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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. Extending Python with C or C++ — Python 2.7.18 documentation</title> <link rel="stylesheet" href="../_static/classic.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/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.18 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="2. Defining New Types" href="newtypes.html" /> <link rel="prev" title="Extending and Embedding the Python Interpreter" href="index.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/extending/extending.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"> This document is for an old version of Python that is <a href="https://devguide.python.org/devcycle/#end-of-life-branches">no longer supported</a>. You should install the python3 and python3-doc packages and read the <a href="file:///usr/share/doc/python3-doc/html/extending/extending.html"> Python documentation for the Python3 version packaged in this release</a>. </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" 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="newtypes.html" title="2. Defining New Types" accesskey="N">next</a> |</li> <li class="right" > <a href="index.html" title="Extending and Embedding the Python Interpreter" 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">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Extending and Embedding the Python Interpreter</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="extending-python-with-c-or-c"> <span id="extending-intro"></span><h1>1. Extending Python with C or C++<a class="headerlink" href="#extending-python-with-c-or-c" title="Permalink to this headline">¶</a></h1> <p>It is quite easy to add new built-in modules to Python, if you know how to program in C. Such <em class="dfn">extension modules</em> can do two things that can’t be done directly in Python: they can implement new built-in object types, and they can call C library functions and system calls.</p> <p>To support extensions, the Python API (Application Programmers Interface) defines a set of functions, macros and variables that provide access to most aspects of the Python run-time system. The Python API is incorporated in a C source file by including the header <code class="docutils literal notranslate"><span class="pre">"Python.h"</span></code>.</p> <p>The compilation of an extension module depends on its intended use as well as on your system setup; details are given in later chapters.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">The C extension interface is specific to CPython, and extension modules do not work on other Python implementations. In many cases, it is possible to avoid writing C extensions and preserve portability to other implementations. For example, if your use case is calling C library functions or system calls, you should consider using the <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> module or the <a class="reference external" href="https://cffi.readthedocs.org">cffi</a> library rather than writing custom C code. These modules let you write Python code to interface with C code and are more portable between implementations of Python than writing and compiling a C extension module.</p> </div> <div class="section" id="a-simple-example"> <span id="extending-simpleexample"></span><h2>1.1. A Simple Example<a class="headerlink" href="#a-simple-example" title="Permalink to this headline">¶</a></h2> <p>Let’s create an extension module called <code class="docutils literal notranslate"><span class="pre">spam</span></code> (the favorite food of Monty Python fans…) and let’s say we want to create a Python interface to the C library function <code class="xref c c-func docutils literal notranslate"><span class="pre">system()</span></code> <a class="footnote-reference" href="#id5" id="id1">[1]</a>. This function takes a null-terminated character string as argument and returns an integer. We want this function to be callable from Python as follows:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="o">>>></span> <span class="n">import</span> <span class="n">spam</span> <span class="o">>>></span> <span class="n">status</span> <span class="o">=</span> <span class="n">spam</span><span class="p">.</span><span class="n">system</span><span class="p">(</span><span class="s">"ls -l"</span><span class="p">)</span> </pre></div> </div> <p>Begin by creating a file <code class="file docutils literal notranslate"><span class="pre">spammodule.c</span></code>. (Historically, if a module is called <code class="docutils literal notranslate"><span class="pre">spam</span></code>, the C file containing its implementation is called <code class="file docutils literal notranslate"><span class="pre">spammodule.c</span></code>; if the module name is very long, like <code class="docutils literal notranslate"><span class="pre">spammify</span></code>, the module name can be just <code class="file docutils literal notranslate"><span class="pre">spammify.c</span></code>.)</p> <p>The first line of our file can be:</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> </pre></div> </div> <p>which pulls in the Python API (you can add a comment describing the purpose of the module and a copyright notice if you like).</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">Since Python may define some pre-processor definitions which affect the standard headers on some systems, you <em>must</em> include <code class="file docutils literal notranslate"><span class="pre">Python.h</span></code> before any standard headers are included.</p> </div> <p>All user-visible symbols defined by <code class="file docutils literal notranslate"><span class="pre">Python.h</span></code> have a prefix of <code class="docutils literal notranslate"><span class="pre">Py</span></code> or <code class="docutils literal notranslate"><span class="pre">PY</span></code>, except those defined in standard header files. For convenience, and since they are used extensively by the Python interpreter, <code class="docutils literal notranslate"><span class="pre">"Python.h"</span></code> includes a few standard header files: <code class="docutils literal notranslate"><span class="pre"><stdio.h></span></code>, <code class="docutils literal notranslate"><span class="pre"><string.h></span></code>, <code class="docutils literal notranslate"><span class="pre"><errno.h></span></code>, and <code class="docutils literal notranslate"><span class="pre"><stdlib.h></span></code>. If the latter header file does not exist on your system, it declares the functions <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">realloc()</span></code> directly.</p> <p>The next thing we add to our module file is the C function that will be called when the Python expression <code class="docutils literal notranslate"><span class="pre">spam.system(string)</span></code> is evaluated (we’ll see shortly how it ends up being called):</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="n">PyObject</span> <span class="o">*</span> <span class="nf">spam_system</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">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">command</span><span class="p">;</span> <span class="kt">int</span> <span class="n">sts</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">"s"</span><span class="p">,</span> <span class="o">&</span><span class="n">command</span><span class="p">))</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="n">sts</span> <span class="o">=</span> <span class="n">system</span><span class="p">(</span><span class="n">command</span><span class="p">);</span> <span class="k">return</span> <span class="n">Py_BuildValue</span><span class="p">(</span><span class="s">"i"</span><span class="p">,</span> <span class="n">sts</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>There is a straightforward translation from the argument list in Python (for example, the single expression <code class="docutils literal notranslate"><span class="pre">"ls</span> <span class="pre">-l"</span></code>) to the arguments passed to the C function. The C function always has two arguments, conventionally named <em>self</em> and <em>args</em>.</p> <p>For module functions, the <em>self</em> argument is <em>NULL</em> or a pointer selected while initializing the module (see <a class="reference internal" href="../c-api/allocation.html#c.Py_InitModule4" title="Py_InitModule4"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_InitModule4()</span></code></a>). For a method, it would point to the object instance.</p> <p>The <em>args</em> argument will be a pointer to a Python tuple object containing the arguments. Each item of the tuple corresponds to an argument in the call’s argument list. The arguments are Python objects — in order to do anything with them in our C function we have to convert them to C values. The function <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> in the Python API checks the argument types and converts them to C values. It uses a template string to determine the required types of the arguments as well as the types of the C variables into which to store the converted values. More about this later.</p> <p><a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> returns true (nonzero) if all arguments have the right type and its components have been stored in the variables whose addresses are passed. It returns false (zero) if an invalid argument list was passed. In the latter case it also raises an appropriate exception so the calling function can return <em>NULL</em> immediately (as we saw in the example).</p> </div> <div class="section" id="intermezzo-errors-and-exceptions"> <span id="extending-errors"></span><h2>1.2. Intermezzo: Errors and Exceptions<a class="headerlink" href="#intermezzo-errors-and-exceptions" title="Permalink to this headline">¶</a></h2> <p>An important convention throughout the Python interpreter is the following: when a function fails, it should set an exception condition and return an error value (usually a <em>NULL</em> pointer). Exceptions are stored in a static global variable inside the interpreter; if this variable is <em>NULL</em> no exception has occurred. A second global variable stores the “associated value” of the exception (the second argument to <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a>). A third variable contains the stack traceback in case the error originated in Python code. These three variables are the C equivalents of the Python variables <code class="docutils literal notranslate"><span class="pre">sys.exc_type</span></code>, <code class="docutils literal notranslate"><span class="pre">sys.exc_value</span></code> and <code class="docutils literal notranslate"><span class="pre">sys.exc_traceback</span></code> (see the section on module <a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">sys</span></code></a> in the Python Library Reference). It is important to know about them to understand how errors are passed around.</p> <p>The Python API defines a number of functions to set various types of exceptions.</p> <p>The most common one is <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_SetString" title="PyErr_SetString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_SetString()</span></code></a>. Its arguments are an exception object and a C string. The exception object is usually a predefined object like <code class="xref c c-data docutils literal notranslate"><span class="pre">PyExc_ZeroDivisionError</span></code>. The C string indicates the cause of the error and is converted to a Python string object and stored as the “associated value” of the exception.</p> <p>Another useful function is <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_SetFromErrno" title="PyErr_SetFromErrno"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_SetFromErrno()</span></code></a>, which only takes an exception argument and constructs the associated value by inspection of the global variable <code class="xref c c-data docutils literal notranslate"><span class="pre">errno</span></code>. The most general function is <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_SetObject" title="PyErr_SetObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_SetObject()</span></code></a>, which takes two object arguments, the exception and its associated value. You don’t need to <a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a> the objects passed to any of these functions.</p> <p>You can test non-destructively whether an exception has been set with <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_Occurred" title="PyErr_Occurred"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_Occurred()</span></code></a>. This returns the current exception object, or <em>NULL</em> if no exception has occurred. You normally don’t need to call <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_Occurred" title="PyErr_Occurred"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_Occurred()</span></code></a> to see whether an error occurred in a function call, since you should be able to tell from the return value.</p> <p>When a function <em>f</em> that calls another function <em>g</em> detects that the latter fails, <em>f</em> should itself return an error value (usually <em>NULL</em> or <code class="docutils literal notranslate"><span class="pre">-1</span></code>). It should <em>not</em> call one of the <code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_*()</span></code> functions — one has already been called by <em>g</em>. <em>f</em>’s caller is then supposed to also return an error indication to <em>its</em> caller, again <em>without</em> calling <code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_*()</span></code>, and so on — the most detailed cause of the error was already reported by the function that first detected it. Once the error reaches the Python interpreter’s main loop, this aborts the currently executing Python code and tries to find an exception handler specified by the Python programmer.