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<div class="section" id="defining-extension-types-assorted-topics">
<h1><span class="section-number">3. </span>Defining Extension Types: Assorted Topics<a class="headerlink" href="#defining-extension-types-assorted-topics" title="Permalink to this headline">¶</a></h1>
<p id="dnt-type-methods">This section aims to give a quick fly-by on the various type methods you can
implement and what they do.</p>
<p>Here is the definition of <a class="reference internal" href="../c-api/type.html#c.PyTypeObject" title="PyTypeObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyTypeObject</span></code></a>, with some fields only used in
debug builds omitted:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">typedef</span> <span class="k">struct</span> <span class="n">_typeobject</span> <span class="p">{</span>
<span class="n">PyObject_VAR_HEAD</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">tp_name</span><span class="p">;</span> <span class="cm">/* For printing, in format "<module>.<name>" */</span>
<span class="n">Py_ssize_t</span> <span class="n">tp_basicsize</span><span class="p">,</span> <span class="n">tp_itemsize</span><span class="p">;</span> <span class="cm">/* For allocation */</span>
<span class="cm">/* Methods to implement standard operations */</span>
<span class="n">destructor</span> <span class="n">tp_dealloc</span><span class="p">;</span>
<span class="n">printfunc</span> <span class="n">tp_print</span><span class="p">;</span>
<span class="n">getattrfunc</span> <span class="n">tp_getattr</span><span class="p">;</span>
<span class="n">setattrfunc</span> <span class="n">tp_setattr</span><span class="p">;</span>
<span class="n">PyAsyncMethods</span> <span class="o">*</span><span class="n">tp_as_async</span><span class="p">;</span> <span class="cm">/* formerly known as tp_compare (Python 2)</span>
<span class="cm"> or tp_reserved (Python 3) */</span>
<span class="n">reprfunc</span> <span class="n">tp_repr</span><span class="p">;</span>
<span class="cm">/* Method suites for standard classes */</span>
<span class="n">PyNumberMethods</span> <span class="o">*</span><span class="n">tp_as_number</span><span class="p">;</span>
<span class="n">PySequenceMethods</span> <span class="o">*</span><span class="n">tp_as_sequence</span><span class="p">;</span>
<span class="n">PyMappingMethods</span> <span class="o">*</span><span class="n">tp_as_mapping</span><span class="p">;</span>
<span class="cm">/* More standard operations (here for binary compatibility) */</span>
<span class="n">hashfunc</span> <span class="n">tp_hash</span><span class="p">;</span>
<span class="n">ternaryfunc</span> <span class="n">tp_call</span><span class="p">;</span>
<span class="n">reprfunc</span> <span class="n">tp_str</span><span class="p">;</span>
<span class="n">getattrofunc</span> <span class="n">tp_getattro</span><span class="p">;</span>
<span class="n">setattrofunc</span> <span class="n">tp_setattro</span><span class="p">;</span>
<span class="cm">/* Functions to access object as input/output buffer */</span>
<span class="n">PyBufferProcs</span> <span class="o">*</span><span class="n">tp_as_buffer</span><span class="p">;</span>
<span class="cm">/* Flags to define presence of optional/expanded features */</span>
<span class="kt">unsigned</span> <span class="kt">long</span> <span class="n">tp_flags</span><span class="p">;</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">tp_doc</span><span class="p">;</span> <span class="cm">/* Documentation string */</span>
<span class="cm">/* call function for all accessible objects */</span>
<span class="n">traverseproc</span> <span class="n">tp_traverse</span><span class="p">;</span>
<span class="cm">/* delete references to contained objects */</span>
<span class="n">inquiry</span> <span class="n">tp_clear</span><span class="p">;</span>
<span class="cm">/* rich comparisons */</span>
<span class="n">richcmpfunc</span> <span class="n">tp_richcompare</span><span class="p">;</span>
<span class="cm">/* weak reference enabler */</span>
<span class="n">Py_ssize_t</span> <span class="n">tp_weaklistoffset</span><span class="p">;</span>
<span class="cm">/* Iterators */</span>
<span class="n">getiterfunc</span> <span class="n">tp_iter</span><span class="p">;</span>
<span class="n">iternextfunc</span> <span class="n">tp_iternext</span><span class="p">;</span>
<span class="cm">/* Attribute descriptor and subclassing stuff */</span>
<span class="k">struct</span> <span class="n">PyMethodDef</span> <span class="o">*</span><span class="n">tp_methods</span><span class="p">;</span>
<span class="k">struct</span> <span class="n">PyMemberDef</span> <span class="o">*</span><span class="n">tp_members</span><span class="p">;</span>
<span class="k">struct</span> <span class="n">PyGetSetDef</span> <span class="o">*</span><span class="n">tp_getset</span><span class="p">;</span>
<span class="k">struct</span> <span class="n">_typeobject</span> <span class="o">*</span><span class="n">tp_base</span><span class="p">;</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">tp_dict</span><span class="p">;</span>
<span class="n">descrgetfunc</span> <span class="n">tp_descr_get</span><span class="p">;</span>
<span class="n">descrsetfunc</span> <span class="n">tp_descr_set</span><span class="p">;</span>
<span class="n">Py_ssize_t</span> <span class="n">tp_dictoffset</span><span class="p">;</span>
<span class="n">initproc</span> <span class="n">tp_init</span><span class="p">;</span>
<span class="n">allocfunc</span> <span class="n">tp_alloc</span><span class="p">;</span>
<span class="n">newfunc</span> <span class="n">tp_new</span><span class="p">;</span>
<span class="n">freefunc</span> <span class="n">tp_free</span><span class="p">;</span> <span class="cm">/* Low-level free-memory routine */</span>
<span class="n">inquiry</span> <span class="n">tp_is_gc</span><span class="p">;</span> <span class="cm">/* For PyObject_IS_GC */</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">tp_bases</span><span class="p">;</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">tp_mro</span><span class="p">;</span> <span class="cm">/* method resolution order */</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">tp_cache</span><span class="p">;</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">tp_subclasses</span><span class="p">;</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">tp_weaklist</span><span class="p">;</span>
<span class="n">destructor</span> <span class="n">tp_del</span><span class="p">;</span>
<span class="cm">/* Type attribute cache version tag. Added in version 2.6 */</span>
<span class="kt">unsigned</span> <span class="kt">int</span> <span class="n">tp_version_tag</span><span class="p">;</span>
<span class="n">destructor</span> <span class="n">tp_finalize</span><span class="p">;</span>
<span class="p">}</span> <span class="n">PyTypeObject</span><span class="p">;</span>
</pre></div>
</div>
<p>Now that’s a <em>lot</em> of methods. Don’t worry too much though – if you have
a type you want to define, the chances are very good that you will only
implement a handful of these.</p>
