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 <div class="section" id="defining-extension-types-tutorial">
<h1>2. Defining Extension Types: Tutorial<a class="headerlink" href="#defining-extension-types-tutorial" title="Permalink to this headline">¶</a></h1>
Python allows the writer of a C extension module to define new types that
can be manipulated from Python code, much like the built-in <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>
and <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a> types. The code for all extension types follows a
pattern, but there are some details that you need to understand before you
can get started. This document is a gentle introduction to the topic.
<div class="section" id="the-basics">
<h2>2.1. The Basics<a class="headerlink" href="#the-basics" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="../glossary.html#term-cpython">CPython</a> runtime sees all Python objects as variables of type
<a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate">PyObject*</code></a>, which serves as a “base type” for all Python objects.
The <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate">PyObject</code></a> structure itself only contains the object’s
<a class="reference internal" href="../glossary.html#term-reference-count">reference count</a> and a pointer to the object’s “type object”.
This is where the action is; the type object determines which (C) functions
get called by the interpreter when, for instance, an attribute gets looked up
on an object, a method called, or it is multiplied by another object. These
C functions are called “type methods”.
So, if you want to define a new extension type, you need to create a new type
object.
This sort of thing can only be explained by example, so here’s a minimal, but
complete, module that defines a new type named <code class="xref py py-class docutils literal notranslate">Custom</code> inside a C
extension module <code class="xref py py-mod docutils literal notranslate">custom</code>:
<div class="admonition note">
Note
What we’re showing here is the traditional way of defining static
extension types. It should be adequate for most uses. The C API also
allows defining heap-allocated extension types using the
<a class="reference internal" href="../c-api/type.html#c.PyType_FromSpec" title="PyType_FromSpec"><code class="xref c c-func docutils literal notranslate">PyType_FromSpec()</code></a> function, which isn’t covered in this tutorial.
</div>
<div class="highlight-c notranslate"><div class="highlight"><pre>#include &lt;Python.h&gt;

typedef struct {
 PyObject_HEAD
 /* Type-specific fields go here. */
} CustomObject;

static PyModuleDef custommodule = {
 PyModuleDef_HEAD_INIT,
 .m_name = &quot;custom&quot;,
 .m_doc = &quot;Example module that creates an extension type.&quot;,
 .m_size = -1,
};

PyMODINIT_FUNC
PyInit_custom(void)
{
 PyObject *m;
 if (PyType_Ready(&amp;CustomType) &lt; 0)
 return NULL;

m = PyModule_Create(&amp;custommodule);
 if (m == NULL)
 return NULL;