</p> <p>(There are situations where a module can actually give a more detailed error message by calling another <code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_*()</span></code> function, and in such cases it is fine to do so. As a general rule, however, this is not necessary, and can cause information about the cause of the error to be lost: most operations can fail for a variety of reasons.)</p> <p>To ignore an exception set by a function call that failed, the exception condition must be cleared explicitly by calling <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_Clear" title="PyErr_Clear"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_Clear()</span></code></a>. The only time C code should call <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_Clear" title="PyErr_Clear"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_Clear()</span></code></a> is if it doesn’t want to pass the error on to the interpreter but wants to handle it completely by itself (possibly by trying something else, or pretending nothing went wrong).</p> <p>Every failing <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> call must be turned into an exception — the direct caller of <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> (or <code class="xref c c-func docutils literal notranslate"><span class="pre">realloc()</span></code>) must call <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_NoMemory" title="PyErr_NoMemory"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_NoMemory()</span></code></a> and return a failure indicator itself. All the object-creating functions (for example, <a class="reference internal" href="../c-api/int.html#c.PyInt_FromLong" title="PyInt_FromLong"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyInt_FromLong()</span></code></a>) already do this, so this note is only relevant to those who call <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> directly.</p> <p>Also note that, with the important exception of <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> and friends, functions that return an integer status usually return a positive value or zero for success and <code class="docutils literal notranslate"><span class="pre">-1</span></code> for failure, like Unix system calls.</p> <p>Finally, be careful to clean up garbage (by making <a class="reference internal" href="../c-api/refcounting.html#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XDECREF()</span></code></a> or <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> calls for objects you have already created) when you return an error indicator!</p> <p>The choice of which exception to raise is entirely yours. There are predeclared C objects corresponding to all built-in Python exceptions, such as <code class="xref c c-data docutils literal notranslate"><span class="pre">PyExc_ZeroDivisionError</span></code>, which you can use directly. Of course, you should choose exceptions wisely — don’t use <code class="xref c c-data docutils literal notranslate"><span class="pre">PyExc_TypeError</span></code> to mean that a file couldn’t be opened (that should probably be <code class="xref c c-data docutils literal notranslate"><span class="pre">PyExc_IOError</span></code>). If something’s wrong with the argument list, the <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> function usually raises <code class="xref c c-data docutils literal notranslate"><span class="pre">PyExc_TypeError</span></code>. If you have an argument whose value must be in a particular range or must satisfy other conditions, <code class="xref c c-data docutils literal notranslate"><span class="pre">PyExc_ValueError</span></code> is appropriate.</p> <p>You can also define a new exception that is unique to your module. For this, you usually declare a static object variable at the beginning of your file:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">SpamError</span><span class="p">;</span> </pre></div> </div> <p>and initialize it in your module’s initialization function (<code class="xref c c-func docutils literal notranslate"><span class="pre">initspam()</span></code>) with an exception object (leaving out the error checking for now):</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyMODINIT_FUNC</span> <span class="nf">initspam</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">m</span><span class="p">;</span> <span class="n">m</span> <span class="o">=</span> <span class="n">Py_InitModule</span><span class="p">(</span><span class="s">"spam"</span><span class="p">,</span> <span class="n">SpamMethods</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">m</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span><span class="p">;</span> <span class="n">SpamError</span> <span class="o">=</span> <span class="n">PyErr_NewException</span><span class="p">(</span><span class="s">"spam.error"</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="n">Py_INCREF</span><span class="p">(</span><span class="n">SpamError</span><span class="p">);</span> <span class="n">PyModule_AddObject</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"error"</span><span class="p">,</span> <span class="n">SpamError</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>Note that the Python name for the exception object is <code class="xref py py-exc docutils literal notranslate"><span class="pre">spam.error</span></code>. The <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_NewException" title="PyErr_NewException"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_NewException()</span></code></a> function may create a class with the base class being <a class="reference internal" href="../library/exceptions.html#exceptions.Exception" title="exceptions.Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a> (unless another class is passed in instead of <em>NULL</em>), described in <a class="reference internal" href="../library/exceptions.html#bltin-exceptions"><span class="std std-ref">Built-in Exceptions</span></a>.</p> <p>Note also that the <code class="xref c c-data docutils literal notranslate"><span class="pre">SpamError</span></code> variable retains a reference to the newly created exception class; this is intentional! Since the exception could be removed from the module by external code, an owned reference to the class is needed to ensure that it will not be discarded, causing <code class="xref c c-data docutils literal notranslate"><span class="pre">SpamError</span></code> to become a dangling pointer. Should it become a dangling pointer, C code which raises the exception could cause a core dump or other unintended side effects.</p> <p>We discuss the use of <code class="docutils literal notranslate"><span class="pre">PyMODINIT_FUNC</span></code> as a function return type later in this sample.</p> <p>The <code class="xref py py-exc docutils literal notranslate"><span class="pre">spam.error</span></code> exception can be raised in your extension module using a call to <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_SetString" title="PyErr_SetString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_SetString()</span></code></a> as shown below:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="n">PyObject</span> <span class="o">*</span> <span class="nf">spam_system</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">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">command</span><span class="p">;</span> <span class="kt">int</span> <span class="n">sts</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">"s"</span><span class="p">,</span> <span class="o">&</span><span class="n">command</span><span class="p">))</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="n">sts</span> <span class="o">=</span> <span class="n">system</span><span class="p">(</span><span class="n">command</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">sts</span> <span class="o"><</span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span> <span class="n">PyErr_SetString</span><span class="p">(</span><span class="n">SpamError</span><span class="p">,</span> <span class="s">"System command failed"</span><span class="p">);</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="p">}</span> <span class="k">return</span> <span class="n">PyLong_FromLong</span><span class="p">(</span><span class="n">sts</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> </div> <div class="section" id="back-to-the-example"> <span id="backtoexample"></span><h2>1.3. Back to the Example<a class="headerlink" href="#back-to-the-example" title="Permalink to this headline">¶</a></h2> <p>Going back to our example function, you should now be able to understand this statement:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></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">"s"</span><span class="p">,</span> <span class="o">&</span><span class="n">command</span><span class="p">))</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> </pre></div> </div> <p>It returns <em>NULL</em> (the error indicator for functions returning object pointers) if an error is detected in the argument list, relying on the exception set by <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>. Otherwise the string value of the argument has been copied to the local variable <code class="xref c c-data docutils literal notranslate"><span class="pre">command</span></code>. This is a pointer assignment and you are not supposed to modify the string to which it points (so in Standard C, the variable <code class="xref c c-data docutils literal notranslate"><span class="pre">command</span></code> should properly be declared as <code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">char</span> <span class="pre">*command</span></code>).</p> <p>The next statement is a call to the Unix function <code class="xref c c-func docutils literal notranslate"><span class="pre">system()</span></code>, passing it the string we just got from <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">sts</span> <span class="o">=</span> <span class="n">system</span><span class="p">(</span><span class="n">command</span><span class="p">);</span> </pre></div> </div> <p>Our <code class="xref py py-func docutils literal notranslate"><span class="pre">spam.system()</span></code> function must return the value of <code class="xref c c-data docutils literal notranslate"><span class="pre">sts</span></code> as a Python object. This is done using the function <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a>, which is something like the inverse of <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>: it takes a format string and an arbitrary number of C values, and returns a new Python object. More info on <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a> is given later.</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">return</span> <span class="nf">Py_BuildValue</span><span class="p">(</span><span class="s">"i"</span><span class="p">,</span> <span class="n">sts</span><span class="p">);</span> </pre></div> </div> <p>In this case, it will return an integer object. (Yes, even integers are objects on the heap in Python!)</p> <p>If you have a C function that returns no useful argument (a function returning <code class="xref c c-type docutils literal notranslate"><span class="pre">void</span></code>), the corresponding Python function must return <code class="docutils literal notranslate"><span class="pre">None</span></code>. You need this idiom to do so (which is implemented by the <a class="reference internal" href="../c-api/none.html#c.Py_RETURN_NONE" title="Py_RETURN_NONE"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_RETURN_NONE</span></code></a> macro):</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">Py_INCREF</span><span class="p">(</span><span class="n">Py_None</span><span class="p">);</span> <span class="k">return</span> <span class="n">Py_None</span><span class="p">;</span> </pre></div> </div> <p><a class="reference internal" href="../c-api/none.html#c.Py_None" title="Py_None"><code class="xref c c-data docutils literal notranslate"><span class="pre">Py_None</span></code></a> is the C name for the special Python object <code class="docutils literal notranslate"><span class="pre">None</span></code>. It is a genuine Python object rather than a <em>NULL</em> pointer, which means “error” in most contexts, as we have seen.</p> </div> <div class="section" id="the-module-s-method-table-and-initialization-function"> <span id="methodtable"></span><h2>1.4. The Module’s Method Table and Initialization Function<a class="headerlink" href="#the-module-s-method-table-and-initialization-function" title="Permalink to this headline">¶</a></h2> <p>I promised to show how <code class="xref c c-func docutils literal notranslate"><span class="pre">spam_system()</span></code> is called from Python programs. First, we need to list its name and address in a “method table”:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="n">PyMethodDef</span> <span class="n">SpamMethods</span><span class="p">[]</span> <span class="o">=</span> <span class="p">{</span> <span class="p">...</span> <span class="p">{</span><span class="s">"system"</span><span class="p">,</span> <span class="n">spam_system</span><span class="p">,</span> <span class="n">METH_VARARGS</span><span class="p">,</span> <span class="s">"Execute a shell command."