<p>As you probably expect by now, we’re going to go over this and give more
information about the various handlers. We won’t go in the order they are
defined in the structure, because there is a lot of historical baggage that
impacts the ordering of the fields. It’s often easiest to find an example
that includes the fields you need and then change the values to suit your new
type.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">tp_name</span><span class="p">;</span> <span class="cm">/* For printing */</span>
</pre></div>
</div>
<p>The name of the type – as mentioned in the previous chapter, this will appear in
various places, almost entirely for diagnostic purposes. Try to choose something
that will be helpful in such a situation!</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">Py_ssize_t</span> <span class="n">tp_basicsize</span><span class="p">,</span> <span class="n">tp_itemsize</span><span class="p">;</span> <span class="cm">/* For allocation */</span>
</pre></div>
</div>
<p>These fields tell the runtime how much memory to allocate when new objects of
this type are created. Python has some built-in support for variable length
structures (think: strings, tuples) which is where the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_itemsize" title="PyTypeObject.tp_itemsize"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_itemsize</span></code></a> field
comes in. This will be dealt with later.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">tp_doc</span><span class="p">;</span>
</pre></div>
</div>
<p>Here you can put a string (or its address) that you want returned when the
Python script references <code class="docutils literal notranslate"><span class="pre">obj.__doc__</span></code> to retrieve the doc string.</p>
<p>Now we come to the basic type methods – the ones most extension types will
implement.</p>
<div class="section" id="finalization-and-de-allocation">
<h2><span class="section-number">3.1. </span>Finalization and De-allocation<a class="headerlink" href="#finalization-and-de-allocation" title="Permalink to this headline">¶</a></h2>
<div class="highlight-c notranslate" id="index-0"><div class="highlight"><pre><span></span><span class="n">destructor</span> <span class="n">tp_dealloc</span><span class="p">;</span>
</pre></div>
</div>
<p>This function is called when the reference count of the instance of your type is
reduced to zero and the Python interpreter wants to reclaim it. If your type
has memory to free or other clean-up to perform, you can put it here. The
object itself needs to be freed here as well. Here is an example of this
function:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="kt">void</span>
<span class="nf">newdatatype_dealloc</span><span class="p">(</span><span class="n">newdatatypeobject</span> <span class="o">*</span><span class="n">obj</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">free</span><span class="p">(</span><span class="n">obj</span><span class="o">-></span><span class="n">obj_UnderlyingDatatypePtr</span><span class="p">);</span>
<span class="n">Py_TYPE</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span><span class="o">-></span><span class="n">tp_free</span><span class="p">(</span><span class="n">obj</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p id="index-1">One important requirement of the deallocator function is that it leaves any
pending exceptions alone. This is important since deallocators are frequently
called as the interpreter unwinds the Python stack; when the stack is unwound
due to an exception (rather than normal returns), nothing is done to protect the
deallocators from seeing that an exception has already been set. Any actions
which a deallocator performs which may cause additional Python code to be
executed may detect that an exception has been set. This can lead to misleading
errors from the interpreter. The proper way to protect against this is to save
a pending exception before performing the unsafe action, and restoring it when
done. This can be done using the <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_Fetch" title="PyErr_Fetch"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_Fetch()</span></code></a> and
<a class="reference internal" href="../c-api/exceptions.html#c.PyErr_Restore" title="PyErr_Restore"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_Restore()</span></code></a> functions:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="kt">void</span>
<span class="nf">my_dealloc</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">obj</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">MyObject</span> <span class="o">*</span><span class="n">self</span> <span class="o">=</span> <span class="p">(</span><span class="n">MyObject</span> <span class="o">*</span><span class="p">)</span> <span class="n">obj</span><span class="p">;</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">cbresult</span><span class="p">;</span>
<span class="k">if</span> <span class="p">(</span><span class="n">self</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="p">{</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">err_type</span><span class="p">,</span> <span class="o">*</span><span class="n">err_value</span><span class="p">,</span> <span class="o">*</span><span class="n">err_traceback</span><span class="p">;</span>
<span class="cm">/* This saves the current exception state */</span>
<span class="n">PyErr_Fetch</span><span class="p">(</span><span class="o">&</span><span class="n">err_type</span><span class="p">,</span> <span class="o">&</span><span class="n">err_value</span><span class="p">,</span> <span class="o">&</span><span class="n">err_traceback</span><span class="p">);</span>
<span class="n">cbresult</span> <span class="o">=</span> <span class="n">PyObject_CallObject</span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">my_callback</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">cbresult</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span>
<span class="n">PyErr_WriteUnraisable</span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">my_callback</span><span class="p">);</span>
<span class="k">else</span>
<span class="n">Py_DECREF</span><span class="p">(</span><span class="n">cbresult</span><span class="p">);</span>
<span class="cm">/* This restores the saved exception state */</span>
<span class="n">PyErr_Restore</span><span class="p">(</span><span class="n">err_type</span><span class="p">,</span> <span class="n">err_value</span><span class="p">,</span> <span class="n">err_traceback</span><span class="p">);</span>
<span class="n">Py_DECREF</span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">my_callback</span><span class="p">);</span>
<span class="p">}</span>
<span class="n">Py_TYPE</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span><span class="o">-></span><span class="n">tp_free</span><span class="p">((</span><span class="n">PyObject</span><span class="o">*</span><span class="p">)</span><span class="n">self</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>There are limitations to what you can safely do in a deallocator function.
