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<ul>
<li><a class="reference internal" href="#">Defining extension modules</a><ul>
<li><a class="reference internal" href="#multiple-module-instances">Multiple module instances</a></li>
<li><a class="reference internal" href="#initialization-function">Initialization function</a></li>
<li><a class="reference internal" href="#multi-phase-initialization">Multi-phase initialization</a></li>
<li><a class="reference internal" href="#legacy-single-phase-initialization">Legacy single-phase initialization</a></li>
</ul>
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<section id="defining-extension-modules">
<span id="extension-modules"></span><h1>Defining extension modules<a class="headerlink" href="#defining-extension-modules" title="Link to this heading">¶</a></h1>
<p>A C extension for CPython is a shared library (for example, a <code class="docutils literal notranslate"><span class="pre">.so</span></code> file
on Linux, <code class="docutils literal notranslate"><span class="pre">.pyd</span></code> DLL on Windows), which is loadable into the Python process
(for example, it is compiled with compatible compiler settings), and which
exports an <a class="reference internal" href="#extension-export-hook"><span class="std std-ref">initialization function</span></a>.</p>
<p>To be importable by default (that is, by
<a class="reference internal" href="../library/importlib.html#importlib.machinery.ExtensionFileLoader" title="importlib.machinery.ExtensionFileLoader"><code class="xref py py-class docutils literal notranslate"><span class="pre">importlib.machinery.ExtensionFileLoader</span></code></a>),
the shared library must be available on <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-attr docutils literal notranslate"><span class="pre">sys.path</span></code></a>,
and must be named after the module name plus an extension listed in
<a class="reference internal" href="../library/importlib.html#importlib.machinery.EXTENSION_SUFFIXES" title="importlib.machinery.EXTENSION_SUFFIXES"><code class="xref py py-attr docutils literal notranslate"><span class="pre">importlib.machinery.EXTENSION_SUFFIXES</span></code></a>.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Building, packaging and distributing extension modules is best done with
third-party tools, and is out of scope of this document.
One suitable tool is Setuptools, whose documentation can be found at
<a class="reference external" href="https://setuptools.pypa.io/en/latest/setuptools.html">https://setuptools.pypa.io/en/latest/setuptools.html</a>.</p>
</div>
<p>Normally, the initialization function returns a module definition initialized
using <a class="reference internal" href="#c.PyModuleDef_Init" title="PyModuleDef_Init"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyModuleDef_Init()</span></code></a>.
This allows splitting the creation process into several phases:</p>
<ul class="simple">
<li><p>Before any substantial code is executed, Python can determine which
capabilities the module supports, and it can adjust the environment or
refuse loading an incompatible extension.</p></li>
<li><p>By default, Python itself creates the module object – that is, it does
the equivalent of <a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__new__()</span></code></a> for classes.
It also sets initial attributes like <a class="reference internal" href="../reference/datamodel.html#module.__package__" title="module.__package__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__package__</span></code></a> and
<a class="reference internal" href="../reference/datamodel.html#module.__loader__" title="module.__loader__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__loader__</span></code></a>.</p></li>
<li><p>Afterwards, the module object is initialized using extension-specific
code – the equivalent of <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> on classes.</p></li>
</ul>
<p>This is called <em>multi-phase initialization</em> to distinguish it from the legacy
(but still supported) <em>single-phase initialization</em> scheme,
where the initialization function returns a fully constructed module.
See the <a class="reference internal" href="#single-phase-initialization"><span class="std std-ref">single-phase-initialization section below</span></a>
for details.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>Added support for multi-phase initialization (<span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-0489/"><strong>PEP 489</strong></a>).</p>
</div>
<section id="multiple-module-instances">
<h2>Multiple module instances<a class="headerlink" href="#multiple-module-instances" title="Link to this heading">¶</a></h2>
<p>By default, extension modules are not singletons.
For example, if the <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-attr docutils literal notranslate"><span class="pre">sys.modules</span></code></a> entry is removed and the module
is re-imported, a new module object is created, and typically populated with
fresh method and type objects.
The old module is subject to normal garbage collection.
This mirrors the behavior of pure-Python modules.</p>
<p>Additional module instances may be created in
<a class="reference internal" href="init.html#sub-interpreter-support"><span class="std std-ref">sub-interpreters</span></a>
or after Python runtime reinitialization
(<a class="reference internal" href="init.html#c.Py_Finalize" title="Py_Finalize"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_Finalize()</span></code></a> and <a class="reference internal" href="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>).