Py_INCREF(&amp;CustomType);
 PyModule_AddObject(m, &quot;Custom&quot;, (PyObject *) &amp;CustomType);
 return m;
}
</pre></div>
</div>
Now that’s quite a bit to take in at once, but hopefully bits will seem familiar
from the previous chapter. This file defines three things:
<ol class="arabic simple">
<li>What a <code class="xref py py-class docutils literal notranslate">Custom</code> object contains: this is the <code class="docutils literal notranslate">CustomObject</code>
struct, which is allocated once for each <code class="xref py py-class docutils literal notranslate">Custom</code> instance.</li>
<li>How the <code class="xref py py-class docutils literal notranslate">Custom</code> type behaves: this is the <code class="docutils literal notranslate">CustomType</code> struct,
which defines a set of flags and function pointers that the interpreter
inspects when specific operations are requested.</li>
<li>How to initialize the <code class="xref py py-mod docutils literal notranslate">custom</code> module: this is the <code class="docutils literal notranslate">PyInit_custom</code>
function and the associated <code class="docutils literal notranslate">custommodule</code> struct.</li>
</ol>
The first bit is:
<div class="highlight-c notranslate"><div class="highlight"><pre>typedef struct {
 PyObject_HEAD
} CustomObject;
</pre></div>
</div>
This is what a Custom object will contain. <code class="docutils literal notranslate">PyObject_HEAD</code> is mandatory
at the start of each object struct and defines a field called <code class="docutils literal notranslate">ob_base</code>
of type <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate">PyObject</code></a>, containing a pointer to a type object and a
reference count (these can be accessed using the macros <a class="reference internal" href="../c-api/structures.html#c.Py_REFCNT" title="Py_REFCNT"><code class="xref c c-macro docutils literal notranslate">Py_REFCNT</code></a>
and <a class="reference internal" href="../c-api/structures.html#c.Py_TYPE" title="Py_TYPE"><code class="xref c c-macro docutils literal notranslate">Py_TYPE</code></a> respectively). The reason for the macro is to
abstract away the layout and to enable additional fields in debug builds.
<div class="admonition note">
Note
There is no semicolon above after the <a class="reference internal" href="../c-api/structures.html#c.PyObject_HEAD" title="PyObject_HEAD"><code class="xref c c-macro docutils literal notranslate">PyObject_HEAD</code></a> macro.
Be wary of adding one by accident: some compilers will complain.
</div>
Of course, objects generally store additional data besides the standard
<code class="docutils literal notranslate">PyObject_HEAD</code> boilerplate; for example, here is the definition for
standard Python floats:
<div class="highlight-c notranslate"><div class="highlight"><pre>typedef struct {
 PyObject_HEAD
 double ob_fval;
} PyFloatObject;
</pre></div>
</div>
The second bit is the definition of the type object.
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyTypeObject CustomType = {
 PyVarObject_HEAD_INIT(NULL, 0)
 .tp_name = &quot;custom.Custom&quot;,
 .tp_doc = &quot;Custom objects&quot;,
 .tp_basicsize = sizeof(CustomObject),
 .tp_itemsize = 0,
 .tp_new = PyType_GenericNew,
};
</pre></div>
</div>
<div class="admonition note">
Note
We recommend using C99-style designated initializers as above, to
avoid listing all the <a class="reference internal" href="../c-api/type.html#c.PyTypeObject" title="PyTypeObject"><code class="xref c c-type docutils literal notranslate">PyTypeObject</code></a> fields that you don’t care
about and also to avoid caring about the fields’ declaration order.
</div>
The actual definition of <a class="reference internal" href="../c-api/type.html#c.PyTypeObject" title="PyTypeObject"><code class="xref c c-type docutils literal notranslate">PyTypeObject</code></a> in <code class="file docutils literal notranslate">object.h</code> has
many more <a class="reference internal" href="../c-api/typeobj.html#type-structs">fields</a> than the definition above. The
remaining fields will be filled with zeros by the C compiler, and it’s
common practice to not specify them explicitly unless you need them.
We’re going to pick it apart, one field at a time:
<div class="highlight-c notranslate"><div class="highlight"><pre>PyVarObject_HEAD_INIT(NULL, 0)
</pre></div>
</div>
This line is mandatory boilerplate to initialize the <code class="docutils literal notranslate">ob_base</code>
field mentioned above.
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_name = &quot;custom.Custom&quot;,
</pre></div>
</div>
The name of our type. This will appear in the default textual representation of
our objects and in some error messages, for example:
<div class="highlight-pycon notranslate"><div class="highlight"><pre>&gt;&gt;&gt; &quot;&quot; + custom.Custom()
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
TypeError: can only concatenate str (not &quot;custom.Custom&quot;) to str
</pre></div>
</div>
Note that the name is a dotted name that includes both the module name and the
name of the type within the module. The module in this case is <code class="xref py py-mod docutils literal notranslate">custom</code> and
the type is <code class="xref py py-class docutils literal notranslate">Custom</code>, so we set the type name to <code class="xref py py-class docutils literal notranslate">custom.Custom</code>.
Using the real dotted import path is important to make your type compatible
with the <a class="reference internal" href="../library/pydoc.html#module-pydoc" title="pydoc: Documentation generator and online help system."><code class="xref py py-mod docutils literal notranslate">pydoc</code></a> and <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> modules.
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
</pre></div>
</div>
This is so that Python knows how much memory to allocate when creating
new <code class="xref py py-class docutils literal notranslate">Custom</code> instances. <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">tp_itemsize</code></a> is
only used for variable-sized objects and should otherwise be zero.
<div class="admonition note">
Note
If you want your type to be subclassable from Python, and your type has the same