First, if your type supports garbage collection (using <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code></a>
and/or <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_clear" title="PyTypeObject.tp_clear"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_clear</span></code></a>), some of the object’s members can have been
cleared or finalized by the time <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_dealloc" title="PyTypeObject.tp_dealloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_dealloc</span></code></a> is called. Second, in
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_dealloc" title="PyTypeObject.tp_dealloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_dealloc</span></code></a>, your object is in an unstable state: its reference
count is equal to zero. Any call to a non-trivial object or API (as in the
example above) might end up calling <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_dealloc" title="PyTypeObject.tp_dealloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_dealloc</span></code></a> again, causing a
double free and a crash.</p>
<p>Starting with Python 3.4, it is recommended not to put any complex
finalization code in <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_dealloc" title="PyTypeObject.tp_dealloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_dealloc</span></code></a>, and instead use the new
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_finalize" title="PyTypeObject.tp_finalize"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_finalize</span></code></a> type method.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><span class="target" id="index-2"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0442"><strong>PEP 442</strong></a> explains the new finalization scheme.</p>
</div>
</div>
</div>
<div class="section" id="object-presentation">
<span id="index-3"></span><h2><span class="section-number">3.2. </span>Object Presentation<a class="headerlink" href="#object-presentation" title="Permalink to this headline">¶</a></h2>
<p>In Python, there are two ways to generate a textual representation of an object:
the <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate"><span class="pre">repr()</span></code></a> function, and the <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a> function. (The <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate"><span class="pre">print()</span></code></a>
function just calls <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a>.) These handlers are both optional.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">reprfunc</span> <span class="n">tp_repr</span><span class="p">;</span>
<span class="n">reprfunc</span> <span class="n">tp_str</span><span class="p">;</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_repr" title="PyTypeObject.tp_repr"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_repr</span></code></a> handler should return a string object containing a
representation of the instance for which it is called. Here is a simple
example:</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">newdatatype_repr</span><span class="p">(</span><span class="n">newdatatypeobject</span> <span class="o">*</span> <span class="n">obj</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">return</span> <span class="n">PyUnicode_FromFormat</span><span class="p">(</span><span class="s">"Repr-ified_newdatatype{{size:%d}}"</span><span class="p">,</span>
<span class="n">obj</span><span class="o">-></span><span class="n">obj_UnderlyingDatatypePtr</span><span class="o">-></span><span class="n">size</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>If no <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_repr" title="PyTypeObject.tp_repr"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_repr</span></code></a> handler is specified, the interpreter will supply a
representation that uses the type’s <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_name" title="PyTypeObject.tp_name"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_name</span></code></a> and a uniquely-identifying
value for the object.</p>
<p>The <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_str" title="PyTypeObject.tp_str"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_str</span></code></a> handler is to <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a> what the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_repr" title="PyTypeObject.tp_repr"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_repr</span></code></a> handler
described above is to <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate"><span class="pre">repr()</span></code></a>; that is, it is called when Python code calls
<a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a> on an instance of your object. Its implementation is very similar
to the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_repr" title="PyTypeObject.tp_repr"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_repr</span></code></a> function, but the resulting string is intended for human
consumption. If <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_str" title="PyTypeObject.tp_str"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_str</span></code></a> is not specified, the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_repr" title="PyTypeObject.tp_repr"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_repr</span></code></a> handler is
used instead.</p>
<p>Here is a simple example:</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">newdatatype_str</span><span class="p">(</span><span class="n">newdatatypeobject</span> <span class="o">*</span> <span class="n">obj</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">return</span> <span class="n">PyUnicode_FromFormat</span><span class="p">(</span><span class="s">"Stringified_newdatatype{{size:%d}}"</span><span class="p">,</span>
<span class="n">obj</span><span class="o">-></span><span class="n">obj_UnderlyingDatatypePtr</span><span class="o">-></span><span class="n">size</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="attribute-management">
<h2><span class="section-number">3.3. </span>Attribute Management<a class="headerlink" href="#attribute-management" title="Permalink to this headline">¶</a></h2>
<p>For every object which can support attributes, the corresponding type must
provide the functions that control how the attributes are resolved. There needs
to be a function which can retrieve attributes (if any are defined), and another
to set attributes (if setting attributes is allowed). Removing an attribute is
a special case, for which the new value passed to the handler is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<p>Python supports two pairs of attribute handlers; a type that supports attributes
only needs to implement the functions for one pair. The difference is that one
pair takes the name of the attribute as a <code class="xref c c-type docutils literal notranslate"><span class="pre">char*</span></code>, while the other
accepts a <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyObject*</span></code></a>. Each type can use whichever pair makes more
sense for the implementation’s convenience.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">getattrfunc</span> <span class="n">tp_getattr</span><span class="p">;</span> <span class="cm">/* char * version */</span>
<span class="n">setattrfunc</span> <span class="n">tp_setattr</span><span class="p">;</span>
<span class="cm">/* ... */</span>
<span class="n">getattrofunc</span> <span class="n">tp_getattro</span><span class="p">;</span> <span class="cm">/* PyObject * version */</span>
<span class="n">setattrofunc</span> <span class="n">tp_setattro</span><span class="p">;</span>
</pre></div>
</div>
<p>If accessing attributes of an object is always a simple operation (this will be
explained shortly), there are generic implementations which can be used to
provide the <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyObject*</span></code></a> version of the attribute management functions.
