<p>
By now you should know how to use Boost.Python to call your C++ code from Python.
However, sometimes you may need to do the reverse: call Python code from the
C++-side. This requires you to <span class="emphasis"><em>embed</em></span> the Python interpreter
into your C++ program.
</p>
<p>
Currently, Boost.Python does not directly support everything you'll need when
embedding. Therefore you'll need to use the <a href="http://www.python.org/doc/current/api/api.html" target="_top">Python/C
API</a> to fill in the gaps. However, Boost.Python already makes embedding
a lot easier and, in a future version, it may become unnecessary to touch the
Python/C API at all. So stay tuned... <span class="inlinemediaobject"><img src="../images/smiley.png" alt="smiley"></span>
</p>
<a name="embedding.building_embedded_programs"></a><h3>
<a name="id773307"></a>
Building embedded programs
</h3>
<p>
To be able to embed python into your programs, you have to link to both Boost.Python's
as well as Python's own runtime library.
</p>
<p>
Boost.Python's library comes in two variants. Both are located in Boost's
<code class="literal">/libs/python/build/bin-stage</code> subdirectory. On Windows, the
variants are called <code class="literal">boost_python.lib</code> (for release builds)
and <code class="literal">boost_python_debug.lib</code> (for debugging). If you can't
find the libraries, you probably haven't built Boost.Python yet. See <a href="../../../../building.html" target="_top">Building and Testing</a> on how to do this.
</p>
<p>
Python's library can be found in the <code class="literal">/libs</code> subdirectory
of your Python directory. On Windows it is called pythonXY.lib where X.Y is
your major Python version number.
</p>
<p>
Additionally, Python's <code class="literal">/include</code> subdirectory has to be added
to your include path.
</p>
<p>
In a Jamfile, all the above boils down to:
</p>
<pre class="programlisting">projectroot c:\projects\embedded_program ; # location of the program
# bring in the rules for python
SEARCH on python.jam = $(BOOST_BUILD_PATH) ;
include python.jam ;
exe embedded_program # name of the executable
: #sources
embedded_program.cpp
: # requirements
<find-library>boost_python <library-path>c:\boost\libs\python
$(PYTHON_PROPERTIES)
<library-path>$(PYTHON_LIB_PATH)
<find-library>$(PYTHON_EMBEDDED_LIBRARY) ;
</pre>
<a name="embedding.getting_started"></a><h3>
<a name="id773391"></a>
Getting started
</h3>
<p>
Being able to build is nice, but there is nothing to build yet. Embedding the
Python interpreter into one of your C++ programs requires these 4 steps:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
#include <code class="literal"><boost/python.hpp></code>
</li>
<li class="listitem">
Call <a href="http://www.python.org/doc/current/api/initialization.html#l2h-652" target="_top">Py_Initialize</a>()
to start the interpreter and create the <code class="literal"><span class="underline">_main</span>_</code>
module.
</li>
<li class="listitem">
Call other Python C API routines to use the interpreter.
</li>
</ol></div>
<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>Note that at this time you must not call <a href="http://www.python.org/doc/current/api/initialization.html#l2h-656" target="_top">Py_Finalize</a>()
to stop the interpreter. This may be fixed in a future version of boost.python.</strong></span>
</p></td></tr>
</table></div>
<p>
(Of course, there can be other C++ code between all of these steps.)
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="emphasis"><em><span class="bold"><strong>Now that we can embed the interpreter in
our programs, lets see how to put it to use...</strong></span></em></span>
</p></blockquote></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.using_the_interpreter"></a>Using the interpreter</h3></div></div></div>
<p>
As you probably already know, objects in Python are reference-counted. Naturally,
the <code class="literal">PyObject</code>s of the Python C API are also reference-counted.
There is a difference however. While the reference-counting is fully automatic
in Python, the Python C API requires you to do it <a href="http://www.python.org/doc/current/c-api/refcounting.html" target="_top">by
hand</a>. This is messy and especially hard to get right in the presence
of C++ exceptions. Fortunately Boost.Python provides the <a href="../../../../v2/handle.html" target="_top">handle</a>
and <a href="../../../../v2/object.html" target="_top">object</a> class templates to
automate the process.
</p>
<a name="using_the_interpreter.running_python_code"></a><h3>
<a name="id773549"></a>
Running Python code
</h3>
<p>
Boost.python provides three related functions to run Python code from C++.
</p>
<pre class="programlisting"><span class="identifier">object</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">expression</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">globals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">(),</span> <span class="identifier">object</span> <span class="identifier">locals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">())</span>
<span class="identifier">object</span> <span class="identifier">exec</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">code</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">globals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">(),</span> <span class="identifier">object</span> <span class="identifier">locals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">())</span>
<span class="identifier">object</span> <span class="identifier">exec_file</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">filename</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">globals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">(),</span> <span class="identifier">object</span> <span class="identifier">locals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">())</span>
</pre>
<p>
eval evaluates the given expression and returns the resulting value. exec
executes the given code (typically a set of statements) returning the result,
and exec_file executes the code contained in the given file.
</p>
<p>
The <code class="literal">globals</code> and <code class="literal">locals</code> parameters are
Python dictionaries containing the globals and locals of the context in which
to run the code. For most intents and purposes you can use the namespace
dictionary of the <code class="literal"><span class="underline">_main</span>_</code>
module for both parameters.
