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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>1. Embedding Python in Another Application — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.6.7 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="Python/C API Reference Manual" href="../c-api/index.html" /> <link rel="prev" title="5. Building C and C++ Extensions on Windows" href="windows.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/extending/embedding.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="../c-api/index.html" title="Python/C API Reference Manual" accesskey="N">next</a> |</li> <li class="right" > <a href="windows.html" title="5. Building C and C++ Extensions on Windows" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Extending and Embedding the Python Interpreter</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="embedding-python-in-another-application"> <span id="embedding"></span><h1>1. Embedding Python in Another Application<a class="headerlink" href="#embedding-python-in-another-application" title="Permalink to this headline">¶</a></h1> <p>The previous chapters discussed how to extend Python, that is, how to extend the functionality of Python by attaching a library of C functions to it. It is also possible to do it the other way around: enrich your C/C++ application by embedding Python in it. Embedding provides your application with the ability to implement some of the functionality of your application in Python rather than C or C++. This can be used for many purposes; one example would be to allow users to tailor the application to their needs by writing some scripts in Python. You can also use it yourself if some of the functionality can be written in Python more easily.</p> <p>Embedding Python is similar to extending it, but not quite. The difference is that when you extend Python, the main program of the application is still the Python interpreter, while if you embed Python, the main program may have nothing to do with Python — instead, some parts of the application occasionally call the Python interpreter to run some Python code.</p> <p>So if you are embedding Python, you are providing your own main program. One of the things this main program has to do is initialize the Python interpreter. At the very least, you have to call the function <a class="reference internal" href="../c-api/init.html#c.Py_Initialize" title="Py_Initialize"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_Initialize()</span></code></a>. There are optional calls to pass command line arguments to Python. Then later you can call the interpreter from any part of the application.</p> <p>There are several different ways to call the interpreter: you can pass a string containing Python statements to <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_SimpleString" title="PyRun_SimpleString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_SimpleString()</span></code></a>, or you can pass a stdio file pointer and a file name (for identification in error messages only) to <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_SimpleFile" title="PyRun_SimpleFile"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_SimpleFile()</span></code></a>. You can also call the lower-level operations described in the previous chapters to construct and use Python objects.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><a class="reference internal" href="../c-api/index.html#c-api-index"><span class="std std-ref">Python/C API Reference Manual</span></a></dt> <dd>The details of Python’s C interface are given in this manual. A great deal of necessary information can be found here.</dd> </dl> </div> <div class="section" id="very-high-level-embedding"> <span id="high-level-embedding"></span><h2>1.1. Very High Level Embedding<a class="headerlink" href="#very-high-level-embedding" title="Permalink to this headline">¶</a></h2> <p>The simplest form of embedding Python is the use of the very high level interface. This interface is intended to execute a Python script without needing to interact with the application directly. This can for example be used to perform some operation on a file.</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf"><Python.h></span><span class="cp"></span> <span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span> <span class="p">{</span> <span class="kt">wchar_t</span> <span class="o">*</span><span class="n">program</span> <span class="o">=</span> <span class="n">Py_DecodeLocale</span><span class="p">(</span><span class="n">argv</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="nb">NULL</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">program</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span> <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"Fatal error: cannot decode argv[0]</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span> <span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span> <span class="p">}</span> <span class="n">Py_SetProgramName</span><span class="p">(</span><span class="n">program</span><span class="p">);</span> <span class="cm">/* optional but recommended */</span> <span class="n">Py_Initialize</span><span class="p">();</span> <span class="n">PyRun_SimpleString</span><span class="p">(</span><span class="s">"from time import time,ctime</span><span class="se">\n</span><span class="s">"</span> <span class="s">"print('Today