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_basicsize" title="PyTypeObject.tp_basicsize"><code class="xref c c-member docutils literal notranslate">tp_basicsize</code></a> as its base type, you may have problems with multiple
inheritance. A Python subclass of your type will have to list your type first
in its <a class="reference internal" href="../library/stdtypes.html#class.__bases__" title="class.__bases__"><code class="xref py py-attr docutils literal notranslate">__bases__</code></a>, or else it will not be able to call your type’s
<a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate">__new__()</code></a> method without getting an error. You can avoid this problem by
ensuring that your type has a larger value for <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_basicsize" title="PyTypeObject.tp_basicsize"><code class="xref c c-member docutils literal notranslate">tp_basicsize</code></a> than its
base type does. Most of the time, this will be true anyway, because either your
base type will be <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>, or else you will be adding data members to
your base type, and therefore increasing its size.
</div>
We set the class flags to <a class="reference internal" href="../c-api/typeobj.html#Py_TPFLAGS_DEFAULT" title="Py_TPFLAGS_DEFAULT"><code class="xref py py-const docutils literal notranslate">Py_TPFLAGS_DEFAULT</code></a>.
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_flags = Py_TPFLAGS_DEFAULT,
</pre></div>
</div>
All types should include this constant in their flags. It enables all of the
members defined until at least Python 3.3. If you need further members,
you will need to OR the corresponding flags.
We provide a doc string for the type in <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_doc" title="PyTypeObject.tp_doc"><code class="xref c c-member docutils literal notranslate">tp_doc</code></a>.
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_doc = &quot;Custom objects&quot;,
</pre></div>
</div>
To enable object creation, we have to provide a <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_new" title="PyTypeObject.tp_new"><code class="xref c c-member docutils literal notranslate">tp_new</code></a>
handler. This is the equivalent of the Python method <a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate">__new__()</code></a>, but
has to be specified explicitly. In this case, we can just use the default
implementation provided by the API function <a class="reference internal" href="../c-api/type.html#c.PyType_GenericNew" title="PyType_GenericNew"><code class="xref c c-func docutils literal notranslate">PyType_GenericNew()</code></a>.
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_new = PyType_GenericNew,
</pre></div>
</div>
Everything else in the file should be familiar, except for some code in
<code class="xref c c-func docutils literal notranslate">PyInit_custom()</code>:
<div class="highlight-c notranslate"><div class="highlight"><pre>if (PyType_Ready(&amp;CustomType) &lt; 0)
 return;
</pre></div>
</div>
This initializes the <code class="xref py py-class docutils literal notranslate">Custom</code> type, filling in a number of members
to the appropriate default values, including <code class="xref py py-attr docutils literal notranslate">ob_type</code> that we initially
set to NULL.
<div class="highlight-c notranslate"><div class="highlight"><pre>PyModule_AddObject(m, &quot;Custom&quot;, (PyObject *) &amp;CustomType);
</pre></div>
</div>
This adds the type to the module dictionary. This allows us to create
<code class="xref py py-class docutils literal notranslate">Custom</code> instances by calling the <code class="xref py py-class docutils literal notranslate">Custom</code> class:
<div class="highlight-pycon notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import custom
&gt;&gt;&gt; mycustom = custom.Custom()
</pre></div>
</div>
That’s it! All that remains is to build it; put the above code in a file called
<code class="file docutils literal notranslate">custom.c</code> and:
<div class="highlight-python notranslate"><div class="highlight"><pre>from distutils.core import setup, Extension
setup(name=&quot;custom&quot;, version=&quot;1.0&quot;,
 ext_modules=[Extension(&quot;custom&quot;, [&quot;custom.c&quot;])])
</pre></div>
</div>
in a file called <code class="file docutils literal notranslate">setup.py</code>; then typing
<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python setup.py build
</pre></div>
</div>
at a shell should produce a file <code class="file docutils literal notranslate">custom.so</code> in a subdirectory; move to
that directory and fire up Python — you should be able to <code class="docutils literal notranslate">import custom</code> and
play around with Custom objects.
That wasn’t so hard, was it?
Of course, the current Custom type is pretty uninteresting. It has no data and
doesn’t do anything. It can’t even be subclassed.
<div class="admonition note">
Note
While this documentation showcases the standard <a class="reference internal" href="../library/distutils.html#module-distutils" title="distutils: Support for building and installing Python modules into an existing Python installation."><code class="xref py py-mod docutils literal notranslate">distutils</code></a> module
for building C extensions, it is recommended in real-world use cases to
use the newer and better-maintained <code class="docutils literal notranslate">setuptools</code> library. Documentation
on how to do this is out of scope for this document and can be found in
the <a class="reference external" href="https://packaging.python.org/tutorials/distributing-packages/">Python Packaging User’s Guide</a>.
</div>
</div>
<div class="section" id="adding-data-and-methods-to-the-basic-example">
<h2>2.2. Adding data and methods to the Basic example<a class="headerlink" href="#adding-data-and-methods-to-the-basic-example" title="Permalink to this headline">¶</a></h2>
Let’s extend the basic example to add some data and methods. Let’s also make
the type usable as a base class. We’ll create a new module, <code class="xref py py-mod docutils literal notranslate">custom2</code> that
adds these capabilities:
<div class="highlight-c notranslate"><div class="highlight"><pre>#include &lt;Python.h&gt;
#include &quot;structmember.h&quot;