The actual need for type-specific attribute handlers almost completely
disappeared starting with Python 2.2, though there are many examples which have
not been updated to use some of the new generic mechanism that is available.</p>
<div class="section" id="generic-attribute-management">
<span id="id1"></span><h3><span class="section-number">3.3.1. </span>Generic Attribute Management<a class="headerlink" href="#generic-attribute-management" title="Permalink to this headline">¶</a></h3>
<p>Most extension types only use <em>simple</em> attributes. So, what makes the
attributes simple? There are only a couple of conditions that must be met:</p>
<ol class="arabic simple">
<li><p>The name of the attributes must be known when <a class="reference internal" href="../c-api/type.html#c.PyType_Ready" title="PyType_Ready"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_Ready()</span></code></a> is
called.</p></li>
<li><p>No special processing is needed to record that an attribute was looked up or
set, nor do actions need to be taken based on the value.</p></li>
</ol>
<p>Note that this list does not place any restrictions on the values of the
attributes, when the values are computed, or how relevant data is stored.</p>
<p>When <a class="reference internal" href="../c-api/type.html#c.PyType_Ready" title="PyType_Ready"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_Ready()</span></code></a> is called, it uses three tables referenced by the
type object to create <a class="reference internal" href="../glossary.html#term-descriptor"><span class="xref std std-term">descriptor</span></a>s which are placed in the dictionary of the
type object. Each descriptor controls access to one attribute of the instance
object. Each of the tables is optional; if all three are <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, instances of
the type will only have attributes that are inherited from their base type, and
should leave the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_getattro" title="PyTypeObject.tp_getattro"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_getattro</span></code></a> and <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_setattro" title="PyTypeObject.tp_setattro"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_setattro</span></code></a> fields <code class="docutils literal notranslate"><span class="pre">NULL</span></code> as
well, allowing the base type to handle attributes.</p>
<p>The tables are declared as three fields of the type object:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">PyMethodDef</span> <span class="o">*</span><span class="n">tp_methods</span><span class="p">;</span>
<span class="k">struct</span> <span class="n">PyMemberDef</span> <span class="o">*</span><span class="n">tp_members</span><span class="p">;</span>
<span class="k">struct</span> <span class="n">PyGetSetDef</span> <span class="o">*</span><span class="n">tp_getset</span><span class="p">;</span>
</pre></div>
</div>
<p>If <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_methods" title="PyTypeObject.tp_methods"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_methods</span></code></a> is not <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, it must refer to 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. Each entry in the table is an instance of this
structure:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">typedef</span> <span class="k">struct</span> <span class="n">PyMethodDef</span> <span class="p">{</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">ml_name</span><span class="p">;</span> <span class="cm">/* method name */</span>
<span class="n">PyCFunction</span> <span class="n">ml_meth</span><span class="p">;</span> <span class="cm">/* implementation function */</span>
<span class="kt">int</span> <span class="n">ml_flags</span><span class="p">;</span> <span class="cm">/* flags */</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">ml_doc</span><span class="p">;</span> <span class="cm">/* docstring */</span>
<span class="p">}</span> <span class="n">PyMethodDef</span><span class="p">;</span>
</pre></div>
</div>
<p>One entry should be defined for each method provided by the type; no entries are
needed for methods inherited from a base type. One additional entry is needed
at the end; it is a sentinel that marks the end of the array. The
<code class="xref py py-attr docutils literal notranslate"><span class="pre">ml_name</span></code> field of the sentinel must be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<p>The second table is used to define attributes which map directly to data stored
in the instance. A variety of primitive C types are supported, and access may
be read-only or read-write. The structures in the table are defined as:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">typedef</span> <span class="k">struct</span> <span class="n">PyMemberDef</span> <span class="p">{</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">name</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">type</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">offset</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">flags</span><span class="p">;</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">doc</span><span class="p">;</span>
<span class="p">}</span> <span class="n">PyMemberDef</span><span class="p">;</span>
</pre></div>
</div>
<p>For each entry in the table, a <a class="reference internal" href="../glossary.html#term-descriptor"><span class="xref std std-term">descriptor</span></a> will be constructed and added to the
type which will be able to extract a value from the instance structure. The
<a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-attr docutils literal notranslate"><span class="pre">type</span></code></a> field should contain one of the type codes defined in the
<code class="file docutils literal notranslate"><span class="pre">structmember.h</span></code> header; the value will be used to determine how to
convert Python values to and from C values. The <code class="xref py py-attr docutils literal notranslate"><span class="pre">flags</span></code> field is used to
store flags which control how the attribute can be accessed.</p>
<p>The following flag constants are defined in <code class="file docutils literal notranslate"><span class="pre">structmember.h</span></code>; they may be
combined using bitwise-OR.</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 37%" />
<col style="width: 63%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">READONLY</span></code></p></td>
<td><p>Never writable.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">READ_RESTRICTED</span></code></p></td>
<td><p>Not readable in restricted mode.</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">WRITE_RESTRICTED</span></code></p></td>
<td><p>Not writable in restricted mode.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">RESTRICTED</span></code></p></td>
<td><p>Not readable or writable in restricted mode.</p></td>
</tr>
</tbody>
</table>
<p id="index-4">An interesting advantage of using the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_members" title="PyTypeObject.tp_members"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_members</span></code></a> table to build
descriptors that are used at runtime is that any attribute defined this way can