is', ctime(time()))</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">Py_FinalizeEx</span><span class="p">()</span> <span class="o"><</span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span> <span class="n">exit</span><span class="p">(</span><span class="mi">120</span><span class="p">);</span> <span class="p">}</span> <span class="n">PyMem_RawFree</span><span class="p">(</span><span class="n">program</span><span class="p">);</span> <span class="k">return</span> <span class="mi">0</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> <p>The <a class="reference internal" href="../c-api/init.html#c.Py_SetProgramName" title="Py_SetProgramName"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_SetProgramName()</span></code></a> function should be called before <a class="reference internal" href="../c-api/init.html#c.Py_Initialize" title="Py_Initialize"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_Initialize()</span></code></a> to inform the interpreter about paths to Python run-time libraries. Next, the Python interpreter is initialized with <a class="reference internal" href="../c-api/init.html#c.Py_Initialize" title="Py_Initialize"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_Initialize()</span></code></a>, followed by the execution of a hard-coded Python script that prints the date and time. Afterwards, the <a class="reference internal" href="../c-api/init.html#c.Py_FinalizeEx" title="Py_FinalizeEx"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_FinalizeEx()</span></code></a> call shuts the interpreter down, followed by the end of the program. In a real program, you may want to get the Python script from another source, perhaps a text-editor routine, a file, or a database. Getting the Python code from a file can better be done by using the <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_SimpleFile" title="PyRun_SimpleFile"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_SimpleFile()</span></code></a> function, which saves you the trouble of allocating memory space and loading the file contents.</p> </div> <div class="section" id="beyond-very-high-level-embedding-an-overview"> <span id="lower-level-embedding"></span><h2>1.2. Beyond Very High Level Embedding: An overview<a class="headerlink" href="#beyond-very-high-level-embedding-an-overview" title="Permalink to this headline">¶</a></h2> <p>The high level interface gives you the ability to execute arbitrary pieces of Python code from your application, but exchanging data values is quite cumbersome to say the least. If you want that, you should use lower level calls. At the cost of having to write more C code, you can achieve almost anything.</p> <p>It should be noted that extending Python and embedding Python is quite the same activity, despite the different intent. Most topics discussed in the previous chapters are still valid. To show this, consider what the extension code from Python to C really does:</p> <ol class="arabic simple"> <li>Convert data values from Python to C,</li> <li>Perform a function call to a C routine using the converted values, and</li> <li>Convert the data values from the call from C to Python.</li> </ol> <p>When embedding Python, the interface code does:</p> <ol class="arabic simple"> <li>Convert data values from C to Python,</li> <li>Perform a function call to a Python interface routine using the converted values, and</li> <li>Convert the data values from the call from Python to C.</li> </ol> <p>As you can see, the data conversion steps are simply swapped to accommodate the different direction of the cross-language transfer. The only difference is the routine that you call between both data conversions. When extending, you call a C routine, when embedding, you call a Python routine.</p> <p>This chapter will not discuss how to convert data from Python to C and vice versa. Also, proper use of references and dealing with errors is assumed to be understood. Since these aspects do not differ from extending the interpreter, you can refer to earlier chapters for the required information.</p> </div> <div class="section" id="pure-embedding"> <span id="id1"></span><h2>1.3. Pure Embedding<a class="headerlink" href="#pure-embedding" title="Permalink to this headline">¶</a></h2> <p>The first program aims to execute a function in a Python script. Like in the section about the very high level interface, the Python interpreter does not directly interact with the application (but that will change in the next section).</p> <p>The code to run a function defined in a Python script is:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf"><Python.h></span><span class="cp"></span> <span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span> <span class="p">{</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">pName</span><span class="p">,</span> <span class="o">*</span><span class="n">pModule</span><span class="p">,</span> <span class="o">*</span><span class="n">pFunc</span><span class="p">;</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">pArgs</span><span class="p">,</span> <span class="o">*</span><span class="n">pValue</span><span class="p">;</span> <span class="kt">int</span> <span class="n">i</span><span class="p">;</span> <span class="k">if</span> <span class="p">(</span><span class="n">argc</span> <span class="o"><</span> <span class="mi">3</span><span class="p">)</span> <span