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<li class="toctree-l1 current"><a class="reference internal" href="guide.html">Porting guide for Python C Extensions</a><ul class="current">
<li class="toctree-l2 current"><a class="current reference internal" href="#">1. Modernization</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#pyobject-structure-members">PyObject Structure Members</a></li>
<li class="toctree-l3"><a class="reference internal" href="#adding-module-level-constants">Adding module-level constants</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#pycobject-to-pycapsule">PyCObject to PyCapsule</a></li>
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<div class="section" id="modernization">
<span id="index-0"></span><h1>Modernization<a class="headerlink" href="#modernization" title="Permalink to this headline">¶</a></h1>
<p>Before porting a C extension to Python 3, you’ll need to make sure that
you’re not using features deprecated even in Python 2.
Also, many of Python 3’s improvements have been backported to Python 2.6,
and using them will make the porting process easier.</p>
<p>For all changes you do, be sure add tests to ensure you do not break anything.</p>
<div class="section" id="pyobject-structure-members">
<span id="index-1"></span><h2>PyObject Structure Members<a class="headerlink" href="#pyobject-structure-members" title="Permalink to this headline">¶</a></h2>
<p>To conform to C’s strict aliasing rules, PyObject_HEAD, which provides
members such as <code class="docutils literal"><span class="pre">ob_type</span></code> and <code class="docutils literal"><span class="pre">ob_refcnt</span></code>, is a separate struct in
Python 3.
Access to these members is provided by macros, which have been ported to
Python 2.6:</p>
<table border="1" class="docutils">
<colgroup>
<col width="50%" />
<col width="50%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Instead of</th>
<th class="head">use</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>obj->ob_type</td>
<td>Py_TYPE(obj)</td>
</tr>
<tr class="row-odd"><td>obj->ob_refcnt</td>
<td>Py_REFCNT(obj)</td>
</tr>
<tr class="row-even"><td>obj->ob_size</td>
<td>Py_SIZE(obj)</td>
</tr>
</tbody>
</table>
<p>And for initialization of type objects, the sequence</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="n">PyObject_HEAD_INIT</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="cm">/* ob_size */</span>
</pre></div>
</div>
<p>must be replaced with</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="n">PyVarObject_HEAD_INIT</span><span class="p">(</span><span class="nb">NULL</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="adding-module-level-constants">
<span id="index-2"></span><h2>Adding module-level constants<a class="headerlink" href="#adding-module-level-constants" title="Permalink to this headline">¶</a></h2>
<p>Often, module initialization uses code like this:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="n">PyModule_AddObject</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"RDWR"</span><span class="p">,</span> <span class="n">PyInt_FromLong</span><span class="p">(</span><span class="n">O_RDWR</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">"__version__"</span><span class="p">,</span> <span class="n">PyString_FromString</span><span class="p">(</span><span class="s">"6.28"</span><span class="p">));</span>
</pre></div>
</div>
<p>Python 2.6 introduced convenience functions, which are shorter to write:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="n">PyModule_AddIntConstant</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"RDWR"</span><span class="p">,</span> <span class="n">O_RDWR</span><span class="p">)</span>
<span class="n">PyModule_AddStringConstant</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"__version__"</span><span class="p">,</span> <span class="s">"6.28"</span><span class="p">)</span>
</pre></div>
</div>
<p>These will use native int and str types in both Python versions.</p>
</div>
<div class="section" id="new-style-classes">
<span id="index-3"></span><h2>New-Style Classes<a class="headerlink" href="#new-style-classes" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference external" href="https://docs.python.org/2/c-api/class.html">old-style classes</a>
(<code class="docutils literal"><span class="pre">PyClass_*</span></code> and <code class="docutils literal"><span class="pre">PyInstance_*</span></code>) will be removed in Python 3.