have an associated doc string simply by providing the text in the table. An
application can use the introspection API to retrieve the descriptor from the
class object, and get the doc string using its <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> attribute.</p>
<p>As with the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_methods" title="PyTypeObject.tp_methods"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_methods</span></code></a> table, a sentinel entry with a <code class="xref py py-attr docutils literal notranslate"><span class="pre">name</span></code> value
of <code class="docutils literal notranslate"><span class="pre">NULL</span></code> is required.</p>
</div>
<div class="section" id="type-specific-attribute-management">
<h3><span class="section-number">3.3.2. </span>Type-specific Attribute Management<a class="headerlink" href="#type-specific-attribute-management" title="Permalink to this headline">¶</a></h3>
<p>For simplicity, only the <code class="xref c c-type docutils literal notranslate"><span class="pre">char*</span></code> version will be demonstrated here; the
type of the name parameter is the only difference between the <code class="xref c c-type docutils literal notranslate"><span class="pre">char*</span></code>
and <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyObject*</span></code></a> flavors of the interface. This example effectively does
the same thing as the generic example above, but does not use the generic
support added in Python 2.2. It explains how the handler functions are
called, so that if you do need to extend their functionality, you’ll understand
what needs to be done.</p>
<p>The <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_getattr" title="PyTypeObject.tp_getattr"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_getattr</span></code></a> handler is called when the object requires an attribute
look-up. It is called in the same situations where the <a class="reference internal" href="../reference/datamodel.html#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a>
method of a class would be called.</p>
<p>Here is an example:</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">newdatatype_getattr</span><span class="p">(</span><span class="n">newdatatypeobject</span> <span class="o">*</span><span class="n">obj</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">name</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="n">strcmp</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="s">"data"</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">return</span> <span class="n">PyLong_FromLong</span><span class="p">(</span><span class="n">obj</span><span class="o">-></span><span class="n">data</span><span class="p">);</span>
<span class="p">}</span>
<span class="n">PyErr_Format</span><span class="p">(</span><span class="n">PyExc_AttributeError</span><span class="p">,</span>
<span class="s">"'%.50s' object has no attribute '%.400s'"</span><span class="p">,</span>
<span class="n">tp</span><span class="o">-></span><span class="n">tp_name</span><span class="p">,</span> <span class="n">name</span><span class="p">);</span>
<span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_setattr" title="PyTypeObject.tp_setattr"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_setattr</span></code></a> handler is called when the <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> or
<a class="reference internal" href="../reference/datamodel.html#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__delattr__()</span></code></a> method of a class instance would be called. When an
attribute should be deleted, the third parameter will be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>. Here is an
example that simply raises an exception; if this were really all you wanted, the
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_setattr" title="PyTypeObject.tp_setattr"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_setattr</span></code></a> handler should be set to <code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</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">newdatatype_setattr</span><span class="p">(</span><span class="n">newdatatypeobject</span> <span class="o">*</span><span class="n">obj</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">name</span><span class="p">,</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">v</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">PyErr_Format</span><span class="p">(</span><span class="n">PyExc_RuntimeError</span><span class="p">,</span> <span class="s">"Read-only attribute: %s"</span><span class="p">,</span> <span class="n">name</span><span class="p">);</span>
<span class="k">return</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="object-comparison">
<h2><span class="section-number">3.4. </span>Object Comparison<a class="headerlink" href="#object-comparison" title="Permalink to this headline">¶</a></h2>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">richcmpfunc</span> <span class="n">tp_richcompare</span><span class="p">;</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_richcompare" title="PyTypeObject.tp_richcompare"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_richcompare</span></code></a> handler is called when comparisons are needed. It is
analogous to the <a class="reference internal" href="../reference/datamodel.html#richcmpfuncs"><span class="std std-ref">rich comparison methods</span></a>, like
<a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a>, and also called by <a class="reference internal" href="../c-api/object.html#c.PyObject_RichCompare" title="PyObject_RichCompare"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_RichCompare()</span></code></a> and
<a class="reference internal" href="../c-api/object.html#c.PyObject_RichCompareBool" title="PyObject_RichCompareBool"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_RichCompareBool()</span></code></a>.</p>
<p>This function is called with two Python objects and the operator as arguments,
where the operator is one of <code class="docutils literal notranslate"><span class="pre">Py_EQ</span></code>, <code class="docutils literal notranslate"><span class="pre">Py_NE</span></code>, <code class="docutils literal notranslate"><span class="pre">Py_LE</span></code>, <code class="docutils literal notranslate"><span class="pre">Py_GT</span></code>,
<code class="docutils literal notranslate"><span class="pre">Py_LT</span></code> or <code class="docutils literal notranslate"><span class="pre">Py_GT</span></code>. It should compare the two objects with respect to the
specified operator and return <code class="docutils literal notranslate"><span class="pre">Py_True</span></code> or <code class="docutils literal notranslate"><span class="pre">Py_False</span></code> if the comparison is
successful, <code class="docutils literal notranslate"><span class="pre">Py_NotImplemented</span></code> to indicate that comparison is not
implemented and the other object’s comparison method should be tried, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code>
if an exception was set.</p>
<p>Here is a sample implementation, for a datatype that is considered equal if the