class="p">{</span> <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="s">"Usage: call pythonfile funcname [args]</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span> <span class="k">return</span> <span class="mi">1</span><span class="p">;</span> <span class="p">}</span> <span class="n">Py_Initialize</span><span class="p">();</span> <span class="n">pName</span> <span class="o">=</span> <span class="n">PyUnicode_DecodeFSDefault</span><span class="p">(</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]);</span> <span class="cm">/* Error checking of pName left out */</span> <span class="n">pModule</span> <span class="o">=</span> <span class="n">PyImport_Import</span><span class="p">(</span><span class="n">pName</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">pName</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">pModule</span> <span class="o">!=</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span> <span class="n">pFunc</span> <span class="o">=</span> <span class="n">PyObject_GetAttrString</span><span class="p">(</span><span class="n">pModule</span><span class="p">,</span> <span class="n">argv</span><span class="p">[</span><span class="mi">2</span><span class="p">]);</span> <span class="cm">/* pFunc is a new reference */</span> <span class="k">if</span> <span class="p">(</span><span class="n">pFunc</span> <span class="o">&&</span> <span class="n">PyCallable_Check</span><span class="p">(</span><span class="n">pFunc</span><span class="p">))</span> <span class="p">{</span> <span class="n">pArgs</span> <span class="o">=</span> <span class="n">PyTuple_New</span><span class="p">(</span><span class="n">argc</span> <span class="o">-</span> <span class="mi">3</span><span class="p">);</span> <span class="k">for</span> <span class="p">(</span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">argc</span> <span class="o">-</span> <span class="mi">3</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span> <span class="p">{</span> <span class="n">pValue</span> <span class="o">=</span> <span class="n">PyLong_FromLong</span><span class="p">(</span><span class="n">atoi</span><span class="p">(</span><span class="n">argv</span><span class="p">[</span><span class="n">i</span> <span class="o">+</span> <span class="mi">3</span><span class="p">]));</span> <span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">pValue</span><span class="p">)</span> <span class="p">{</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">pArgs</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">pModule</span><span class="p">);</span> <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"Cannot convert argument</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span> <span class="k">return</span> <span class="mi">1</span><span class="p">;</span> <span class="p">}</span> <span class="cm">/* pValue reference stolen here: */</span> <span class="n">PyTuple_SetItem</span><span class="p">(</span><span class="n">pArgs</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">pValue</span><span class="p">);</span> <span class="p">}</span> <span class="n">pValue</span> <span class="o">=</span> <span class="n">PyObject_CallObject</span><span class="p">(</span><span class="n">pFunc</span><span class="p">,</span> <span class="n">pArgs</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">pArgs</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">pValue</span> <span class="o">!=</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span> <span class="n">printf</span><span class="p">(</span><span class="s">"Result of call: %ld</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">PyLong_AsLong</span><span class="p">(</span><span class="n">pValue</span><span class="p">));</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">pValue</span><span class="p">);</span> <span class="p">}</span> <span class="k">else</span> <span class="p">{</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">pFunc</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">pModule</span><span class="p">);</span> <span class="n">PyErr_Print</span><span class="p">();</span> <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="s">"Call failed</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span> <span class="k">return</span> <span class="mi">1</span><span class="p">;</span> <span class="p">}</span> <span class="p">}</span> <span class="k">else</span> <span class="p">{</span> <span class="k">if</span> <span class="p">(</span><span class="n">PyErr_Occurred</span><span class="p">())</span> <span class="n">PyErr_Print</span><span class="p">();</span> <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"Cannot find function </span><span class="se">\"</span><span class="s">%s</span><span class="se">\"\n</span><span class="s">"</span><span class="p">,</span> <span class="n">argv</span><span class="p">[</span><span class="mi">2</span><span class="p">]);</span> <span class="p">}</span> <span class="n">Py_XDECREF</span><span class="p">(</span><span class="n">pFunc</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">pModule</span><span class="p">);</span> <span class="p">}</span> <span class="k">else</span> <span class="p">{</span> <span class="n">PyErr_Print</span><span class="p">();</span> <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"Failed to load </span><span class="se">\"</span><span class="s">%s</span><span class="se">\"\n</span><span