</span><span class="p">},</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="cm">/* Sentinel */</span> <span class="p">};</span> </pre></div> </div> <p>Note the third entry (<code class="docutils literal notranslate"><span class="pre">METH_VARARGS</span></code>). This is a flag telling the interpreter the calling convention to be used for the C function. It should normally always be <code class="docutils literal notranslate"><span class="pre">METH_VARARGS</span></code> or <code class="docutils literal notranslate"><span class="pre">METH_VARARGS</span> <span class="pre">|</span> <span class="pre">METH_KEYWORDS</span></code>; a value of <code class="docutils literal notranslate"><span class="pre">0</span></code> means that an obsolete variant of <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> is used.</p> <p>When using only <code class="docutils literal notranslate"><span class="pre">METH_VARARGS</span></code>, the function should expect the Python-level parameters to be passed in as a tuple acceptable for parsing via <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>; more information on this function is provided below.</p> <p>The <a class="reference internal" href="../c-api/structures.html#METH_KEYWORDS" title="METH_KEYWORDS"><code class="xref py py-const docutils literal notranslate"><span class="pre">METH_KEYWORDS</span></code></a> bit may be set in the third field if keyword arguments should be passed to the function. In this case, the C function should accept a third <code class="docutils literal notranslate"><span class="pre">PyObject</span> <span class="pre">*</span></code> parameter which will be a dictionary of keywords. Use <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTupleAndKeywords" title="PyArg_ParseTupleAndKeywords"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTupleAndKeywords()</span></code></a> to parse the arguments to such a function.</p> <p>The method table must be passed to the interpreter in the module’s initialization function. The initialization function must be named <code class="xref c c-func docutils literal notranslate"><span class="pre">initname()</span></code>, where <em>name</em> is the name of the module, and should be the only non-<code class="docutils literal notranslate"><span class="pre">static</span></code> item defined in the module file:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyMODINIT_FUNC</span> <span class="nf">initspam</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">Py_InitModule</span><span class="p">(</span><span class="s">"spam"</span><span class="p">,</span> <span class="n">SpamMethods</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>Note that PyMODINIT_FUNC declares the function as <code class="docutils literal notranslate"><span class="pre">void</span></code> return type, declares any special linkage declarations required by the platform, and for C++ declares the function as <code class="docutils literal notranslate"><span class="pre">extern</span> <span class="pre">"C"</span></code>.</p> <p>When the Python program imports module <code class="xref py py-mod docutils literal notranslate"><span class="pre">spam</span></code> for the first time, <code class="xref c c-func docutils literal notranslate"><span class="pre">initspam()</span></code> is called. (See below for comments about embedding Python.) It calls <a class="reference internal" href="../c-api/allocation.html#c.Py_InitModule" title="Py_InitModule"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_InitModule()</span></code></a>, which creates a “module object” (which is inserted in the dictionary <code class="docutils literal notranslate"><span class="pre">sys.modules</span></code> under the key <code class="docutils literal notranslate"><span class="pre">"spam"</span></code>), and inserts built-in function objects into the newly created module based upon the table (an array of <a class="reference internal" href="../c-api/structures.html#c.PyMethodDef" title="PyMethodDef"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyMethodDef</span></code></a> structures) that was passed as its second argument. <a class="reference internal" href="../c-api/allocation.html#c.Py_InitModule" title="Py_InitModule"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_InitModule()</span></code></a> returns a pointer to the module object that it creates (which is unused here). It may abort with a fatal error for certain errors, or return <em>NULL</em> if the module could not be initialized satisfactorily.</p> <p>When embedding Python, the <code class="xref c c-func docutils literal notranslate"><span class="pre">initspam()</span></code> function is not called automatically unless there’s an entry in the <code class="xref c c-data docutils literal notranslate"><span class="pre">_PyImport_Inittab</span></code> table. The easiest way to handle this is to statically initialize your statically-linked modules by directly calling <code class="xref c c-func docutils literal notranslate"><span class="pre">initspam()</span></code> after 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="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="cm">/* Pass argv[0] to the Python interpreter */</span> <span class="n">Py_SetProgramName</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="cm">/* Initialize the Python interpreter. Required. */</span> <span class="n">Py_Initialize</span><span class="p">();</span> <span class="cm">/* Add a static module */</span> <span class="n">initspam</span><span class="p">();</span> <span class="p">...</span> </pre></div> </div> <p>An example may be found in the file <code class="file docutils literal notranslate"><span class="pre">Demo/embed/demo.c</span></code> in the Python source distribution.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">Removing entries from <code class="docutils literal notranslate"><span class="pre">sys.modules</span></code> or importing compiled modules into multiple interpreters within a process (or following a <code class="xref c c-func docutils literal notranslate"><span class="pre">fork()</span></code> without an intervening <code class="xref c c-func docutils literal notranslate"><span class="pre">exec()</span></code>) can create problems for some extension modules. Extension module authors should exercise caution when initializing internal data structures. Note also that the <a class="reference internal" href="../library/functions.html#reload" title="reload"><code class="xref py py-func docutils literal notranslate"><span class="pre">reload()</span></code></a> function can be used with extension modules, and will call the module initialization function (<code class="xref c c-func docutils literal notranslate"><span class="pre">initspam()</span></code> in the example), but will not load the module again if it was loaded from a dynamically loadable object file (<code class="file docutils literal notranslate"><span class="pre">.so</span></code> on Unix, <code class="file docutils literal notranslate"><span class="pre">.dll</span></code> on Windows).</p> </div> <p>A more substantial example module is included in the Python source distribution as <code class="file docutils literal notranslate"><span class="pre">Modules/xxmodule.c</span></code>. This file may be used as a template or simply read as an example.</p> </div> <div class="section" id="compilation-and-linkage"> <span id="compilation"></span><h2>1.5. Compilation and Linkage<a class="headerlink" href="#compilation-and-linkage" title="Permalink to this headline">¶</a></h2> <p>There are two more things to do before you can use your new extension: compiling and linking it with the Python system. If you use dynamic loading, the details may depend on the style of dynamic loading your system uses; see the chapters about building extension modules (chapter <a class="reference internal" href="building.html#building"><span class="std std-ref">Building C and C++ Extensions with distutils</span></a>) and additional information that pertains only to building on Windows (chapter <a class="reference internal" href="windows.html#building-on-windows"><span class="std std-ref">Building C and C++ Extensions on Windows</span></a>) for more information about this.</p> <p>If you can’t use dynamic loading, or if you want to make your module a permanent part of the Python interpreter, you will have to change the configuration setup and rebuild the interpreter. Luckily, this is very simple on Unix: just place your file (<code class="file docutils literal notranslate"><span class="pre">spammodule.c</span></code> for example) in the <code class="file docutils literal notranslate"><span class="pre">Modules/</span></code> directory of an unpacked source distribution, add a line to the file <code class="file docutils literal notranslate"><span class="pre">Modules/Setup.local</span></code> describing your file:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">spam</span> <span class="n">spammodule</span><span class="p">.</span><span class="n">o</span> </pre></div> </div> <p>and rebuild the interpreter by running <strong class="program">make</strong> in the toplevel directory. You can also run <strong class="program">make</strong> in the <code class="file docutils literal notranslate"><span class="pre">Modules/</span></code> subdirectory, but then you must first rebuild <code class="file docutils literal notranslate"><span class="pre">Makefile</span></code> there by running ‘<strong class="program">make</strong> Makefile’. (This is necessary each time you change the <code class="file docutils literal notranslate"><span class="pre">Setup</span></code> file.)</p> <p>If your module requires additional libraries to link with, these can be listed on the line in the configuration file as well, for instance:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">spam</span> <span class="n">spammodule</span><span class="p">.</span><span class="n">o</span> <span class="o">-</span><span class="n">lX11</span> </pre></div> </div> </div> <div class="section" id="calling-python-functions-from-c"> <span id="callingpython"></span><h2>1.6. Calling Python Functions from C<a class="headerlink" href="#calling-python-functions-from-c" title="Permalink to this headline">¶</a></h2> <p>So far we have concentrated on making C functions callable from Python. The reverse is also useful: calling Python functions from C. This is especially the case for libraries that support so-called “callback” functions. If a C interface makes use of callbacks, the equivalent Python often needs to provide a callback mechanism to the Python programmer; the implementation will require calling the Python callback functions from a C callback. Other uses are also imaginable.</p> <p>Fortunately, the Python interpreter is easily called recursively, and there is a standard interface to call a Python function. (I won’t dwell on how to call the Python parser with a particular string as input — if you’re interested, have a look at the implementation of the <a class="reference internal" href="../using/cmdline.html#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></a> command line option in <code class="file docutils literal notranslate"><span class="pre">Modules/main.c</span></code> from the Python source code.)</p> <p>Calling a Python function is easy. First, the Python program must somehow pass you the Python function object. You should provide a function (or some other interface) to do this. When this function is called, save a pointer to the Python function object (be careful to <a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a> it!) in a global variable — or wherever you see fit. For example, the following function might be part of a module definition:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">my_callback</span> <span class="o">=</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">my_set_callback</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">dummy</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="n">PyObject</span> <span class="o">*</span><span class="n">result</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">temp</span><span class="p">;</span> <span class="k">if</span> <span class="p">(</span><span class="n">PyArg_ParseTuple</span><span class="p">(</span><span class="n">args</span><span class="p">,</span> <span class="s">"O:set_callback"</span><span class="p">,</span> <span class="o">&</span><span class="n">temp</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">PyCallable_Check</span><span class="p">(</span><span class="n">temp</span><span class="p">))</span> <span class="p">{</span> <span class="n">PyErr_SetString</span><span class="p">(</span><span class="n">PyExc_TypeError</span><span class="p">,</span> <span class="s">"parameter must be callable"</span><span class="p">);</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="p">}</span> <span class="n">Py_XINCREF</span><span class="p">(</span><span class="n">temp</span><span class="p">);</span> <span class="cm">/* Add a reference to new callback */</span> <span class="n">Py_XDECREF</span><span class="p">(</span><span class="n">my_callback</span><span class="p">);</span> <span class="cm">/* Dispose of previous callback */</span> <span class="n">my_callback</span> <span class="o">=</span> <span class="n">temp</span><span class="p">;</span> <span class="cm">/* Remember new callback */</span> <span class="cm">/* Boilerplate to return "None" */</span> <span class="n">Py_INCREF</span><span class="p">(</span><span class="n">Py_None</span><span class="p">);</span> <span class="n">result</span> <span class="o">=</span> <span class="n">Py_None</span><span class="p">;</span> <span class="p">}</span> <span class="k">return</span> <span class="n">result</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> <p>This function must be registered with the interpreter using the <a class="reference internal" href="../c-api/structures.html#METH_VARARGS" title="METH_VARARGS"><code class="xref py py-const docutils literal notranslate"><span class="pre">METH_VARARGS</span></code></a> flag; this is described in section <a class="reference internal" href="#methodtable"><span class="std std-ref">The Module’s Method Table and Initialization Function</span></a>. The <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> function and its arguments are documented in section <a class="reference internal" href="#parsetuple"><span class="std std-ref">Extracting Parameters in Extension Functions</span></a>.