typedef struct {
 PyObject_HEAD
 PyObject *first; /* first name */
 PyObject *last; /* last name */
 int number;
} CustomObject;

static void
Custom_dealloc(CustomObject *self)
{
 Py_XDECREF(self-&gt;first);
 Py_XDECREF(self-&gt;last);
 Py_TYPE(self)-&gt;tp_free((PyObject *) self);
}

static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
 CustomObject *self;
 self = (CustomObject *) type-&gt;tp_alloc(type, 0);
 if (self != NULL) {
 self-&gt;first = PyUnicode_FromString(&quot;&quot;);
 if (self-&gt;first == NULL) {
 Py_DECREF(self);
 return NULL;
 }
 self-&gt;last = PyUnicode_FromString(&quot;&quot;);
 if (self-&gt;last == NULL) {
 Py_DECREF(self);
 return NULL;
 }
 self-&gt;number = 0;
 }
 return (PyObject *) self;
}

static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
 static char *kwlist[] = {&quot;first&quot;, &quot;last&quot;, &quot;number&quot;, NULL};
 PyObject *first = NULL, *last = NULL, *tmp;

if (!PyArg_ParseTupleAndKeywords(args, kwds, &quot;|OOi&quot;, kwlist,
 &amp;first, &amp;last,
 &amp;self-&gt;number))
 return -1;

static PyMemberDef Custom_members[] = {
 {&quot;first&quot;, T_OBJECT_EX, offsetof(CustomObject, first), 0,
 &quot;first name&quot;},
 {&quot;last&quot;, T_OBJECT_EX, offsetof(CustomObject, last), 0,
 &quot;last name&quot;},
 {&quot;number&quot;, T_INT, offsetof(CustomObject, number), 0,
 &quot;custom number&quot;},
 {NULL} /* Sentinel */
};

static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
 if (self-&gt;first == NULL) {
 PyErr_SetString(PyExc_AttributeError, &quot;first&quot;);
 return NULL;
 }
 if (self-&gt;last == NULL) {
 PyErr_SetString(PyExc_AttributeError, &quot;last&quot;);
 return NULL;
 }
 return PyUnicode_FromFormat(&quot;%S %S&quot;, self-&gt;first, self-&gt;last);
}

static PyMethodDef Custom_methods[] = {
 {&quot;name&quot;, (PyCFunction) Custom_name, METH_NOARGS,
 &quot;Return the name, combining the first and last name&quot;
 },
 {NULL} /* Sentinel */
};

static PyTypeObject CustomType = {
 PyVarObject_HEAD_INIT(NULL, 0)
 .tp_name = &quot;custom2.Custom&quot;,
 .tp_doc = &quot;Custom objects&quot;,
 .tp_basicsize = sizeof(CustomObject),
 .tp_itemsize = 0,
 .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
 .tp_new = Custom_new,
 .tp_init = (initproc) Custom_init,
 .tp_dealloc = (destructor) Custom_dealloc,
 .tp_members = Custom_members,
 .tp_methods = Custom_methods,
};

static PyModuleDef custommodule = {
 PyModuleDef_HEAD_INIT,
 .m_name = &quot;custom2&quot;,
 .m_doc = &quot;Example module that creates an extension type.&quot;,
 .m_size = -1,
};

PyMODINIT_FUNC
PyInit_custom2(void)
{
 PyObject *m;
 if (PyType_Ready(&amp;CustomType) &lt; 0)
 return NULL;