class="s">"</span><span class="p">,</span> <span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]);</span> <span class="k">return</span> <span class="mi">1</span><span class="p">;</span> <span class="p">}</span> <span class="k">if</span> <span class="p">(</span><span class="n">Py_FinalizeEx</span><span class="p">()</span> <span class="o"><</span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="mi">120</span><span class="p">;</span> <span class="p">}</span> <span class="k">return</span> <span class="mi">0</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> <p>This code loads a Python script using <code class="docutils literal notranslate"><span class="pre">argv[1]</span></code>, and calls the function named in <code class="docutils literal notranslate"><span class="pre">argv[2]</span></code>. Its integer arguments are the other values of the <code class="docutils literal notranslate"><span class="pre">argv</span></code> array. If you <a class="reference internal" href="#compiling"><span class="std std-ref">compile and link</span></a> this program (let’s call the finished executable <strong class="program">call</strong>), and use it to execute a Python script, such as:</p> <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">multiply</span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="n">b</span><span class="p">):</span> <span class="k">print</span><span class="p">(</span><span class="s2">"Will compute"</span><span class="p">,</span> <span class="n">a</span><span class="p">,</span> <span class="s2">"times"</span><span class="p">,</span> <span class="n">b</span><span class="p">)</span> <span class="n">c</span> <span class="o">=</span> <span class="mi">0</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">a</span><span class="p">):</span> <span class="n">c</span> <span class="o">=</span> <span class="n">c</span> <span class="o">+</span> <span class="n">b</span> <span class="k">return</span> <span class="n">c</span> </pre></div> </div> <p>then the result should be:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> call multiply multiply <span class="m">3</span> <span class="m">2</span> <span class="go">Will compute 3 times 2</span> <span class="go">Result of call: 6</span> </pre></div> </div> <p>Although the program is quite large for its functionality, most of the code is for data conversion between Python and C, and for error reporting. The interesting part with respect to embedding Python starts with</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">Py_Initialize</span><span class="p">();</span> <span class="n">pName</span> <span class="o">=</span> <span class="n">PyUnicode_DecodeFSDefault</span><span class="p">(</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]);</span> <span class="cm">/* Error checking of pName left out */</span> <span class="n">pModule</span> <span class="o">=</span> <span class="n">PyImport_Import</span><span class="p">(</span><span class="n">pName</span><span class="p">);</span> </pre></div> </div> <p>After initializing the interpreter, the script is loaded using <a class="reference internal" href="../c-api/import.html#c.PyImport_Import" title="PyImport_Import"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyImport_Import()</span></code></a>. This routine needs a Python string as its argument, which is constructed using the <a class="reference internal" href="../c-api/unicode.html#c.PyUnicode_FromString" title="PyUnicode_FromString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyUnicode_FromString()</span></code></a> data conversion routine.</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">pFunc</span> <span class="o">=</span> <span class="n">PyObject_GetAttrString</span><span class="p">(</span><span class="n">pModule</span><span class="p">,</span> <span class="n">argv</span><span class="p">[</span><span class="mi">2</span><span class="p">]);</span> <span class="cm">/* pFunc is a new reference */</span> <span class="k">if</span> <span class="p">(</span><span class="n">pFunc</span> <span class="o">&&</span> <span class="n">PyCallable_Check</span><span class="p">(</span><span class="n">pFunc</span><span class="p">))</span> <span class="p">{</span> <span class="p">...</span> <span class="p">}</span> <span class="n">Py_XDECREF</span><span class="p">(</span><span class="n">pFunc</span><span class="p">);</span> </pre></div> </div> <p>Once the script is loaded, the name we’re looking for is retrieved using <a class="reference internal" href="../c-api/object.html#c.PyObject_GetAttrString" title="PyObject_GetAttrString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_GetAttrString()</span></code></a>. If the name exists, and the object returned is callable, you can safely assume that it is a function. The program then proceeds by constructing a tuple of arguments as normal. The call to the Python function is then made with:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">pValue</span> <span class="o">=</span> <span class="n">PyObject_CallObject</span><span class="p">(</span><span class="n">pFunc</span><span class="p">,</span> <span class="n">pArgs</span><span class="p">);</span> </pre></div> </div> <p>Upon return of the function, <code class="docutils literal notranslate"><span class="pre">pValue</span></code> is either <em>NULL</em> or it contains a reference to the return value of the function. Be sure to release the reference after examining the value.