</p> <p>The macros <a class="reference internal" href="../c-api/refcounting.html#c.Py_XINCREF" title="Py_XINCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XINCREF()</span></code></a> and <a class="reference internal" href="../c-api/refcounting.html#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XDECREF()</span></code></a> increment/decrement the reference count of an object and are safe in the presence of <em>NULL</em> pointers (but note that <em>temp</em> will not be <em>NULL</em> in this context). More info on them in section <a class="reference internal" href="#refcounts"><span class="std std-ref">Reference Counts</span></a>.</p> <p id="index-0">Later, when it is time to call the function, you call the C function <a class="reference internal" href="../c-api/object.html#c.PyObject_CallObject" title="PyObject_CallObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallObject()</span></code></a>. This function has two arguments, both pointers to arbitrary Python objects: the Python function, and the argument list. The argument list must always be a tuple object, whose length is the number of arguments. To call the Python function with no arguments, pass in NULL, or an empty tuple; to call it with one argument, pass a singleton tuple. <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a> returns a tuple when its format string consists of zero or more format codes between parentheses. For example:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span> <span class="n">arg</span><span class="p">;</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">arglist</span><span class="p">;</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">result</span><span class="p">;</span> <span class="p">...</span> <span class="n">arg</span> <span class="o">=</span> <span class="mi">123</span><span class="p">;</span> <span class="p">...</span> <span class="cm">/* Time to call the callback */</span> <span class="n">arglist</span> <span class="o">=</span> <span class="n">Py_BuildValue</span><span class="p">(</span><span class="s">"(i)"</span><span class="p">,</span> <span class="n">arg</span><span class="p">);</span> <span class="n">result</span> <span class="o">=</span> <span class="n">PyObject_CallObject</span><span class="p">(</span><span class="n">my_callback</span><span class="p">,</span> <span class="n">arglist</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">arglist</span><span class="p">);</span> </pre></div> </div> <p><a class="reference internal" href="../c-api/object.html#c.PyObject_CallObject" title="PyObject_CallObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallObject()</span></code></a> returns a Python object pointer: this is the return value of the Python function. <a class="reference internal" href="../c-api/object.html#c.PyObject_CallObject" title="PyObject_CallObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallObject()</span></code></a> is “reference-count-neutral” with respect to its arguments. In the example a new tuple was created to serve as the argument list, which is <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>-ed immediately after the <a class="reference internal" href="../c-api/object.html#c.PyObject_CallObject" title="PyObject_CallObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallObject()</span></code></a> call.</p> <p>The return value of <a class="reference internal" href="../c-api/object.html#c.PyObject_CallObject" title="PyObject_CallObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallObject()</span></code></a> is “new”: either it is a brand new object, or it is an existing object whose reference count has been incremented. So, unless you want to save it in a global variable, you should somehow <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> the result, even (especially!) if you are not interested in its value.</p> <p>Before you do this, however, it is important to check that the return value isn’t <em>NULL</em>. If it is, the Python function terminated by raising an exception. If the C code that called <a class="reference internal" href="../c-api/object.html#c.PyObject_CallObject" title="PyObject_CallObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallObject()</span></code></a> is called from Python, it should now return an error indication to its Python caller, so the interpreter can print a stack trace, or the calling Python code can handle the exception. If this is not possible or desirable, the exception should be cleared by calling <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_Clear" title="PyErr_Clear"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_Clear()</span></code></a>. For example:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="p">(</span><span class="n">result</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="cm">/* Pass error back */</span> <span class="p">...</span><span class="n">use</span> <span class="n">result</span><span class="p">...</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">result</span><span class="p">);</span> </pre></div> </div> <p>Depending on the desired interface to the Python callback function, you may also have to provide an argument list to <a class="reference internal" href="../c-api/object.html#c.PyObject_CallObject" title="PyObject_CallObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallObject()</span></code></a>. In some cases the argument list is also provided by the Python program, through the same interface that specified the callback function. It can then be saved and used in the same manner as the function object. In other cases, you may have to construct a new tuple to pass as the argument list. The simplest way to do this is to call <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a>. For example, if you want to pass an integral event code, you might use the following code:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyObject</span> <span class="o">*</span><span class="n">arglist</span><span class="p">;</span> <span class="p">...</span> <span class="n">arglist</span> <span class="o">=</span> <span class="n">Py_BuildValue</span><span class="p">(</span><span class="s">"(l)"</span><span class="p">,</span> <span class="n">eventcode</span><span class="p">);</span> <span class="n">result</span> <span class="o">=</span> <span class="n">PyObject_CallObject</span><span class="p">(</span><span class="n">my_callback</span><span class="p">,</span> <span class="n">arglist</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">arglist</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">result</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="cm">/* Pass error back */</span> <span class="cm">/* Here maybe use the result */</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">result</span><span class="p">);</span> </pre></div> </div> <p>Note the placement of <code class="docutils literal notranslate"><span class="pre">Py_DECREF(arglist)</span></code> immediately after the call, before the error check! Also note that strictly speaking this code is not complete: <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a> may run out of memory, and this should be checked.</p> <p>You may also call a function with keyword arguments by using <a class="reference internal" href="../c-api/object.html#c.PyObject_Call" title="PyObject_Call"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Call()</span></code></a>, which supports arguments and keyword arguments. As in the above example, we use <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a> to construct the dictionary.</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyObject</span> <span class="o">*</span><span class="n">dict</span><span class="p">;</span> <span class="p">...</span> <span class="n">dict</span> <span class="o">=</span> <span class="n">Py_BuildValue</span><span class="p">(</span><span class="s">"{s:i}"</span><span class="p">,</span> <span class="s">"name"</span><span class="p">,</span> <span class="n">val</span><span class="p">);</span> <span class="n">result</span> <span class="o">=</span> <span class="n">PyObject_Call</span><span class="p">(</span><span class="n">my_callback</span><span class="p">,</span> <span class="nb">NULL</span><span class="p">,</span> <span class="n">dict</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">dict</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">result</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="cm">/* Pass error back */</span> <span class="cm">/* Here maybe use the result */</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">result</span><span class="p">);</span> </pre></div> </div> </div> <div class="section" id="extracting-parameters-in-extension-functions"> <span id="parsetuple"></span><h2>1.7. Extracting Parameters in Extension Functions<a class="headerlink" href="#extracting-parameters-in-extension-functions" title="Permalink to this headline">¶</a></h2> <p id="index-1">The <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> function is declared as follows:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span> <span class="nf">PyArg_ParseTuple</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">arg</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">format</span><span class="p">,</span> <span class="p">...);</span> </pre></div> </div> <p>The <em>arg</em> argument must be a tuple object containing an argument list passed from Python to a C function. The <em>format</em> argument must be a format string, whose syntax is explained in <a class="reference internal" href="../c-api/arg.html#arg-parsing"><span class="std std-ref">Parsing arguments and building values</span></a> in the Python/C API Reference Manual. The remaining arguments must be addresses of variables whose type is determined by the format string.</p> <p>Note that while <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> checks that the Python arguments have the required types, it cannot check the validity of the addresses of C variables passed to the call: if you make mistakes there, your code will probably crash or at least overwrite random bits in memory. So be careful!</p> <p>Note that any Python object references which are provided to the caller are <em>borrowed</em> references; do not decrement their reference count!</p> <p>Some example calls:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span> <span class="n">ok</span><span class="p">;</span> <span class="kt">int</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">;</span> <span class="kt">long</span> <span class="n">k</span><span class="p">,</span> <span class="n">l</span><span class="p">;</span> <span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">s</span><span class="p">;</span> <span class="kt">int</span> <span class="n">size</span><span class="p">;</span> <span class="n">ok</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">""</span><span class="p">);</span> <span class="cm">/* No arguments */</span> <span class="cm">/* Python call: f() */</span> </pre></div> </div> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">ok</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">"s"</span><span class="p">,</span> <span class="o">&</span><span class="n">s</span><span class="p">);</span> <span class="cm">/* A string */</span> <span class="cm">/* Possible Python call: f('whoops!') */</span> </pre></div> </div> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">ok</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">"lls"</span><span class="p">,</span> <span class="o">&</span><span class="n">k</span><span class="p">,</span> <span class="o">&</span><span class="n">l</span><span class="p">,</span> <span class="o">&</span><span class="n">s</span><span class="p">);</span> <span class="cm">/* Two longs and a string */</span> <span class="cm">/* Possible Python call: f(1, 2, 'three') */</span> </pre></div> </div> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">ok</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">"(ii)s#"</span><span class="p">,</span> <span class="o">&</span><span class="n">i</span><span class="p">,</span> <span class="o">&</span><span class="n">j</span><span class="p">,</span> <span class="o">&</span><span class="n">s</span><span class="p">,</span> <span class="o">&</span><span class="n">size</span><span class="p">);</span> <span class="cm">/* A pair of ints and a string, whose size is also returned */</span> <span class="cm">/* Possible Python call: f((1, 2), 'three') */</span> </pre></div> </div> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="p">{</span> <span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">file</span><span class="p">;</span> <span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">mode</span> <span class="o">=</span> <span class="s">"r"</span><span class="p">;</span> <span class="kt">int</span> <span