In these cases, sharing Python objects between module instances would likely
cause crashes or undefined behavior.</p>
<p>To avoid such issues, each instance of an extension module should
be <em>isolated</em>: changes to one instance should not implicitly affect the others,
and all state owned by the module, including references to Python objects,
should be specific to a particular module instance.
See <a class="reference internal" href="../howto/isolating-extensions.html#isolating-extensions-howto"><span class="std std-ref">Isolating Extension Modules</span></a> for more details and a practical guide.</p>
<p>A simpler way to avoid these issues is
<a class="reference internal" href="../howto/isolating-extensions.html#isolating-extensions-optout"><span class="std std-ref">raising an error on repeated initialization</span></a>.</p>
<p>All modules are expected to support
<a class="reference internal" href="init.html#sub-interpreter-support"><span class="std std-ref">sub-interpreters</span></a>, or otherwise explicitly
signal a lack of support.
This is usually achieved by isolation or blocking repeated initialization,
as above.
A module may also be limited to the main interpreter using
the <a class="reference internal" href="module.html#c.Py_mod_multiple_interpreters" title="Py_mod_multiple_interpreters"><code class="xref c c-data docutils literal notranslate"><span class="pre">Py_mod_multiple_interpreters</span></code></a> slot.</p>
</section>
<section id="initialization-function">
<span id="extension-export-hook"></span><h2>Initialization function<a class="headerlink" href="#initialization-function" title="Link to this heading">¶</a></h2>
<p>The initialization function defined by an extension module has the
following signature:</p>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyInit_modulename">
<a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyInit_modulename</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyInit_modulename" title="Link to this definition">¶</a><br /></dt>
<dd></dd></dl>
<p>Its name should be <code class="samp docutils literal notranslate"><span class="pre">PyInit_</span><em><span class="pre"><name></span></em></code>, with <code class="docutils literal notranslate"><span class="pre"><name></span></code> replaced by the
name of the module.</p>
<p>For modules with ASCII-only names, the function must instead be named
<code class="samp docutils literal notranslate"><span class="pre">PyInit_</span><em><span class="pre"><name></span></em></code>, with <code class="docutils literal notranslate"><span class="pre"><name></span></code> replaced by the name of the module.
When using <a class="reference internal" href="#multi-phase-initialization"><span class="std std-ref">Multi-phase initialization</span></a>, non-ASCII module names
are allowed. In this case, the initialization function name is
<code class="samp docutils literal notranslate"><span class="pre">PyInitU_</span><em><span class="pre"><name></span></em></code>, with <code class="docutils literal notranslate"><span class="pre"><name></span></code> encoded using Python’s
<em>punycode</em> encoding with hyphens replaced by underscores. In Python:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">initfunc_name</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">suffix</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">'_'</span> <span class="o">+</span> <span class="n">name</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s1">'ascii'</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">UnicodeEncodeError</span><span class="p">:</span>
<span class="n">suffix</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">'U_'</span> <span class="o">+</span> <span class="n">name</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s1">'punycode'</span><span class="p">)</span><span class="o">.</span><span class="n">replace</span><span class="p">(</span><span class="sa">b</span><span class="s1">'-'</span><span class="p">,</span> <span class="sa">b</span><span class="s1">'_'</span><span class="p">)</span>
<span class="k">return</span> <span class="sa">b</span><span class="s1">'PyInit'</span> <span class="o">+</span> <span class="n">suffix</span>
</pre></div>
</div>
<p>It is recommended to define the initialization function using a helper macro:</p>
<dl class="c macro">
<dt class="sig sig-object c" id="c.PyMODINIT_FUNC">
<span class="sig-name descname"><span class="n"><span class="pre">PyMODINIT_FUNC</span></span></span><a class="headerlink" href="#c.PyMODINIT_FUNC" title="Link to this definition">¶</a><br /></dt>
<dd><p>Declare an extension module initialization function.