</p> </div> <div class="section" id="extending-embedded-python"> <span id="extending-with-embedding"></span><h2>1.4. Extending Embedded Python<a class="headerlink" href="#extending-embedded-python" title="Permalink to this headline">¶</a></h2> <p>Until now, the embedded Python interpreter had no access to functionality from the application itself. The Python API allows this by extending the embedded interpreter. That is, the embedded interpreter gets extended with routines provided by the application. While it sounds complex, it is not so bad. Simply forget for a while that the application starts the Python interpreter. Instead, consider the application to be a set of subroutines, and write some glue code that gives Python access to those routines, just like you would write a normal Python extension. For example:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="kt">int</span> <span class="n">numargs</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span> <span class="cm">/* Return the number of arguments of the application command line */</span> <span class="k">static</span> <span class="n">PyObject</span><span class="o">*</span> <span class="nf">emb_numargs</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">self</span><span class="p">,</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">args</span><span class="p">)</span> <span class="p">{</span> <span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">PyArg_ParseTuple</span><span class="p">(</span><span class="n">args</span><span class="p">,</span> <span class="s">":numargs"</span><span class="p">))</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="k">return</span> <span class="n">PyLong_FromLong</span><span class="p">(</span><span class="n">numargs</span><span class="p">);</span> <span class="p">}</span> <span class="k">static</span> <span class="n">PyMethodDef</span> <span class="n">EmbMethods</span><span class="p">[]</span> <span class="o">=</span> <span class="p">{</span> <span class="p">{</span><span class="s">"numargs"</span><span class="p">,</span> <span class="n">emb_numargs</span><span class="p">,</span> <span class="n">METH_VARARGS</span><span class="p">,</span> <span class="s">"Return the number of arguments received by the process."</span><span class="p">},</span> <span class="p">{</span><span class="nb">NULL</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">}</span> <span class="p">};</span> <span class="k">static</span> <span class="n">PyModuleDef</span> <span class="n">EmbModule</span> <span class="o">=</span> <span class="p">{</span> <span class="n">PyModuleDef_HEAD_INIT</span><span class="p">,</span> <span class="s">"emb"</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="n">EmbMethods</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">,</span> <span class="nb">NULL</span> <span class="p">};</span> <span class="k">static</span> <span class="n">PyObject</span><span class="o">*</span> <span class="nf">PyInit_emb</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">PyModule_Create</span><span class="p">(</span><span class="o">&</span><span class="n">EmbModule</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>Insert the above code just above the <code class="xref c c-func docutils literal notranslate"><span class="pre">main()</span></code> function. Also, insert the following two statements before the call to <a class="reference internal" href="../c-api/init.html#c.Py_Initialize" title="Py_Initialize"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_Initialize()</span></code></a>:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">numargs</span> <span class="o">=</span> <span class="n">argc</span><span class="p">;</span> <span class="n">PyImport_AppendInittab</span><span class="p">(</span><span class="s">"emb"</span><span class="p">,</span> <span class="o">&</span><span class="n">PyInit_emb</span><span class="p">);</span> </pre></div> </div> <p>These two lines initialize the <code class="docutils literal notranslate"><span class="pre">numargs</span></code> variable, and make the <code class="xref py py-func docutils literal notranslate"><span class="pre">emb.numargs()</span></code> function accessible to the embedded Python interpreter. With these extensions, the Python script can do things like</p> <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">emb</span> <span class="k">print</span><span class="p">(</span><span class="s2">"Number of arguments"</span><span class="p">,</span> <span class="n">emb</span><span class="o">.</span><span class="n">numargs</span><span class="p">())</span> </pre></div> </div> <p>In a real application, the methods will expose an API of the application to Python.</p> </div> <div class="section" id="embedding-python-in-c"> <span id="embeddingincplusplus"></span><h2>1.5. Embedding Python in C++<a class="headerlink" href="#embedding-python-in-c" title="Permalink to this headline">¶</a></h2> <p>It is also possible to embed Python in a C++ program; precisely how this is done will depend on the details of the C++ system used; in general you will need to write the main program in C++, and use the C++ compiler to compile and link your program. There is no need to recompile Python itself using C++.