class="n">bufsize</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">ok</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">"s|si"</span><span class="p">,</span> <span class="o">&</span><span class="n">file</span><span class="p">,</span> <span class="o">&</span><span class="n">mode</span><span class="p">,</span> <span class="o">&</span><span class="n">bufsize</span><span class="p">);</span> <span class="cm">/* A string, and optionally another string and an integer */</span> <span class="cm">/* Possible Python calls:</span> <span class="cm"> f('spam')</span> <span class="cm"> f('spam', 'w')</span> <span class="cm"> f('spam', 'wb', 100000) */</span> <span class="p">}</span> </pre></div> </div> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="p">{</span> <span class="kt">int</span> <span class="n">left</span><span class="p">,</span> <span class="n">top</span><span class="p">,</span> <span class="n">right</span><span class="p">,</span> <span class="n">bottom</span><span class="p">,</span> <span class="n">h</span><span class="p">,</span> <span class="n">v</span><span class="p">;</span> <span class="n">ok</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">"((ii)(ii))(ii)"</span><span class="p">,</span> <span class="o">&</span><span class="n">left</span><span class="p">,</span> <span class="o">&</span><span class="n">top</span><span class="p">,</span> <span class="o">&</span><span class="n">right</span><span class="p">,</span> <span class="o">&</span><span class="n">bottom</span><span class="p">,</span> <span class="o">&</span><span class="n">h</span><span class="p">,</span> <span class="o">&</span><span class="n">v</span><span class="p">);</span> <span class="cm">/* A rectangle and a point */</span> <span class="cm">/* Possible Python call:</span> <span class="cm"> f(((0, 0), (400, 300)), (10, 10)) */</span> <span class="p">}</span> </pre></div> </div> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="p">{</span> <span class="n">Py_complex</span> <span class="n">c</span><span class="p">;</span> <span class="n">ok</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">"D:myfunction"</span><span class="p">,</span> <span class="o">&</span><span class="n">c</span><span class="p">);</span> <span class="cm">/* a complex, also providing a function name for errors */</span> <span class="cm">/* Possible Python call: myfunction(1+2j) */</span> <span class="p">}</span> </pre></div> </div> </div> <div class="section" id="keyword-parameters-for-extension-functions"> <span id="parsetupleandkeywords"></span><h2>1.8. Keyword Parameters for Extension Functions<a class="headerlink" href="#keyword-parameters-for-extension-functions" title="Permalink to this headline">¶</a></h2> <p id="index-2">The <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTupleAndKeywords" title="PyArg_ParseTupleAndKeywords"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTupleAndKeywords()</span></code></a> function is declared as follows:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span> <span class="nf">PyArg_ParseTupleAndKeywords</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">arg</span><span class="p">,</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">kwdict</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">format</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">kwlist</span><span class="p">[],</span> <span class="p">...);</span> </pre></div> </div> <p>The <em>arg</em> and <em>format</em> parameters are identical to those of the <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> function. The <em>kwdict</em> parameter is the dictionary of keywords received as the third parameter from the Python runtime. The <em>kwlist</em> parameter is a <em>NULL</em>-terminated list of strings which identify the parameters; the names are matched with the type information from <em>format</em> from left to right. On success, <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTupleAndKeywords" title="PyArg_ParseTupleAndKeywords"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTupleAndKeywords()</span></code></a> returns true, otherwise it returns false and raises an appropriate exception.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">Nested tuples cannot be parsed when using keyword arguments! Keyword parameters passed in which are not present in the <em>kwlist</em> will cause <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> to be raised.</p> </div> <p id="index-3">Here is an example module which uses keywords, based on an example by Geoff Philbrick (<a class="reference external" href="mailto:philbrick%40hks.com">philbrick<span>@</span>hks<span>.</span>com</a>):</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="k">static</span> <span class="n">PyObject</span> <span class="o">*</span> <span class="nf">keywdarg_parrot</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="n">PyObject</span> <span class="o">*</span><span class="n">keywds</span><span class="p">)</span> <span class="p">{</span> <span class="kt">int</span> <span class="n">voltage</span><span class="p">;</span> <span class="kt">char</span> <span class="o">*</span><span class="n">state</span> <span class="o">=</span> <span class="s">"a stiff"</span><span class="p">;</span> <span class="kt">char</span> <span class="o">*</span><span class="n">action</span> <span class="o">=</span> <span class="s">"voom"</span><span class="p">;</span> <span class="kt">char</span> <span class="o">*</span><span class="n">type</span> <span class="o">=</span> <span class="s">"Norwegian Blue"</span><span class="p">;</span> <span class="k">static</span> <span class="kt">char</span> <span class="o">*</span><span class="n">kwlist</span><span class="p">[]</span> <span class="o">=</span> <span class="p">{</span><span class="s">"voltage"</span><span class="p">,</span> <span class="s">"state"</span><span class="p">,</span> <span class="s">"action"</span><span class="p">,</span> <span class="s">"type"</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="o">!</span><span class="n">PyArg_ParseTupleAndKeywords</span><span class="p">(</span><span class="n">args</span><span class="p">,</span> <span class="n">keywds</span><span class="p">,</span> <span class="s">"i|sss"</span><span class="p">,</span> <span class="n">kwlist</span><span class="p">,</span> <span class="o">&</span><span class="n">voltage</span><span class="p">,</span> <span class="o">&</span><span class="n">state</span><span class="p">,</span> <span class="o">&</span><span class="n">action</span><span class="p">,</span> <span class="o">&</span><span class="n">type</span><span class="p">))</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="n">printf</span><span class="p">(</span><span class="s">"-- This parrot wouldn't %s if you put %i Volts through it.</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">action</span><span class="p">,</span> <span class="n">voltage</span><span class="p">);</span> <span class="n">printf</span><span class="p">(</span><span class="s">"-- Lovely plumage, the %s -- It's %s!</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">type</span><span class="p">,</span> <span class="n">state</span><span class="p">);</span> <span class="n">Py_INCREF</span><span class="p">(</span><span class="n">Py_None</span><span class="p">);</span> <span class="k">return</span> <span class="n">Py_None</span><span class="p">;</span> <span class="p">}</span> <span class="k">static</span> <span class="n">PyMethodDef</span> <span class="n">keywdarg_methods</span><span class="p">[]</span> <span class="o">=</span> <span class="p">{</span> <span class="cm">/* The cast of the function is necessary since PyCFunction values</span> <span class="cm"> * only take two PyObject* parameters, and keywdarg_parrot() takes</span> <span class="cm"> * three.</span> <span class="cm"> */</span> <span class="p">{</span><span class="s">"parrot"</span><span class="p">,</span> <span class="p">(</span><span class="n">PyCFunction</span><span class="p">)</span><span class="n">keywdarg_parrot</span><span class="p">,</span> <span class="n">METH_VARARGS</span> <span class="o">|</span> <span class="n">METH_KEYWORDS</span><span class="p">,</span> <span class="s">"Print a lovely skit to standard output."</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="cm">/* sentinel */</span> <span class="p">};</span> </pre></div> </div> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">initkeywdarg</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* Create the module and add the functions */</span> <span class="n">Py_InitModule</span><span class="p">(</span><span class="s">"keywdarg"</span><span class="p">,</span> <span class="n">keywdarg_methods</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> </div> <div class="section" id="building-arbitrary-values"> <span id="buildvalue"></span><h2>1.9. Building Arbitrary Values<a class="headerlink" href="#building-arbitrary-values" title="Permalink to this headline">¶</a></h2> <p>This function is the counterpart to <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>. It is declared as follows:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyObject</span> <span class="o">*</span><span class="nf">Py_BuildValue</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">format</span><span class="p">,</span> <span class="p">...);</span> </pre></div> </div> <p>It recognizes a set of format units similar to the ones recognized by <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>, but the arguments (which are input to the function, not output) must not be pointers, just values. It returns a new Python object, suitable for returning from a C function called from Python.</p> <p>One difference with <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>: while the latter requires its first argument to be a tuple (since Python argument lists are always represented as tuples internally), <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a> does not always build a tuple. It builds a tuple only if its format string contains two or more format units. If the format string is empty, it returns <code class="docutils literal notranslate"><span class="pre">None</span></code>; if it contains exactly one format unit, it returns whatever object is described by that format unit. To force it to return a tuple of size 0 or one, parenthesize the format string.</p> <p>Examples (to the left the call, to the right the resulting Python value):</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>Py_BuildValue("") None Py_BuildValue("i", 123) 123 Py_BuildValue("iii", 123, 456, 789) (123, 456, 789) Py_BuildValue("s", "hello") 'hello' Py_BuildValue("ss", "hello", "world") ('hello', 'world') Py_BuildValue("s#", "hello", 4) 'hell' Py_BuildValue("()") () Py_BuildValue("(i)", 123) (123,) Py_BuildValue("(ii)", 123, 456) (123, 456) Py_BuildValue("(i,i)", 123, 456) (123, 456) Py_BuildValue("[i,i]", 123, 456) [123, 456] Py_BuildValue("{s:i,s:i}", "abc", 123, "def", 456) {'abc': 123, 'def': 456} Py_BuildValue("((ii)(ii)) (ii)", 1, 2, 3, 4, 5, 6) (((1, 2), (3, 4)), (5, 6)) </pre></div> </div> </div> <div class="section" id="reference-counts"> <span id="refcounts"></span><h2>1.10. Reference Counts<a class="headerlink" href="#reference-counts" title="Permalink to this headline">¶</a></h2> <p>In languages like C or C++, the programmer is responsible for dynamic allocation and deallocation of memory on the heap. In C, this is done using the functions <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code>. In C++, the operators <code class="docutils literal notranslate"><span class="pre">new</span></code> and <code class="docutils literal notranslate"><span class="pre">delete</span></code> are used with essentially the same meaning and we’ll restrict the following discussion to the C case.</p> <p>Every block of memory allocated with <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> should eventually be returned to the pool of available memory by exactly one call to <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code>. It is important to call <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> at the right time. If a block’s address is forgotten but <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> is not called for it, the memory it occupies cannot be reused until the program terminates. This is called a <em class="dfn">memory leak</em>. On the other hand, if a program calls <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> for a block and then continues to use the block, it creates a conflict with re-use of the block through another <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> call. This is called <em class="dfn">using freed memory</em>. It has the same bad consequences as referencing uninitialized data — core dumps, wrong results, mysterious crashes.</p> <p>Common causes of memory leaks are unusual paths through the code. For instance, a function may allocate a block of memory, do some calculation, and then free the block again. Now a change in the requirements for the function may add a test to the calculation that detects an error condition and can return prematurely from the function. It’s easy to forget to free the allocated memory block when taking this premature exit, especially when it is added later to the code. Such leaks, once introduced, often go undetected for a long time: the error exit is taken only in a small fraction of all calls, and most modern machines have plenty of virtual memory, so the leak only becomes apparent in a long-running process that uses the leaking function frequently. Therefore, it’s important to prevent leaks from happening by having a coding convention or strategy that minimizes this kind of errors.