This macro:</p>
<ul class="simple">
<li><p>specifies the <span class="c-expr sig sig-inline c"><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n">PyObject</span></a><span class="p">*</span></span> return type,</p></li>
<li><p>adds any special linkage declarations required by the platform, and</p></li>
<li><p>for C++, declares the function as <code class="docutils literal notranslate"><span class="pre">extern</span> <span class="pre">"C"</span></code>.</p></li>
</ul>
</dd></dl>
<p>For example, a module called <code class="docutils literal notranslate"><span class="pre">spam</span></code> would be defined like this:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">PyModuleDef</span><span class="w"> </span><span class="n">spam_module</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">.</span><span class="n">m_base</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">PyModuleDef_HEAD_INIT</span><span class="p">,</span>
<span class="w"> </span><span class="p">.</span><span class="n">m_name</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"spam"</span><span class="p">,</span>
<span class="w"> </span><span class="p">...</span>
<span class="p">};</span>
<span class="n">PyMODINIT_FUNC</span>
<span class="nf">PyInit_spam</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">PyModuleDef_Init</span><span class="p">(</span><span class="o">&</span><span class="n">spam_module</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>It is possible to export multiple modules from a single shared library by
defining multiple initialization functions. However, importing them requires
using symbolic links or a custom importer, because by default only the
function corresponding to the filename is found.
See the <a class="reference external" href="https://peps.python.org/pep-0489/#multiple-modules-in-one-library">Multiple modules in one library</a>
section in <span class="target" id="index-1"></span><a class="pep reference external" href="https://peps.python.org/pep-0489/"><strong>PEP 489</strong></a> for details.</p>
<p>The initialization function is typically the only non-<code class="docutils literal notranslate"><span class="pre">static</span></code>
item defined in the module’s C source.</p>
</section>
<section id="multi-phase-initialization">
<span id="id1"></span><h2>Multi-phase initialization<a class="headerlink" href="#multi-phase-initialization" title="Link to this heading">¶</a></h2>
<p>Normally, the <a class="reference internal" href="#extension-export-hook"><span class="std std-ref">initialization function</span></a>
(<code class="docutils literal notranslate"><span class="pre">PyInit_modulename</span></code>) returns a <a class="reference internal" href="module.html#c.PyModuleDef" title="PyModuleDef"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyModuleDef</span></code></a> instance with
non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> <a class="reference internal" href="module.html#c.PyModuleDef.m_slots" title="PyModuleDef.m_slots"><code class="xref c c-member docutils literal notranslate"><span class="pre">m_slots</span></code></a>.
Before it is returned, the <code class="docutils literal notranslate"><span class="pre">PyModuleDef</span></code> instance must be initialized
using the following function:</p>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyModuleDef_Init">
<a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyModuleDef_Init</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="module.html#c.PyModuleDef" title="PyModuleDef"><span class="n"><span class="pre">PyModuleDef</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">def</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyModuleDef_Init" title="Link to this definition">¶</a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a> since version 3.5.</em><p>Ensure a module definition is a properly initialized Python object that
correctly reports its type and a reference count.</p>
<p>Return <em>def</em> cast to <code class="docutils literal notranslate"><span class="pre">PyObject*</span></code>, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> if an error occurred.</p>
<p>Calling this function is required for <a class="reference internal" href="#multi-phase-initialization"><span class="std std-ref">Multi-phase initialization</span></a>.
It should not be used in other contexts.</p>
<p>Note that Python assumes that <code class="docutils literal notranslate"><span class="pre">PyModuleDef</span></code> structures are statically
allocated.
This function may return either a new reference or a borrowed one;
this reference must not be released.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.5.</span></p>
</div>
</dd></dl>
</section>
<section id="legacy-single-phase-initialization">
<span id="single-phase-initialization"></span><h2>Legacy single-phase initialization<a class="headerlink" href="#legacy-single-phase-initialization" title="Link to this heading">¶</a></h2>
<div class="admonition attention">
<p class="admonition-title">Attention</p>
<p>Single-phase initialization is a legacy mechanism to initialize extension
modules, with known drawbacks and design flaws. Extension module authors
are encouraged to use multi-phase initialization instead.</p>
</div>
<p>In single-phase initialization, the
<a class="reference internal" href="#extension-export-hook"><span class="std std-ref">initialization function</span></a> (<code class="docutils literal notranslate"><span class="pre">PyInit_modulename</span></code>)
should create, populate and return a module object.