</p>
<p>
Boost.python provides a function to import a module:
</p>
<pre class="programlisting"><span class="identifier">object</span> <span class="identifier">import</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">name</span><span class="special">)</span>
</pre>
<p>
import imports a python module (potentially loading it into the running process
first), and returns it.
</p>
<p>
Let's import the <code class="literal"><span class="underline">_main</span>_</code>
module and run some Python code in its namespace:
</p>
<pre class="programlisting"><span class="identifier">object</span> <span class="identifier">main_module</span> <span class="special">=</span> <span class="identifier">import</span><span class="special">(</span><span class="string">"__main__"</span><span class="special">);</span>
<span class="identifier">object</span> <span class="identifier">main_namespace</span> <span class="special">=</span> <span class="identifier">main_module</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">object</span> <span class="identifier">ignored</span> <span class="special">=</span> <span class="identifier">exec</span><span class="special">(</span><span class="string">"hello = file('hello.txt', 'w')\n"</span>
<span class="string">"hello.write('Hello world!')\n"</span>
<span class="string">"hello.close()"</span><span class="special">,</span>
<span class="identifier">main_namespace</span><span class="special">);</span>
</pre>
<p>
This should create a file called 'hello.txt' in the current directory containing
a phrase that is well-known in programming circles.
</p>
<a name="using_the_interpreter.manipulating_python_objects"></a><h3>
<a name="id774064"></a>
Manipulating Python objects
</h3>
<p>
Often we'd like to have a class to manipulate Python objects. But we have
already seen such a class above, and in the <a href="object.html" target="_top">previous
section</a>: the aptly named <code class="literal">object</code> class and its
derivatives. We've already seen that they can be constructed from a <code class="literal">handle</code>.
The following examples should further illustrate this fact:
</p>
<pre class="programlisting"><span class="identifier">object</span> <span class="identifier">main_module</span> <span class="special">=</span> <span class="identifier">import</span><span class="special">(</span><span class="string">"__main__"</span><span class="special">);</span>
<span class="identifier">object</span> <span class="identifier">main_namespace</span> <span class="special">=</span> <span class="identifier">main_module</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">object</span> <span class="identifier">ignored</span> <span class="special">=</span> <span class="identifier">exec</span><span class="special">(</span><span class="string">"result = 5 ** 2"</span><span class="special">,</span> <span class="identifier">main_namespace</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">five_squared</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">main_namespace</span><span class="special">[</span><span class="string">"result"</span><span class="special">]);</span>
</pre>
<p>
Here we create a dictionary object for the <code class="literal"><span class="underline">_main</span>_</code>
module's namespace. Then we assign 5 squared to the result variable and read
this variable from the dictionary. Another way to achieve the same result
is to use eval instead, which returns the result directly:
</p>
<pre class="programlisting"><span class="identifier">object</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">eval</span><span class="special">(</span><span class="string">"5 ** 2"</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">five_squared</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">result</span><span class="special">);</span>
</pre>
<a name="using_the_interpreter.exception_handling"></a><h3>
<a name="id774390"></a>
Exception handling
</h3>
<p>
If an exception occurs in the evaluation of the python expression, <a href="../../../../v2/errors.html#error_already_set-spec" target="_top">error_already_set</a>
is thrown:
</p>
<pre class="programlisting"><span class="keyword">try</span>
<span class="special">{</span>
<span class="identifier">object</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">eval</span><span class="special">(</span><span class="string">"5/0"</span><span class="special">);</span>
<span class="comment">// execution will never get here:
</span> <span class="keyword">int</span> <span class="identifier">five_divided_by_zero</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">result</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">catch</span><span class="special">(</span><span class="identifier">error_already_set</span> <span class="keyword">const</span> <span class="special">&)</span>
<span class="special">{</span>
<span class="comment">// handle the exception in some way
</span><span class="special">}</span>
</pre>
<p>
The <code class="literal">error_already_set</code> exception class doesn't carry any
information in itself. To find out more about the Python exception that occurred,
you need to use the <a href="http://www.python.org/doc/api/exceptionHandling.html" target="_top">exception
handling functions</a> of the Python C API in your catch-statement. This
can be as simple as calling <a href="http://www.python.org/doc/api/exceptionHandling.html#l2h-70" target="_top">PyErr_Print()</a>
to print the exception's traceback to the console, or comparing the type
of the exception with those of the <a href="http://www.python.org/doc/api/standardExceptions.html" target="_top">standard
exceptions</a>:
</p>
<pre class="programlisting"><span class="keyword">catch</span><span class="special">(</span><span class="identifier">error_already_set</span> <span class="keyword">const</span> <span class="special">&)</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">PyErr_ExceptionMatches</span><span class="special">(</span><span class="identifier">PyExc_ZeroDivisionError</span><span class="special">))</span>
<span class="special">{</span>
<span class="comment">// handle ZeroDivisionError specially
</span> <span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="comment">// print all other errors to stderr
</span> <span class="identifier">PyErr_Print</span><span class="special">();</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
(To retrieve even more information from the exception you can use some of
the other exception handling functions listed <a href="http://www.python.org/doc/api/exceptionHandling.html" target="_top">here</a>.)
</p>
</div>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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