</p> <p>Since Python makes heavy use of <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code>, it needs a strategy to avoid memory leaks as well as the use of freed memory. The chosen method is called <em class="dfn">reference counting</em>. The principle is simple: every object contains a counter, which is incremented when a reference to the object is stored somewhere, and which is decremented when a reference to it is deleted. When the counter reaches zero, the last reference to the object has been deleted and the object is freed.</p> <p>An alternative strategy is called <em class="dfn">automatic garbage collection</em>. (Sometimes, reference counting is also referred to as a garbage collection strategy, hence my use of “automatic” to distinguish the two.) The big advantage of automatic garbage collection is that the user doesn’t need to call <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> explicitly. (Another claimed advantage is an improvement in speed or memory usage — this is no hard fact however.) The disadvantage is that for C, there is no truly portable automatic garbage collector, while reference counting can be implemented portably (as long as the functions <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> are available — which the C Standard guarantees). Maybe some day a sufficiently portable automatic garbage collector will be available for C. Until then, we’ll have to live with reference counts.</p> <p>While Python uses the traditional reference counting implementation, it also offers a cycle detector that works to detect reference cycles. This allows applications to not worry about creating direct or indirect circular references; these are the weakness of garbage collection implemented using only reference counting. Reference cycles consist of objects which contain (possibly indirect) references to themselves, so that each object in the cycle has a reference count which is non-zero. Typical reference counting implementations are not able to reclaim the memory belonging to any objects in a reference cycle, or referenced from the objects in the cycle, even though there are no further references to the cycle itself.</p> <p>The cycle detector is able to detect garbage cycles and can reclaim them so long as there are no finalizers implemented in Python (<a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods). When there are such finalizers, the detector exposes the cycles through the <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module (specifically, the <a class="reference internal" href="../library/gc.html#gc.garbage" title="gc.garbage"><code class="xref py py-attr docutils literal notranslate"><span class="pre">garbage</span></code></a> variable in that module). The <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module also exposes a way to run the detector (the <a class="reference internal" href="../library/gc.html#gc.collect" title="gc.collect"><code class="xref py py-func docutils literal notranslate"><span class="pre">collect()</span></code></a> function), as well as configuration interfaces and the ability to disable the detector at runtime. The cycle detector is considered an optional component; though it is included by default, it can be disabled at build time using the <code class="xref std std-option docutils literal notranslate"><span class="pre">--without-cycle-gc</span></code> option to the <strong class="program">configure</strong> script on Unix platforms (including Mac OS X) or by removing the definition of <code class="docutils literal notranslate"><span class="pre">WITH_CYCLE_GC</span></code> in the <code class="file docutils literal notranslate"><span class="pre">pyconfig.h</span></code> header on other platforms. If the cycle detector is disabled in this way, the <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module will not be available.</p> <div class="section" id="reference-counting-in-python"> <span id="refcountsinpython"></span><h3>1.10.1. Reference Counting in Python<a class="headerlink" href="#reference-counting-in-python" title="Permalink to this headline">¶</a></h3> <p>There are two macros, <code class="docutils literal notranslate"><span class="pre">Py_INCREF(x)</span></code> and <code class="docutils literal notranslate"><span class="pre">Py_DECREF(x)</span></code>, which handle the incrementing and decrementing of the reference count. <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> also frees the object when the count reaches zero. For flexibility, it doesn’t call <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> directly — rather, it makes a call through a function pointer in the object’s <em class="dfn">type object</em>. For this purpose (and others), every object also contains a pointer to its type object.</p> <p>The big question now remains: when to use <code class="docutils literal notranslate"><span class="pre">Py_INCREF(x)</span></code> and <code class="docutils literal notranslate"><span class="pre">Py_DECREF(x)</span></code>? Let’s first introduce some terms. Nobody “owns” an object; however, you can <em class="dfn">own a reference</em> to an object. An object’s reference count is now defined as the number of owned references to it. The owner of a reference is responsible for calling <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> when the reference is no longer needed. Ownership of a reference can be transferred. There are three ways to dispose of an owned reference: pass it on, store it, or call <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>. Forgetting to dispose of an owned reference creates a memory leak.</p> <p>It is also possible to <em class="dfn">borrow</em> <a class="footnote-reference" href="#id6" id="id2">[2]</a> a reference to an object. The borrower of a reference should not call <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>. The borrower must not hold on to the object longer than the owner from which it was borrowed. Using a borrowed reference after the owner has disposed of it risks using freed memory and should be avoided completely <a class="footnote-reference" href="#id7" id="id3">[3]</a>.</p> <p>The advantage of borrowing over owning a reference is that you don’t need to take care of disposing of the reference on all possible paths through the code — in other words, with a borrowed reference you don’t run the risk of leaking when a premature exit is taken. The disadvantage of borrowing over owning is that there are some subtle situations where in seemingly correct code a borrowed reference can be used after the owner from which it was borrowed has in fact disposed of it.</p> <p>A borrowed reference can be changed into an owned reference by calling <a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a>. This does not affect the status of the owner from which the reference was borrowed — it creates a new owned reference, and gives full owner responsibilities (the new owner must dispose of the reference properly, as well as the previous owner).</p> </div> <div class="section" id="ownership-rules"> <span id="ownershiprules"></span><h3>1.10.2. Ownership Rules<a class="headerlink" href="#ownership-rules" title="Permalink to this headline">¶</a></h3> <p>Whenever an object reference is passed into or out of a function, it is part of the function’s interface specification whether ownership is transferred with the reference or not.</p> <p>Most functions that return a reference to an object pass on ownership with the reference. In particular, all functions whose function it is to create a new object, such as <a class="reference internal" href="../c-api/int.html#c.PyInt_FromLong" title="PyInt_FromLong"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyInt_FromLong()</span></code></a> and <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a>, pass ownership to the receiver. Even if the object is not actually new, you still receive ownership of a new reference to that object. For instance, <a class="reference internal" href="../c-api/int.html#c.PyInt_FromLong" title="PyInt_FromLong"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyInt_FromLong()</span></code></a> maintains a cache of popular values and can return a reference to a cached item.</p> <p>Many functions that extract objects from other objects also transfer ownership with the reference, for instance <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>. The picture is less clear, here, however, since a few common routines are exceptions: <a class="reference internal" href="../c-api/tuple.html#c.PyTuple_GetItem" title="PyTuple_GetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyTuple_GetItem()</span></code></a>, <a class="reference internal" href="../c-api/list.html#c.PyList_GetItem" title="PyList_GetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyList_GetItem()</span></code></a>, <a class="reference internal" href="../c-api/dict.html#c.PyDict_GetItem" title="PyDict_GetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyDict_GetItem()</span></code></a>, and <a class="reference internal" href="../c-api/dict.html#c.PyDict_GetItemString" title="PyDict_GetItemString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyDict_GetItemString()</span></code></a> all return references that you borrow from the tuple, list or dictionary.</p> <p>The function <a class="reference internal" href="../c-api/import.html#c.PyImport_AddModule" title="PyImport_AddModule"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyImport_AddModule()</span></code></a> also returns a borrowed reference, even though it may actually create the object it returns: this is possible because an owned reference to the object is stored in <code class="docutils literal notranslate"><span class="pre">sys.modules</span></code>.</p> <p>When you pass an object reference into another function, in general, the function borrows the reference from you — if it needs to store it, it will use <a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a> to become an independent owner. There are exactly two important exceptions to this rule: <a class="reference internal" href="../c-api/tuple.html#c.PyTuple_SetItem" title="PyTuple_SetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyTuple_SetItem()</span></code></a> and <a class="reference internal" href="../c-api/list.html#c.PyList_SetItem" title="PyList_SetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyList_SetItem()</span></code></a>. These functions take over ownership of the item passed to them — even if they fail! (Note that <a class="reference internal" href="../c-api/dict.html#c.PyDict_SetItem" title="PyDict_SetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyDict_SetItem()</span></code></a> and friends don’t take over ownership — they are “normal.”)</p> <p>When a C function is called from Python, it borrows references to its arguments from the caller. The caller owns a reference to the object, so the borrowed reference’s lifetime is guaranteed until the function returns. Only when such a borrowed reference must be stored or passed on, it must be turned into an owned reference by calling <a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a>.</p> <p>The object reference returned from a C function that is called from Python must be an owned reference — ownership is transferred from the function to its caller.</p> </div> <div class="section" id="thin-ice"> <span id="thinice"></span><h3>1.10.3. Thin Ice<a class="headerlink" href="#thin-ice" title="Permalink to this headline">¶</a></h3> <p>There are a few situations where seemingly harmless use of a borrowed reference can lead to problems. These all have to do with implicit invocations of the interpreter, which can cause the owner of a reference to dispose of it.</p> <p>The first and most important case to know about is using <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> on an unrelated object while borrowing a reference to a list item. For instance:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">bug</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">list</span><span class="p">)</span> <span class="p">{</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">item</span> <span class="o">=</span> <span class="n">PyList_GetItem</span><span class="p">(</span><span class="n">list</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span> <span class="n">PyList_SetItem</span><span class="p">(</span><span class="n">list</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="n">PyInt_FromLong</span><span class="p">(</span><span class="mi">0L</span><span class="p">));</span> <span class="n">PyObject_Print</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="n">stdout</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span> <span class="cm">/* BUG! */</span> <span class="p">}</span> </pre></div> </div> <p>This function first borrows a reference to <code class="docutils literal notranslate"><span class="pre">list[0]</span></code>, then replaces <code class="docutils literal notranslate"><span class="pre">list[1]</span></code> with the value <code class="docutils literal notranslate"><span class="pre">0</span></code>, and finally prints the borrowed reference. Looks harmless, right? But it’s not!</p> <p>Let’s follow the control flow into <a class="reference internal" href="../c-api/list.html#c.PyList_SetItem" title="PyList_SetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyList_SetItem()</span></code></a>. The list owns references to all its items, so when item 1 is replaced, it has to dispose of the original item 1. Now let’s suppose the original item 1 was an instance of a user-defined class, and let’s further suppose that the class defined a <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method. If this class instance has a reference count of 1, disposing of it will call its <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method.