Py_INCREF(&amp;CustomType);
 PyModule_AddObject(m, &quot;Custom&quot;, (PyObject *) &amp;CustomType);
 return m;
}
</pre></div>
</div>
This version of the module has a number of changes.
We’ve added an extra include:
<div class="highlight-c notranslate"><div class="highlight"><pre>#include &lt;structmember.h&gt;
</pre></div>
</div>
This include provides declarations that we use to handle attributes, as
described a bit later.
The <code class="xref py py-class docutils literal notranslate">Custom</code> type now has three data attributes in its C struct,
first, last, and number. The first and last variables are Python
strings containing first and last names. The number attribute is a C integer.
The object structure is updated accordingly:
<div class="highlight-c notranslate"><div class="highlight"><pre>typedef struct {
 PyObject_HEAD
 PyObject *first; /* first name */
 PyObject *last; /* last name */
 int number;
} CustomObject;
</pre></div>
</div>
Because we now have data to manage, we have to be more careful about object
allocation and deallocation. At a minimum, we need a deallocation method:
<div class="highlight-c notranslate"><div class="highlight"><pre>static void
Custom_dealloc(CustomObject *self)
{
 Py_XDECREF(self-&gt;first);
 Py_XDECREF(self-&gt;last);
 Py_TYPE(self)-&gt;tp_free((PyObject *) self);
}
</pre></div>
</div>
which is assigned to the <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">tp_dealloc</code></a> member:
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_dealloc = (destructor) Custom_dealloc,
</pre></div>
</div>
This method first clears the reference counts of the two Python attributes.
<a class="reference internal" href="../c-api/refcounting.html#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate">Py_XDECREF()</code></a> correctly handles the case where its argument is
NULL (which might happen here if <code class="docutils literal notranslate">tp_new</code> failed midway). It then
calls the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_free" title="PyTypeObject.tp_free"><code class="xref c c-member docutils literal notranslate">tp_free</code></a> member of the object’s type
(computed by <code class="docutils literal notranslate">Py_TYPE(self)</code>) to free the object’s memory. Note that
the object’s type might not be <code class="xref py py-class docutils literal notranslate">CustomType</code>, because the object may
be an instance of a subclass.
<div class="admonition note">
Note
The explicit cast to <code class="docutils literal notranslate">destructor</code> above is needed because we defined
<code class="docutils literal notranslate">Custom_dealloc</code> to take a <code class="docutils literal notranslate">CustomObject *</code> argument, but the <code class="docutils literal notranslate">tp_dealloc</code>
function pointer expects to receive a <code class="docutils literal notranslate">PyObject *</code> argument. Otherwise,
the compiler will emit a warning. This is object-oriented polymorphism,
in C!
</div>
We want to make sure that the first and last names are initialized to empty
strings, so we provide a <code class="docutils literal notranslate">tp_new</code> implementation:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
 CustomObject *self;
 self = (CustomObject *) type-&gt;tp_alloc(type, 0);
 if (self != NULL) {
 self-&gt;first = PyUnicode_FromString(&quot;&quot;);
 if (self-&gt;first == NULL) {
 Py_DECREF(self);
 return NULL;
 }
 self-&gt;last = PyUnicode_FromString(&quot;&quot;);
 if (self-&gt;last == NULL) {
 Py_DECREF(self);
 return NULL;
 }
 self-&gt;number = 0;
 }
 return (PyObject *) self;
}
</pre></div>
</div>
and install it in the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_new" title="PyTypeObject.tp_new"><code class="xref c c-member docutils literal notranslate">tp_new</code></a> member:
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_new = Custom_new,
</pre></div>
</div>
The <code class="docutils literal notranslate">tp_new</code> handler is responsible for creating (as opposed to initializing)
objects of the type. It is exposed in Python as the <a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate">__new__()</code></a> method.