This is typically done using <a class="reference internal" href="module.html#c.PyModule_Create" title="PyModule_Create"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyModule_Create()</span></code></a> and functions like
<a class="reference internal" href="module.html#c.PyModule_AddObjectRef" title="PyModule_AddObjectRef"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyModule_AddObjectRef()</span></code></a>.</p>
<p>Single-phase initialization differs from the <a class="reference internal" href="#multi-phase-initialization"><span class="std std-ref">default</span></a>
in the following ways:</p>
<ul>
<li><p>Single-phase modules are, or rather <em>contain</em>, “singletons”.</p>
<p>When the module is first initialized, Python saves the contents of
the module’s <code class="docutils literal notranslate"><span class="pre">__dict__</span></code> (that is, typically, the module’s functions and
types).</p>
<p>For subsequent imports, Python does not call the initialization function
again.
Instead, it creates a new module object with a new <code class="docutils literal notranslate"><span class="pre">__dict__</span></code>, and copies
the saved contents to it.
For example, given a single-phase module <code class="docutils literal notranslate"><span class="pre">_testsinglephase</span></code>
<a class="footnote-reference brackets" href="#testsinglephase" id="id2" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a> that defines a function <code class="docutils literal notranslate"><span class="pre">sum</span></code> and an exception class
<code class="docutils literal notranslate"><span class="pre">error</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span><span class="w"> </span><span class="nn">sys</span>
<span class="gp">>>> </span><span class="kn">import</span><span class="w"> </span><span class="nn">_testsinglephase</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">one</span>
<span class="gp">>>> </span><span class="k">del</span> <span class="n">sys</span><span class="o">.</span><span class="n">modules</span><span class="p">[</span><span class="s1">'_testsinglephase'</span><span class="p">]</span>
<span class="gp">>>> </span><span class="kn">import</span><span class="w"> </span><span class="nn">_testsinglephase</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">two</span>
<span class="gp">>>> </span><span class="n">one</span> <span class="ow">is</span> <span class="n">two</span>
<span class="go">False</span>
<span class="gp">>>> </span><span class="n">one</span><span class="o">.</span><span class="vm">__dict__</span> <span class="ow">is</span> <span class="n">two</span><span class="o">.</span><span class="vm">__dict__</span>
<span class="go">False</span>
<span class="gp">>>> </span><span class="n">one</span><span class="o">.</span><span class="n">sum</span> <span class="ow">is</span> <span class="n">two</span><span class="o">.</span><span class="n">sum</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="n">one</span><span class="o">.</span><span class="n">error</span> <span class="ow">is</span> <span class="n">two</span><span class="o">.</span><span class="n">error</span>
<span class="go">True</span>
</pre></div>
</div>
<p>The exact behavior should be considered a CPython implementation detail.</p>
</li>
<li><p>To work around the fact that <code class="docutils literal notranslate"><span class="pre">PyInit_modulename</span></code> does not take a <em>spec</em>
argument, some state of the import machinery is saved and applied to the
first suitable module created during the <code class="docutils literal notranslate"><span class="pre">PyInit_modulename</span></code> call.
Specifically, when a sub-module is imported, this mechanism prepends the
parent package name to the name of the module.</p>
<p>A single-phase <code class="docutils literal notranslate"><span class="pre">PyInit_modulename</span></code> function should create “its” module
object as soon as possible, before any other module objects can be created.</p>
</li>
<li><p>Non-ASCII module names (<code class="docutils literal notranslate"><span class="pre">PyInitU_modulename</span></code>) are not supported.</p></li>
<li><p>Single-phase modules support module lookup functions like
<a class="reference internal" href="module.html#c.PyState_FindModule" title="PyState_FindModule"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyState_FindModule()</span></code></a>.</p></li>
</ul>
<aside class="footnote-list brackets">
<aside class="footnote brackets" id="testsinglephase" role="doc-footnote">
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id2">1</a><span class="fn-bracket">]</span></span>
<p><code class="docutils literal notranslate"><span class="pre">_testsinglephase</span></code> is an internal module used
in CPython’s self-test suite; your installation may or may not
include it.</p>
</aside>
</aside>
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<li><a class="reference internal" href="#">Defining extension modules</a><ul>
<li><a class="reference internal" href="#multiple-module-instances">Multiple module instances</a></li>
<li><a class="reference internal" href="#initialization-function">Initialization function</a></li>
<li><a class="reference internal" href="#multi-phase-initialization">Multi-phase initialization</a></li>
<li><a class="reference internal" href="#legacy-single-phase-initialization">Legacy single-phase initialization</a></li>
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