</p> <p>Since it is written in Python, the <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method can execute arbitrary Python code. Could it perhaps do something to invalidate the reference to <code class="docutils literal notranslate"><span class="pre">item</span></code> in <code class="xref c c-func docutils literal notranslate"><span class="pre">bug()</span></code>? You bet! Assuming that the list passed into <code class="xref c c-func docutils literal notranslate"><span class="pre">bug()</span></code> is accessible to the <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method, it could execute a statement to the effect of <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">list[0]</span></code>, and assuming this was the last reference to that object, it would free the memory associated with it, thereby invalidating <code class="docutils literal notranslate"><span class="pre">item</span></code>.</p> <p>The solution, once you know the source of the problem, is easy: temporarily increment the reference count. The correct version of the function reads:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">no_bug</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">list</span><span class="p">)</span> <span class="p">{</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">item</span> <span class="o">=</span> <span class="n">PyList_GetItem</span><span class="p">(</span><span class="n">list</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span> <span class="n">Py_INCREF</span><span class="p">(</span><span class="n">item</span><span class="p">);</span> <span class="n">PyList_SetItem</span><span class="p">(</span><span class="n">list</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="n">PyInt_FromLong</span><span class="p">(</span><span class="mi">0L</span><span class="p">));</span> <span class="n">PyObject_Print</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="n">stdout</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">item</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>This is a true story. An older version of Python contained variants of this bug and someone spent a considerable amount of time in a C debugger to figure out why his <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods would fail…</p> <p>The second case of problems with a borrowed reference is a variant involving threads. Normally, multiple threads in the Python interpreter can’t get in each other’s way, because there is a global lock protecting Python’s entire object space. However, it is possible to temporarily release this lock using the macro <a class="reference internal" href="../c-api/init.html#c.Py_BEGIN_ALLOW_THREADS" title="Py_BEGIN_ALLOW_THREADS"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_BEGIN_ALLOW_THREADS</span></code></a>, and to re-acquire it using <a class="reference internal" href="../c-api/init.html#c.Py_END_ALLOW_THREADS" title="Py_END_ALLOW_THREADS"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_END_ALLOW_THREADS</span></code></a>. This is common around blocking I/O calls, to let other threads use the processor while waiting for the I/O to complete. Obviously, the following function has the same problem as the previous one:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">bug</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">list</span><span class="p">)</span> <span class="p">{</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">item</span> <span class="o">=</span> <span class="n">PyList_GetItem</span><span class="p">(</span><span class="n">list</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span> <span class="n">Py_BEGIN_ALLOW_THREADS</span> <span class="p">...</span><span class="n">some</span> <span class="n">blocking</span> <span class="n">I</span><span class="o">/</span><span class="n">O</span> <span class="n">call</span><span class="p">...</span> <span class="n">Py_END_ALLOW_THREADS</span> <span class="n">PyObject_Print</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="n">stdout</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span> <span class="cm">/* BUG! */</span> <span class="p">}</span> </pre></div> </div> </div> <div class="section" id="null-pointers"> <span id="nullpointers"></span><h3>1.10.4. NULL Pointers<a class="headerlink" href="#null-pointers" title="Permalink to this headline">¶</a></h3> <p>In general, functions that take object references as arguments do not expect you to pass them <em>NULL</em> pointers, and will dump core (or cause later core dumps) if you do so. Functions that return object references generally return <em>NULL</em> only to indicate that an exception occurred. The reason for not testing for <em>NULL</em> arguments is that functions often pass the objects they receive on to other function — if each function were to test for <em>NULL</em>, there would be a lot of redundant tests and the code would run more slowly.</p> <p>It is better to test for <em>NULL</em> only at the “source:” when a pointer that may be <em>NULL</em> is received, for example, from <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> or from a function that may raise an exception.</p> <p>The macros <a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a> and <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> do not check for <em>NULL</em> pointers — however, their variants <a class="reference internal" href="../c-api/refcounting.html#c.Py_XINCREF" title="Py_XINCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XINCREF()</span></code></a> and <a class="reference internal" href="../c-api/refcounting.html#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XDECREF()</span></code></a> do.</p> <p>The macros for checking for a particular object type (<code class="docutils literal notranslate"><span class="pre">Pytype_Check()</span></code>) don’t check for <em>NULL</em> pointers — again, there is much code that calls several of these in a row to test an object against various different expected types, and this would generate redundant tests. There are no variants with <em>NULL</em> checking.</p> <p>The C function calling mechanism guarantees that the argument list passed to C functions (<code class="docutils literal notranslate"><span class="pre">args</span></code> in the examples) is never <em>NULL</em> — in fact it guarantees that it is always a tuple <a class="footnote-reference" href="#id8" id="id4">[4]</a>.</p> <p>It is a severe error to ever let a <em>NULL</em> pointer “escape” to the Python user.</p> </div> </div> <div class="section" id="writing-extensions-in-c"> <span id="cplusplus"></span><h2>1.11. Writing Extensions in C++<a class="headerlink" href="#writing-extensions-in-c" title="Permalink to this headline">¶</a></h2> <p>It is possible to write extension modules in C++. Some restrictions apply. If the main program (the Python interpreter) is compiled and linked by the C compiler, global or static objects with constructors cannot be used. This is not a problem if the main program is linked by the C++ compiler. Functions that will be called by the Python interpreter (in particular, module initialization functions) have to be declared using <code class="docutils literal notranslate"><span class="pre">extern</span> <span class="pre">"C"</span></code>. It is unnecessary to enclose the Python header files in <code class="docutils literal notranslate"><span class="pre">extern</span> <span class="pre">"C"</span> <span class="pre">{...}</span></code> — they use this form already if the symbol <code class="docutils literal notranslate"><span class="pre">__cplusplus</span></code> is defined (all recent C++ compilers define this symbol).</p> </div> <div class="section" id="providing-a-c-api-for-an-extension-module"> <span id="using-capsules"></span><h2>1.12. Providing a C API for an Extension Module<a class="headerlink" href="#providing-a-c-api-for-an-extension-module" title="Permalink to this headline">¶</a></h2> <p>Many extension modules just provide new functions and types to be used from Python, but sometimes the code in an extension module can be useful for other extension modules. For example, an extension module could implement a type “collection” which works like lists without order. Just like the standard Python list type has a C API which permits extension modules to create and manipulate lists, this new collection type should have a set of C functions for direct manipulation from other extension modules.</p> <p>At first sight this seems easy: just write the functions (without declaring them <code class="docutils literal notranslate"><span class="pre">static</span></code>, of course), provide an appropriate header file, and document the C API. And in fact this would work if all extension modules were always linked statically with the Python interpreter. When modules are used as shared libraries, however, the symbols defined in one module may not be visible to another module. The details of visibility depend on the operating system; some systems use one global namespace for the Python interpreter and all extension modules (Windows, for example), whereas others require an explicit list of imported symbols at module link time (AIX is one example), or offer a choice of different strategies (most Unices). And even if symbols are globally visible, the module whose functions one wishes to call might not have been loaded yet!</p> <p>Portability therefore requires not to make any assumptions about symbol visibility. This means that all symbols in extension modules should be declared <code class="docutils literal notranslate"><span class="pre">static</span></code>, except for the module’s initialization function, in order to avoid name clashes with other extension modules (as discussed in section <a class="reference internal" href="#methodtable"><span class="std std-ref">The Module’s Method Table and Initialization Function</span></a>). And it means that symbols that <em>should</em> be accessible from other extension modules must be exported in a different way.</p> <p>Python provides a special mechanism to pass C-level information (pointers) from one extension module to another one: Capsules. A Capsule is a Python data type which stores a pointer (<code class="xref c c-type docutils literal notranslate"><span class="pre">void</span> <span class="pre">*</span></code>). Capsules can only be created and accessed via their C API, but they can be passed around like any other Python object. In particular, they can be assigned to a name in an extension module’s namespace. Other extension modules can then import this module, retrieve the value of this name, and then retrieve the pointer from the Capsule.</p> <p>There are many ways in which Capsules can be used to export the C API of an extension module. Each function could get its own Capsule, or all C API pointers could be stored in an array whose address is published in a Capsule. And the various tasks of storing and retrieving the pointers can be distributed in different ways between the module providing the code and the client modules.</p> <p>Whichever method you choose, it’s important to name your Capsules properly. The function <a class="reference internal" href="../c-api/capsule.html#c.PyCapsule_New" title="PyCapsule_New"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyCapsule_New()</span></code></a> takes a name parameter (<code class="xref c c-type docutils literal notranslate"><span class="pre">const</span> <span class="pre">char</span> <span class="pre">*</span></code>); you’re permitted to pass in a <em>NULL</em> name, but we strongly encourage you to specify a name. Properly named Capsules provide a degree of runtime type-safety; there is no feasible way to tell one unnamed Capsule from another.</p> <p>In particular, Capsules used to expose C APIs should be given a name following this convention:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">modulename</span><span class="p">.</span><span class="n">attributename</span> </pre></div> </div> <p>The convenience function <a class="reference internal" href="../c-api/capsule.html#c.PyCapsule_Import" title="PyCapsule_Import"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyCapsule_Import()</span></code></a> makes it easy to load a C API provided via a Capsule, but only if the Capsule’s name matches this convention. This behavior gives C API users a high degree of certainty that the Capsule they load contains the correct C API.</p> <p>The following example demonstrates an approach that puts most of the burden on the writer of the exporting module, which is appropriate for commonly used library modules. It stores all C API pointers (just one in the example!) in an array of <code class="xref c c-type docutils literal notranslate"><span class="pre">void</span></code> pointers which becomes the value of a Capsule. The header file corresponding to the module provides a macro that takes care of importing the module and retrieving its C API pointers; client modules only have to call this macro before accessing the C API.</p> <p>The exporting module is a modification of the <code class="xref py py-mod docutils literal notranslate"><span class="pre">spam</span></code> module from section <a class="reference internal" href="#extending-simpleexample"><span class="std std-ref">A Simple Example</span></a>. The function <code class="xref py py-func docutils literal notranslate"><span class="pre">spam.system()</span></code> does not call the C library function <code class="xref c c-func docutils literal notranslate"><span class="pre">system()</span></code> directly, but a function <code class="xref c c-func docutils literal notranslate"><span class="pre">PySpam_System()</span></code>, which would of course do something more complicated in reality (such as adding “spam” to every command). This function <code class="xref c c-func docutils literal notranslate"><span class="pre">PySpam_System()</span></code> is also exported to other extension modules.