It is not required to define a <code class="docutils literal notranslate">tp_new</code> member, and indeed many extension
types will simply reuse <a class="reference internal" href="../c-api/type.html#c.PyType_GenericNew" title="PyType_GenericNew"><code class="xref c c-func docutils literal notranslate">PyType_GenericNew()</code></a> as done in the first
version of the <code class="docutils literal notranslate">Custom</code> type above. In this case, we use the <code class="docutils literal notranslate">tp_new</code>
handler to initialize the <code class="docutils literal notranslate">first</code> and <code class="docutils literal notranslate">last</code> attributes to non-NULL
default values.
<code class="docutils literal notranslate">tp_new</code> is passed the type being instantiated (not necessarily <code class="docutils literal notranslate">CustomType</code>,
if a subclass is instantiated) and any arguments passed when the type was
called, and is expected to return the instance created. <code class="docutils literal notranslate">tp_new</code> handlers
always accept positional and keyword arguments, but they often ignore the
arguments, leaving the argument handling to initializer (a.k.a. <code class="docutils literal notranslate">tp_init</code>
in C or <code class="docutils literal notranslate">__init__</code> in Python) methods.
<div class="admonition note">
Note
<code class="docutils literal notranslate">tp_new</code> shouldn’t call <code class="docutils literal notranslate">tp_init</code> explicitly, as the interpreter
will do it itself.
</div>
The <code class="docutils literal notranslate">tp_new</code> implementation calls the <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">tp_alloc</code></a>
slot to allocate memory:
<div class="highlight-c notranslate"><div class="highlight"><pre>self = (CustomObject *) type-&gt;tp_alloc(type, 0);
</pre></div>
</div>
Since memory allocation may fail, we must check the <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">tp_alloc</code></a>
result against NULL before proceeding.
<div class="admonition note">
Note
We didn’t fill the <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">tp_alloc</code></a> slot ourselves. Rather
<a class="reference internal" href="../c-api/type.html#c.PyType_Ready" title="PyType_Ready"><code class="xref c c-func docutils literal notranslate">PyType_Ready()</code></a> fills it for us by inheriting it from our base class,
which is <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a> by default. Most types use the default allocation
strategy.
</div>
<div class="admonition note">
Note
If you are creating a co-operative <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_new" title="PyTypeObject.tp_new"><code class="xref c c-member docutils literal notranslate">tp_new</code></a> (one
that calls a base type’s <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_new" title="PyTypeObject.tp_new"><code class="xref c c-member docutils literal notranslate">tp_new</code></a> or <a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate">__new__()</code></a>),
you must not try to determine what method to call using method resolution
order at runtime. Always statically determine what type you are going to
call, and call its <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_new" title="PyTypeObject.tp_new"><code class="xref c c-member docutils literal notranslate">tp_new</code></a> directly, or via
<code class="docutils literal notranslate">type-&gt;tp_base-&gt;tp_new</code>. If you do not do this, Python subclasses of your
type that also inherit from other Python-defined classes may not work correctly.
(Specifically, you may not be able to create instances of such subclasses
without getting a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.)
</div>
We also define an initialization function which accepts arguments to provide
initial values for our instance:
<div class="highlight-c notranslate"><div class="highlight"><pre>static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
 static char *kwlist[] = {&quot;first&quot;, &quot;last&quot;, &quot;number&quot;, NULL};
 PyObject *first = NULL, *last = NULL, *tmp;