Instead, use <a class="reference external" href="https://docs.python.org/2/c-api/type.html#typeobjects">type objects</a>,
which have been available since Python 2.2.</p>
</div>
<div class="section" id="pycobject-to-pycapsule">
<span id="pycapsule-porting"></span><span id="index-4"></span><h2>PyCObject to PyCapsule<a class="headerlink" href="#pycobject-to-pycapsule" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference external" href="https://docs.python.org/3.1/c-api/cobject.html">PyCObject API</a> has been
removed in Python 3.3.
You should instead use its replacement, <a class="reference external" href="https://docs.python.org/3/c-api/capsule.html#capsules">PyCapsule</a>,
which is available in Python 2.7 and 3.1+.
For the rationale behind Capsules, see <a class="reference external" href="https://bugs.python.org/issue5630">CPython issue 5630</a>.</p>
<p>If you need to support Python 2.6, you can use <code class="docutils literal"><span class="pre">capsulethunk.h</span></code>, which
implements the PyCapsule API (with some limitations) in terms of PyCObject.
For instructions, see the chapter <a class="reference internal" href="capsulethunk.html"><span class="doc">PyCapsule API for Python 2.6</span></a>.</p>
<p>The port to PyCapsule API should be straightforward:</p>
<ul class="simple">
<li>Instead of
<code class="xref c c-func docutils literal"><span class="pre">PyCObject_FromVoidPtr(obj,</span> <span class="pre">destr)</span></code>, use
<code class="xref c c-func docutils literal"><span class="pre">PyCapsule_New(obj,</span> <span class="pre">name,</span> <span class="pre">destr)</span></code>.
If the capsule will be available as a module attribute, use
<code class="docutils literal"><span class="pre">"<modulename>.<attrname>"</span></code> for <em>name</em>.
Otherwise, use your best judgment, but try making the name unique.</li>
<li>Instead of <code class="xref c c-func docutils literal"><span class="pre">PyCObject_FromVoidPtrAndDesc(obj,</span> <span class="pre">desc,</span> <span class="pre">destr)</span></code>,
use <code class="xref c c-func docutils literal"><span class="pre">py3:PyCapsule_New()</span></code> as above; then call
<code class="xref c c-func docutils literal"><span class="pre">PyCapsule_SetContext(obj,</span> <span class="pre">desc)</span></code>.</li>
<li>Instead of <code class="xref c c-func docutils literal"><span class="pre">PyCObject_AsVoidPtr(obj)</span></code>,
use <code class="xref c c-func docutils literal"><span class="pre">PyCapsule_GetPointer(obj,</span> <span class="pre">name)</span></code>.
You will need to provide a capsule name, which is checked at runtime
as a form of type safety.</li>
<li>Instead of <code class="xref c c-func docutils literal"><span class="pre">py2:PyCObject_GetDesc()</span></code>,
use <code class="xref c c-func docutils literal"><span class="pre">py3:PyCapsule_GetContext()</span></code>.</li>
<li>Instead of <code class="xref c c-func docutils literal"><span class="pre">py2:PyCObject_SetVoidPtr()</span></code>,
use <code class="xref c c-func docutils literal"><span class="pre">py3:PyCapsule_SetPointer()</span></code>.</li>
<li>Change all CObject destructors to <code class="xref c c-type docutils literal"><span class="pre">PyCapsule</span> <span class="pre">destructors</span></code>,
which take the PyCapsule object as their only argument.</li>
</ul>
</div>
<div class="section" id="comparisons">
<span id="index-5"></span><h2>Comparisons<a class="headerlink" href="#comparisons" title="Permalink to this headline">¶</a></h2>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">In previous versions, this chapter talked about rich comparison,
but those changes are better left to <a class="reference internal" href="guide-porting.html#comparison-porting"><span class="std std-ref">porting</span></a>.</p>
</div>
</div>
<div class="section" id="done">
<h2>Done!<a class="headerlink" href="#done" title="Permalink to this headline">¶</a></h2>
<p>When your project is sufficiently modernized, and the tests still pass under
Python 2, you’re ready to start the actual <a class="reference internal" href="guide-porting.html"><span class="doc">Porting</span></a>.</p>
</div>
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