size of an internal pointer is equal:</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">newdatatype_richcmp</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">obj1</span><span class="p">,</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">obj2</span><span class="p">,</span> <span class="kt">int</span> <span class="n">op</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="p">;</span>
<span class="kt">int</span> <span class="n">c</span><span class="p">,</span> <span class="n">size1</span><span class="p">,</span> <span class="n">size2</span><span class="p">;</span>
<span class="cm">/* code to make sure that both arguments are of type</span>
<span class="cm"> newdatatype omitted */</span>
<span class="n">size1</span> <span class="o">=</span> <span class="n">obj1</span><span class="o">-></span><span class="n">obj_UnderlyingDatatypePtr</span><span class="o">-></span><span class="n">size</span><span class="p">;</span>
<span class="n">size2</span> <span class="o">=</span> <span class="n">obj2</span><span class="o">-></span><span class="n">obj_UnderlyingDatatypePtr</span><span class="o">-></span><span class="n">size</span><span class="p">;</span>
<span class="k">switch</span> <span class="p">(</span><span class="n">op</span><span class="p">)</span> <span class="p">{</span>
<span class="k">case</span> <span class="nl">Py_LT</span><span class="p">:</span> <span class="n">c</span> <span class="o">=</span> <span class="n">size1</span> <span class="o"><</span> <span class="n">size2</span><span class="p">;</span> <span class="k">break</span><span class="p">;</span>
<span class="k">case</span> <span class="nl">Py_LE</span><span class="p">:</span> <span class="n">c</span> <span class="o">=</span> <span class="n">size1</span> <span class="o"><=</span> <span class="n">size2</span><span class="p">;</span> <span class="k">break</span><span class="p">;</span>
<span class="k">case</span> <span class="nl">Py_EQ</span><span class="p">:</span> <span class="n">c</span> <span class="o">=</span> <span class="n">size1</span> <span class="o">==</span> <span class="n">size2</span><span class="p">;</span> <span class="k">break</span><span class="p">;</span>
<span class="k">case</span> <span class="nl">Py_NE</span><span class="p">:</span> <span class="n">c</span> <span class="o">=</span> <span class="n">size1</span> <span class="o">!=</span> <span class="n">size2</span><span class="p">;</span> <span class="k">break</span><span class="p">;</span>
<span class="k">case</span> <span class="nl">Py_GT</span><span class="p">:</span> <span class="n">c</span> <span class="o">=</span> <span class="n">size1</span> <span class="o">></span> <span class="n">size2</span><span class="p">;</span> <span class="k">break</span><span class="p">;</span>
<span class="k">case</span> <span class="nl">Py_GE</span><span class="p">:</span> <span class="n">c</span> <span class="o">=</span> <span class="n">size1</span> <span class="o">>=</span> <span class="n">size2</span><span class="p">;</span> <span class="k">break</span><span class="p">;</span>
<span class="p">}</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">c</span> <span class="o">?</span> <span class="nl">Py_True</span> <span class="p">:</span> <span class="n">Py_False</span><span class="p">;</span>
<span class="n">Py_INCREF</span><span class="p">(</span><span class="n">result</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>
</div>
<div class="section" id="abstract-protocol-support">
<h2><span class="section-number">3.5. </span>Abstract Protocol Support<a class="headerlink" href="#abstract-protocol-support" title="Permalink to this headline">¶</a></h2>
<p>Python supports a variety of <em>abstract</em> ‘protocols;’ the specific interfaces
provided to use these interfaces are documented in <a class="reference internal" href="../c-api/abstract.html#abstract"><span class="std std-ref">Abstract Objects Layer</span></a>.</p>
<p>A number of these abstract interfaces were defined early in the development of
the Python implementation. In particular, the number, mapping, and sequence
protocols have been part of Python since the beginning. Other protocols have
been added over time. For protocols which depend on several handler routines
from the type implementation, the older protocols have been defined as optional
blocks of handlers referenced by the type object. For newer protocols there are
additional slots in the main type object, with a flag bit being set to indicate
that the slots are present and should be checked by the interpreter. (The flag
bit does not indicate that the slot values are non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code>. The flag may be set
to indicate the presence of a slot, but a slot may still be unfilled.)</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyNumberMethods</span> <span class="o">*</span><span class="n">tp_as_number</span><span class="p">;</span>
<span class="n">PySequenceMethods</span> <span class="o">*</span><span class="n">tp_as_sequence</span><span class="p">;</span>
<span class="n">PyMappingMethods</span> <span class="o">*</span><span class="n">tp_as_mapping</span><span class="p">;</span>
</pre></div>
</div>
<p>If you wish your object to be able to act like a number, a sequence, or a
mapping object, then you place the address of a structure that implements the C
type <a class="reference internal" href="../c-api/typeobj.html#c.PyNumberMethods" title="PyNumberMethods"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyNumberMethods</span></code></a>, <a class="reference internal" href="../c-api/typeobj.html#c.PySequenceMethods" title="PySequenceMethods"><code class="xref c c-type docutils literal notranslate"><span class="pre">PySequenceMethods</span></code></a>, or
<a class="reference internal" href="../c-api/typeobj.html#c.PyMappingMethods" title="PyMappingMethods"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyMappingMethods</span></code></a>, respectively. It is up to you to fill in this
structure with appropriate values. You can find examples of the use of each of
these in the <code class="file docutils literal notranslate"><span class="pre">Objects</span></code> directory of the Python source distribution.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">hashfunc</span> <span class="n">tp_hash</span><span class="p">;</span>
</pre></div>
</div>
<p>This function, if you choose to provide it, should return a hash number for an
instance of your data type. Here is a simple example:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="n">Py_hash_t</span>
<span class="nf">newdatatype_hash</span><span class="p">(</span><span class="n">newdatatypeobject</span> <span class="o">*</span><span class="n">obj</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">Py_hash_t</span> <span class="n">result</span><span class="p">;</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">obj</span><span class="o">-></span><span class="n">some_size</span> <span class="o">+</span> <span class="mi">32767</span> <span class="o">*</span> <span class="n">obj</span><span class="o">-></span><span class="n">some_number</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="o">-</span><span class="mi">1</span><span class="p">)</span>
<span class="n">result</span> <span class="o">=</span> <span class="o">-</span><span class="mi">2</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><code class="xref c c-type docutils literal notranslate"><span class="pre">Py_hash_t</span></code> is a signed integer type with a platform-varying width.