</p> <p>The function <code class="xref c c-func docutils literal notranslate"><span class="pre">PySpam_System()</span></code> is a plain C function, declared <code class="docutils literal notranslate"><span class="pre">static</span></code> like everything else:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="kt">int</span> <span class="nf">PySpam_System</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">command</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">system</span><span class="p">(</span><span class="n">command</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>The function <code class="xref c c-func docutils literal notranslate"><span class="pre">spam_system()</span></code> is modified in a trivial way:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="n">PyObject</span> <span class="o">*</span> <span class="nf">spam_system</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">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">command</span><span class="p">;</span> <span class="kt">int</span> <span class="n">sts</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">"s"</span><span class="p">,</span> <span class="o">&</span><span class="n">command</span><span class="p">))</span> <span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span> <span class="n">sts</span> <span class="o">=</span> <span class="n">PySpam_System</span><span class="p">(</span><span class="n">command</span><span class="p">);</span> <span class="k">return</span> <span class="n">Py_BuildValue</span><span class="p">(</span><span class="s">"i"</span><span class="p">,</span> <span class="n">sts</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>In the beginning of the module, right after the line</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> </pre></div> </div> <p>two more lines must be added:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#define SPAM_MODULE</span> <span class="cp">#include</span> <span class="cpf">"spammodule.h"</span><span class="cp"></span> </pre></div> </div> <p>The <code class="docutils literal notranslate"><span class="pre">#define</span></code> is used to tell the header file that it is being included in the exporting module, not a client module. Finally, the module’s initialization function must take care of initializing the C API pointer array:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyMODINIT_FUNC</span> <span class="nf">initspam</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">m</span><span class="p">;</span> <span class="k">static</span> <span class="kt">void</span> <span class="o">*</span><span class="n">PySpam_API</span><span class="p">[</span><span class="n">PySpam_API_pointers</span><span class="p">];</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">c_api_object</span><span class="p">;</span> <span class="n">m</span> <span class="o">=</span> <span class="n">Py_InitModule</span><span class="p">(</span><span class="s">"spam"</span><span class="p">,</span> <span class="n">SpamMethods</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">m</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span><span class="p">;</span> <span class="cm">/* Initialize the C API pointer array */</span> <span class="n">PySpam_API</span><span class="p">[</span><span class="n">PySpam_System_NUM</span><span class="p">]</span> <span class="o">=</span> <span class="p">(</span><span class="kt">void</span> <span class="o">*</span><span class="p">)</span><span class="n">PySpam_System</span><span class="p">;</span> <span class="cm">/* Create a Capsule containing the API pointer array's address */</span> <span class="n">c_api_object</span> <span class="o">=</span> <span class="n">PyCapsule_New</span><span class="p">((</span><span class="kt">void</span> <span class="o">*</span><span class="p">)</span><span class="n">PySpam_API</span><span class="p">,</span> <span class="s">"spam._C_API"</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">c_api_object</span> <span class="o">!=</span> <span class="nb">NULL</span><span class="p">)</span> <span class="n">PyModule_AddObject</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"_C_API"</span><span class="p">,</span> <span class="n">c_api_object</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>Note that <code class="docutils literal notranslate"><span class="pre">PySpam_API</span></code> is declared <code class="docutils literal notranslate"><span class="pre">static</span></code>; otherwise the pointer array would disappear when <code class="xref py py-func docutils literal notranslate"><span class="pre">initspam()</span></code> terminates!</p> <p>The bulk of the work is in the header file <code class="file docutils literal notranslate"><span class="pre">spammodule.h</span></code>, which looks like this:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#ifndef Py_SPAMMODULE_H</span> <span class="cp">#define Py_SPAMMODULE_H</span> <span class="cp">#ifdef __cplusplus</span> <span class="k">extern</span> <span class="s">"C"</span> <span class="p">{</span> <span class="cp">#endif</span> <span class="cm">/* Header file for spammodule */</span> <span class="cm">/* C API functions */</span> <span class="cp">#define PySpam_System_NUM 0</span> <span class="cp">#define PySpam_System_RETURN int</span> <span class="cp">#define PySpam_System_PROTO (const char *command)</span> <span class="cm">/* Total number of C API pointers */</span> <span class="cp">#define PySpam_API_pointers 1</span> <span class="cp">#ifdef SPAM_MODULE</span> <span class="cm">/* This section is used when compiling spammodule.c */</span> <span class="k">static</span> <span class="n">PySpam_System_RETURN</span> <span class="n">PySpam_System</span> <span class="n">PySpam_System_PROTO</span><span class="p">;</span> <span class="cp">#else</span> <span class="cm">/* This section is used in modules that use spammodule's API */</span> <span class="k">static</span> <span class="kt">void</span> <span class="o">**</span><span class="n">PySpam_API</span><span class="p">;</span> <span class="cp">#define PySpam_System \</span> <span class="cp"> (*(PySpam_System_RETURN (*)PySpam_System_PROTO) PySpam_API[PySpam_System_NUM])</span> <span class="cm">/* Return -1 on error, 0 on success.</span> <span class="cm"> * PyCapsule_Import will set an exception if there's an error.</span> <span class="cm"> */</span> <span class="k">static</span> <span class="kt">int</span> <span class="nf">import_spam</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="n">PySpam_API</span> <span class="o">=</span> <span class="p">(</span><span class="kt">void</span> <span class="o">**</span><span class="p">)</span><span class="n">PyCapsule_Import</span><span class="p">(</span><span class="s">"spam._C_API"</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span> <span class="k">return</span> <span class="p">(</span><span class="n">PySpam_API</span> <span class="o">!=</span> <span class="nb">NULL</span><span class="p">)</span> <span class="o">?</span> <span class="mi">0</span> <span class="o">:</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span> <span class="p">}</span> <span class="cp">#endif</span> <span class="cp">#ifdef __cplusplus</span> <span class="p">}</span> <span class="cp">#endif</span> <span class="cp">#endif </span><span class="cm">/* !defined(Py_SPAMMODULE_H) */</span><span class="cp"></span> </pre></div> </div> <p>All that a client module must do in order to have access to the function <code class="xref c c-func docutils literal notranslate"><span class="pre">PySpam_System()</span></code> is to call the function (or rather macro) <code class="xref c c-func docutils literal notranslate"><span class="pre">import_spam()</span></code> in its initialization function:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyMODINIT_FUNC</span> <span class="nf">initclient</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">m</span><span class="p">;</span> <span class="n">m</span> <span class="o">=</span> <span class="n">Py_InitModule</span><span class="p">(</span><span class="s">"client"</span><span class="p">,</span> <span class="n">ClientMethods</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">m</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span><span class="p">;</span> <span class="k">if</span> <span class="p">(</span><span class="n">import_spam</span><span class="p">()</span> <span class="o"><</span> <span class="mi">0</span><span class="p">)</span> <span class="k">return</span><span class="p">;</span> <span class="cm">/* additional initialization can happen here */</span> <span class="p">}</span> </pre></div> </div> <p>The main disadvantage of this approach is that the file <code class="file docutils literal notranslate"><span class="pre">spammodule.h</span></code> is rather complicated. However, the basic structure is the same for each function that is exported, so it has to be learned only once.</p> <p>Finally it should be mentioned that Capsules offer additional functionality, which is especially useful for memory allocation and deallocation of the pointer stored in a Capsule. The details are described in the Python/C API Reference Manual in the section <a class="reference internal" href="../c-api/capsule.html#capsules"><span class="std std-ref">Capsules</span></a> and in the implementation of Capsules (files <code class="file docutils literal notranslate"><span class="pre">Include/pycapsule.h</span></code> and <code class="file docutils literal notranslate"><span class="pre">Objects/pycapsule.c</span></code> in the Python source code distribution).</p> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id5" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>An interface for this function already exists in the standard module <a class="reference internal" href="../library/os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> — it was chosen as a simple and straightforward example.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id6" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>The metaphor of “borrowing” a reference is not completely correct: the owner still has a copy of the reference.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id7" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id3">[3]</a></td><td>Checking that the reference count is at least 1 <strong>does not work</strong> — the reference count itself could be in freed memory and may thus be reused for another object!</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id8" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id4">[4]</a></td><td>These guarantees don’t hold when you use the “old” style calling convention — this is still found in much existing code.</td></tr> </tbody> </table> </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. Extending Python with C or C++</a><ul> <li><a class="reference internal" href="#a-simple-example">1.1. A Simple Example</a></li> <li><a class="reference internal" href="#intermezzo-errors-and-exceptions">1.2. Intermezzo: Errors and Exceptions</a></li> <li><a class="reference internal" href="#back-to-the-example">1.3. Back to the Example</a></li> <li><a class="reference internal" href="#the-module-s-method-table-and-initialization-function">1.4. The Module’s Method Table and Initialization Function</a></li> <li><a class="reference internal" href="#compilation-and-linkage">1.5. Compilation and Linkage</a></li> <li><a class="reference internal" href="#calling-python-functions-from-c">1.6. Calling Python Functions from C</a></li> <li><a class="reference internal" href="#extracting-parameters-in-extension-functions">1.7. Extracting Parameters in Extension Functions</a></li> <li><a class="reference internal" href="#keyword-parameters-for-extension-functions">1.8. Keyword Parameters for Extension Functions</a></li> <li><a class="reference internal" href="#building-arbitrary-values">1.9. Building Arbitrary Values</a></li> <li><a class="reference internal" href="#reference-counts">1.10. Reference Counts</a><ul> <li><a class="reference internal" href="#reference-counting-in-python">1.10.1. Reference Counting in Python</a></li> <li><a class="reference internal" href="#ownership-rules">1.10.2. Ownership Rules</a></li> <li><a class="reference internal" href="#thin-ice">1.10.3. Thin Ice</a></li> <li><a class="reference internal" href="#null-pointers">1.10.4. NULL Pointers</a></li> </ul> </li> <li><a class="reference internal" href="#writing-extensions-in-c">1.11. Writing Extensions in C++</a></li> <li><a class="reference internal" href="#providing-a-c-api-for-an-extension-module">1.12. Providing a C API for an Extension Module</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="index.html" title="previous chapter">Extending and Embedding the Python Interpreter</a></p> <h4>Next topic</h4> <p class="topless"><a href="newtypes.html" title="next chapter">2. Defining New Types</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/extending/extending.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> <input 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> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </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="newtypes.html" title="2. 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