</p> </div> <div class="section" id="compiling-and-linking-under-unix-like-systems"> <span id="compiling"></span><h2>1.6. Compiling and Linking under Unix-like systems<a class="headerlink" href="#compiling-and-linking-under-unix-like-systems" title="Permalink to this headline">¶</a></h2> <p>It is not necessarily trivial to find the right flags to pass to your compiler (and linker) in order to embed the Python interpreter into your application, particularly because Python needs to load library modules implemented as C dynamic extensions (<code class="file docutils literal notranslate"><span class="pre">.so</span></code> files) linked against it.</p> <p>To find out the required compiler and linker flags, you can execute the <code class="file docutils literal notranslate"><span class="pre">python</span><em><span class="pre">X.Y</span></em><span class="pre">-config</span></code> script which is generated as part of the installation process (a <code class="file docutils literal notranslate"><span class="pre">python3-config</span></code> script may also be available). This script has several options, of which the following will be directly useful to you:</p> <ul> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">pythonX.Y-config</span> <span class="pre">--cflags</span></code> will give you the recommended flags when compiling:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> /opt/bin/python3.4-config --cflags <span class="go">-I/opt/include/python3.4m -I/opt/include/python3.4m -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes</span> </pre></div> </div> </li> <li><p class="first"><code class="docutils literal notranslate"><span class="pre">pythonX.Y-config</span> <span class="pre">--ldflags</span></code> will give you the recommended flags when linking:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> /opt/bin/python3.4-config --ldflags <span class="go">-L/opt/lib/python3.4/config-3.4m -lpthread -ldl -lutil -lm -lpython3.4m -Xlinker -export-dynamic</span> </pre></div> </div> </li> </ul> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">To avoid confusion between several Python installations (and especially between the system Python and your own compiled Python), it is recommended that you use the absolute path to <code class="file docutils literal notranslate"><span class="pre">python</span><em><span class="pre">X.Y</span></em><span class="pre">-config</span></code>, as in the above example.</p> </div> <p>If this procedure doesn’t work for you (it is not guaranteed to work for all Unix-like platforms; however, we welcome <a class="reference internal" href="../bugs.html#reporting-bugs"><span class="std std-ref">bug reports</span></a>) you will have to read your system’s documentation about dynamic linking and/or examine Python’s <code class="file docutils literal notranslate"><span class="pre">Makefile</span></code> (use <a class="reference internal" href="../library/sysconfig.html#sysconfig.get_makefile_filename" title="sysconfig.get_makefile_filename"><code class="xref py py-func docutils literal notranslate"><span class="pre">sysconfig.get_makefile_filename()</span></code></a> to find its location) and compilation options. In this case, the <a class="reference internal" href="../library/sysconfig.html#module-sysconfig" title="sysconfig: Python's configuration information"><code class="xref py py-mod docutils literal notranslate"><span class="pre">sysconfig</span></code></a> module is a useful tool to programmatically extract the configuration values that you will want to combine together. For example:</p> <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sysconfig</span> <span class="gp">>>> </span><span class="n">sysconfig</span><span class="o">.</span><span class="n">get_config_var</span><span class="p">(</span><span class="s1">'LIBS'</span><span class="p">)</span> <span class="go">'-lpthread -ldl -lutil'</span> <span class="gp">>>> </span><span class="n">sysconfig</span><span class="o">.</span><span class="n">get_config_var</span><span class="p">(</span><span class="s1">'LINKFORSHARED'</span><span class="p">)</span> <span class="go">'-Xlinker -export-dynamic'</span> </pre></div> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">1. Embedding Python in Another Application</a><ul> <li><a class="reference internal" href="#very-high-level-embedding">1.1. Very High Level Embedding</a></li> <li><a class="reference internal" href="#beyond-very-high-level-embedding-an-overview">1.2. Beyond Very High Level Embedding: An overview</a></li> <li><a class="reference internal" href="#pure-embedding">1.3. Pure Embedding</a></li> <li><a class="reference internal" href="#extending-embedded-python">1.4. Extending Embedded Python</a></li> <li><a class="reference internal" href="#embedding-python-in-c">1.5. Embedding Python in C++</a></li> <li><a class="reference internal" href="#compiling-and-linking-under-unix-like-systems">1.6. Compiling and Linking under Unix-like systems</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="windows.html" title="previous chapter">5. Building C and C++ Extensions on Windows</a></p> <h4>Next topic</h4> <p class="topless"><a href="../c-api/index.html" title="next chapter">Python/C API Reference Manual</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/3.6/Doc/extending/embedding.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="../c-api/index.html" title="Python/C API Reference Manual" >next</a> |</li> <li class="right" > <a href="windows.html" title="5. 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