Returning <code class="docutils literal notranslate"><span class="pre">-1</span></code> from <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_hash" title="PyTypeObject.tp_hash"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_hash</span></code></a> indicates an error,
which is why you should be careful to avoid returning it when hash computation
is successful, as seen above.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">ternaryfunc</span> <span class="n">tp_call</span><span class="p">;</span>
</pre></div>
</div>
<p>This function is called when an instance of your data type is “called”, for
example, if <code class="docutils literal notranslate"><span class="pre">obj1</span></code> is an instance of your data type and the Python script
contains <code class="docutils literal notranslate"><span class="pre">obj1('hello')</span></code>, the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_call" title="PyTypeObject.tp_call"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_call</span></code></a> handler is invoked.</p>
<p>This function takes three arguments:</p>
<ol class="arabic simple">
<li><p><em>self</em> is the instance of the data type which is the subject of the call.
If the call is <code class="docutils literal notranslate"><span class="pre">obj1('hello')</span></code>, then <em>self</em> is <code class="docutils literal notranslate"><span class="pre">obj1</span></code>.</p></li>
<li><p><em>args</em> is a tuple containing the arguments to the call. You can use
<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> to extract the arguments.</p></li>
<li><p><em>kwds</em> is a dictionary of keyword arguments that were passed. If this is
non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> and you support keyword arguments, 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 extract the arguments. If you
do not want to support keyword arguments and this is non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code>, raise a
<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> with a message saying that keyword arguments are not supported.</p></li>
</ol>
<p>Here is a toy <code class="docutils literal notranslate"><span class="pre">tp_call</span></code> implementation:</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">newdatatype_call</span><span class="p">(</span><span class="n">newdatatypeobject</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">kwds</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="p">;</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">arg1</span><span class="p">;</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">arg2</span><span class="p">;</span>
<span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">arg3</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">"sss:call"</span><span class="p">,</span> <span class="o">&</span><span class="n">arg1</span><span class="p">,</span> <span class="o">&</span><span class="n">arg2</span><span class="p">,</span> <span class="o">&</span><span class="n">arg3</span><span class="p">))</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">result</span> <span class="o">=</span> <span class="n">PyUnicode_FromFormat</span><span class="p">(</span>
<span class="s">"Returning -- value: [%d] arg1: [%s] arg2: [%s] arg3: [%s]</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span>
<span class="n">obj</span><span class="o">-></span><span class="n">obj_UnderlyingDatatypePtr</span><span class="o">-></span><span class="n">size</span><span class="p">,</span>
<span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">,</span> <span class="n">arg3</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>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cm">/* Iterators */</span>
<span class="n">getiterfunc</span> <span class="n">tp_iter</span><span class="p">;</span>
<span class="n">iternextfunc</span> <span class="n">tp_iternext</span><span class="p">;</span>
</pre></div>
</div>
<p>These functions provide support for the iterator protocol. Both handlers
take exactly one parameter, the instance for which they are being called,
and return a new reference. In the case of an error, they should set an
exception and return <code class="docutils literal notranslate"><span class="pre">NULL</span></code>. <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iter" title="PyTypeObject.tp_iter"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iter</span></code></a> corresponds
to the Python <a class="reference internal" href="../reference/datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__iter__()</span></code></a> method, while <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iternext" title="PyTypeObject.tp_iternext"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iternext</span></code></a>
corresponds to the Python <a class="reference internal" href="../library/stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> method.</p>
<p>Any <a class="reference internal" href="../glossary.html#term-iterable"><span class="xref std std-term">iterable</span></a> object must implement the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iter" title="PyTypeObject.tp_iter"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iter</span></code></a>
handler, which must return an <a class="reference internal" href="../glossary.html#term-iterator"><span class="xref std std-term">iterator</span></a> object. Here the same guidelines
apply as for Python classes:</p>
<ul class="simple">
<li><p>For collections (such as lists and tuples) which can support multiple
independent iterators, a new iterator should be created and returned by
each call to <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iter" title="PyTypeObject.tp_iter"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iter</span></code></a>.</p></li>
<li><p>Objects which can only be iterated over once (usually due to side effects of
iteration, such as file objects) can implement <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iter" title="PyTypeObject.tp_iter"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iter</span></code></a>
by returning a new reference to themselves – and should also therefore
implement the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iternext" title="PyTypeObject.tp_iternext"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iternext</span></code></a> handler.</p></li>
</ul>
<p>Any <a class="reference internal" href="../glossary.html#term-iterator"><span class="xref std std-term">iterator</span></a> object should implement both <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iter" title="PyTypeObject.tp_iter"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iter</span></code></a>
and <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iternext" title="PyTypeObject.tp_iternext"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iternext</span></code></a>. An iterator’s
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iter" title="PyTypeObject.tp_iter"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iter</span></code></a> handler should return a new reference
to the iterator. Its <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iternext" title="PyTypeObject.tp_iternext"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iternext</span></code></a> handler should
return a new reference to the next object in the iteration, if there is one.
If the iteration has reached the end, <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iternext" title="PyTypeObject.tp_iternext"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iternext</span></code></a>
may return <code class="docutils literal notranslate"><span class="pre">NULL</span></code> without setting an exception, or it may set
<a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> <em>in addition</em> to returning <code class="docutils literal notranslate"><span class="pre">NULL</span></code>; avoiding
the exception can yield slightly better performance. If an actual error
occurs, <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_iternext" title="PyTypeObject.tp_iternext"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_iternext</span></code></a> should always set an exception
and return <code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
</div>
<div class="section" id="weak-reference-support">
<span id="weakref-support"></span><h2><span class="section-number">3.6. </span>Weak Reference Support<a class="headerlink" href="#weak-reference-support" title="Permalink to this headline">¶</a></h2>
<p>One of the goals of Python’s weak reference implementation is to allow any type
to participate in the weak reference mechanism without incurring the overhead on
performance-critical objects (such as numbers).</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p>Documentation for the <a class="reference internal" href="../library/weakref.html#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate"><span class="pre">weakref</span></code></a> module.</p>
</div>
<p>For an object to be weakly referencable, the extension type must do two things:</p>
<ol class="arabic simple">
<li><p>Include a <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyObject*</span></code></a> field in the C object structure dedicated to
the weak reference mechanism. The object’s constructor should leave it
<code class="docutils literal notranslate"><span class="pre">NULL</span></code> (which is automatic when using the default
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_alloc" title="PyTypeObject.tp_alloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_alloc</span></code></a>).</p></li>
<li><p>Set the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_weaklistoffset" title="PyTypeObject.tp_weaklistoffset"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_weaklistoffset</span></code></a> type member
to the offset of the aforementioned field in the C object structure,
so that the interpreter knows how to access and modify that field.</p></li>
</ol>
<p>Concretely, here is how a trivial object structure would be augmented
with the required field:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">typedef</span> <span class="k">struct</span> <span class="p">{</span>
<span class="n">PyObject_HEAD</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">weakreflist</span><span class="p">;</span> <span class="cm">/* List of weak references */</span>
<span class="p">}</span> <span class="n">TrivialObject</span><span class="p">;</span>
</pre></div>
</div>
<p>And the corresponding member in the statically-declared type object:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="n">PyTypeObject</span> <span class="n">TrivialType</span> <span class="o">=</span> <span class="p">{</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>
<span class="cm">/* ... other members omitted for brevity ... */</span>
<span class="p">.</span><span class="n">tp_weaklistoffset</span> <span class="o">=</span> <span class="n">offsetof</span><span class="p">(</span><span class="n">TrivialObject</span><span class="p">,</span> <span class="n">weakreflist</span><span class="p">),</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The only further addition is that <code class="docutils literal notranslate"><span class="pre">tp_dealloc</span></code> needs to clear any weak
references (by calling <code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_ClearWeakRefs()</span></code>) if the field is
non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code>:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="kt">void</span>
<span class="nf">Trivial_dealloc</span><span class="p">(</span><span class="n">TrivialObject</span> <span class="o">*</span><span class="n">self</span><span class="p">)</span>
<span class="p">{</span>
<span class="cm">/* Clear weakrefs first before calling any destructors */</span>
<span class="k">if</span> <span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">weakreflist</span> <span class="o">!=</span> <span class="nb">NULL</span><span class="p">)</span>
<span class="n">PyObject_ClearWeakRefs</span><span class="p">((</span><span class="n">PyObject</span> <span class="o">*</span><span class="p">)</span> <span class="n">self</span><span class="p">);</span>
<span class="cm">/* ... remainder of destruction code omitted for brevity ... */</span>
<span class="n">Py_TYPE</span><span class="p">(</span><span class="n">self</span><span class="p">)</span><span class="o">-></span><span class="n">tp_free</span><span class="p">((</span><span class="n">PyObject</span> <span class="o">*</span><span class="p">)</span> <span class="n">self</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="more-suggestions">
<h2><span class="section-number">3.7. </span>More Suggestions<a class="headerlink" href="#more-suggestions" title="Permalink to this headline">¶</a></h2>
<p>In order to learn how to implement any specific method for your new data type,
get the <a class="reference internal" href="../glossary.html#term-cpython"><span class="xref std std-term">CPython</span></a> source code. Go to the <code class="file docutils literal notranslate"><span class="pre">Objects</span></code> directory,
then search the C source files for <code class="docutils literal notranslate"><span class="pre">tp_</span></code> plus the function you want
(for example, <code class="docutils literal notranslate"><span class="pre">tp_richcompare</span></code>). You will find examples of the function
you want to implement.</p>
<p>When you need to verify that an object is a concrete instance of the type you
are implementing, use the <a class="reference internal" href="../c-api/object.html#c.PyObject_TypeCheck" title="PyObject_TypeCheck"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_TypeCheck()</span></code></a> function. A sample of
its use might be something like the following:</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">PyObject_TypeCheck</span><span class="p">(</span><span class="n">some_object</span><span class="p">,</span> <span class="o">&</span><span class="n">MyType</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">"arg #1 not a mything"</span><span class="p">);</span>
<span class="k">return</span> <span class="nb">NULL</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt>Download CPython source releases.</dt><dd><p><a class="reference external" href="https://www.python.org/downloads/source/">https://www.python.org/downloads/source/</a></p>
</dd>
<dt>The CPython project on GitHub, where the CPython source code is developed.</dt><dd><p><a class="reference external" href="https://github.com/python/cpython">https://github.com/python/cpython</a></p>
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<li><a class="reference internal" href="#">3. Defining Extension Types: Assorted Topics</a><ul>
<li><a class="reference internal" href="#finalization-and-de-allocation">3.1. Finalization and De-allocation</a></li>
<li><a class="reference internal" href="#object-presentation">3.2. Object Presentation</a></li>
<li><a class="reference internal" href="#attribute-management">3.3. Attribute Management</a><ul>
<li><a class="reference internal" href="#generic-attribute-management">3.3.1. Generic Attribute Management</a></li>
<li><a class="reference internal" href="#type-specific-attribute-management">3.3.2. Type-specific Attribute Management</a></li>
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<li><a class="reference internal" href="#object-comparison">3.4. Object Comparison</a></li>
<li><a class="reference internal" href="#abstract-protocol-support">3.5. Abstract Protocol Support</a></li>
<li><a class="reference internal" href="#weak-reference-support">3.6. Weak Reference Support</a></li>
<li><a class="reference internal" href="#more-suggestions">3.7. More Suggestions</a></li>
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