if (first) {
 tmp = self-&gt;first;
 Py_INCREF(first);
 self-&gt;first = first;
 Py_XDECREF(tmp);
 }
 if (last) {
 tmp = self-&gt;last;
 Py_INCREF(last);
 self-&gt;last = last;
 Py_XDECREF(tmp);
 }
 return 0;
}
</pre></div>
</div>
by filling the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_init" title="PyTypeObject.tp_init"><code class="xref c c-member docutils literal notranslate">tp_init</code></a> slot.
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_init = (initproc) Custom_init,
</pre></div>
</div>
The <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_init" title="PyTypeObject.tp_init"><code class="xref c c-member docutils literal notranslate">tp_init</code></a> slot is exposed in Python as the
<a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method. It is used to initialize an object after it’s
created. Initializers always accept positional and keyword arguments,
and they should return either <code class="docutils literal notranslate">0</code> on success or <code class="docutils literal notranslate">-1</code> on error.
Unlike the <code class="docutils literal notranslate">tp_new</code> handler, there is no guarantee that <code class="docutils literal notranslate">tp_init</code>
is called at all (for example, the <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> module by default
doesn’t call <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> on unpickled instances). It can also be
called multiple times. Anyone can call the <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method on
our objects. For this reason, we have to be extra careful when assigning
the new attribute values. We might be tempted, for example to assign the
<code class="docutils literal notranslate">first</code> member like this:
<div class="highlight-c notranslate"><div class="highlight"><pre>if (first) {
 Py_XDECREF(self-&gt;first);
 Py_INCREF(first);
 self-&gt;first = first;
}
</pre></div>
</div>
But this would be risky. Our type doesn’t restrict the type of the
<code class="docutils literal notranslate">first</code> member, so it could be any kind of object. It could have a
destructor that causes code to be executed that tries to access the
<code class="docutils literal notranslate">first</code> member; or that destructor could release the
<a class="reference internal" href="../glossary.html#term-global-interpreter-lock">Global interpreter Lock</a> and let arbitrary code run in other
threads that accesses and modifies our object.
To be paranoid and protect ourselves against this possibility, we almost
always reassign members before decrementing their reference counts. When
don’t we have to do this?
<ul class="simple">
<li>when we absolutely know that the reference count is greater than 1;</li>
<li>when we know that deallocation of the object <a class="footnote-reference" href="#id5" id="id1">[1]</a> will neither release
the <a class="reference internal" href="../glossary.html#term-gil">GIL</a> nor cause any calls back into our type’s code;</li>
<li>when decrementing a reference count in a <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">tp_dealloc</code></a>
handler on a type which doesn’t support cyclic garbage collection <a class="footnote-reference" href="#id6" id="id2">[2]</a>.</li>
</ul>
We want to expose our instance variables as attributes. There are a
number of ways to do that. The simplest way is to define member definitions:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyMemberDef Custom_members[] = {
 {&quot;first&quot;, T_OBJECT_EX, offsetof(CustomObject, first), 0,
 &quot;first name&quot;},
 {&quot;last&quot;, T_OBJECT_EX, offsetof(CustomObject, last), 0,
 &quot;last name&quot;},
 {&quot;number&quot;, T_INT, offsetof(CustomObject, number), 0,
 &quot;custom number&quot;},
 {NULL} /* Sentinel */
};
</pre></div>
</div>
and put the definitions in 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">tp_members</code></a> slot:
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_members = Custom_members,
</pre></div>
</div>
Each member definition has a member name, type, offset, access flags and
documentation string. See the <a class="reference internal" href="newtypes.html#generic-attribute-management">Generic Attribute Management</a> section
below for details.
A disadvantage of this approach is that it doesn’t provide a way to restrict the
types of objects that can be assigned to the Python attributes. We expect the
first and last names to be strings, but any Python objects can be assigned.
Further, the attributes can be deleted, setting the C pointers to NULL. Even
though we can make sure the members are initialized to non-NULL values, the
members can be set to NULL if the attributes are deleted.
We define a single method, <code class="xref py py-meth docutils literal notranslate">Custom.name()</code>, that outputs the objects name as the
concatenation of the first and last names.
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyObject *
Custom_name(CustomObject *self)
{
 if (self-&gt;first == NULL) {
 PyErr_SetString(PyExc_AttributeError, &quot;first&quot;);
 return NULL;
 }
 if (self-&gt;last == NULL) {
 PyErr_SetString(PyExc_AttributeError, &quot;last&quot;);
 return NULL;
 }
 return PyUnicode_FromFormat(&quot;%S %S&quot;, self-&gt;first, self-&gt;last);
}
</pre></div>
</div>
The method is implemented as a C function that takes a <code class="xref py py-class docutils literal notranslate">Custom</code> (or
<code class="xref py py-class docutils literal notranslate">Custom</code> subclass) instance as the first argument. Methods always take an
instance as the first argument. Methods often take positional and keyword
arguments as well, but in this case we don’t take any and don’t need to accept
a positional argument tuple or keyword argument dictionary. This method is
equivalent to the Python method:
<div class="highlight-python notranslate"><div class="highlight"><pre>def name(self):
 return &quot;%s %s&quot; % (self.first, self.last)
</pre></div>
</div>
Note that we have to check for the possibility that our <code class="xref py py-attr docutils literal notranslate">first</code> and
<code class="xref py py-attr docutils literal notranslate">last</code> members are NULL. This is because they can be deleted, in which
case they are set to NULL. It would be better to prevent deletion of these
attributes and to restrict the attribute values to be strings. We’ll see how to
do that in the next section.
Now that we’ve defined the method, we need to create an array of method
definitions:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyMethodDef Custom_methods[] = {
 {&quot;name&quot;, (PyCFunction) Custom_name, METH_NOARGS,
 &quot;Return the name, combining the first and last name&quot;
 },
 {NULL} /* Sentinel */
};
</pre></div>
</div>
(note that we used the <a class="reference internal" href="../c-api/structures.html#METH_NOARGS" title="METH_NOARGS"><code class="xref py py-const docutils literal notranslate">METH_NOARGS</code></a> flag to indicate that the method
is expecting no arguments other than self)
and assign it to 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">tp_methods</code></a> slot:
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_methods = Custom_methods,
</pre></div>
</div>
Finally, we’ll make our type usable as a base class for subclassing. We’ve
written our methods carefully so far so that they don’t make any assumptions
about the type of the object being created or used, so all we need to do is
to add the <a class="reference internal" href="../c-api/typeobj.html#Py_TPFLAGS_BASETYPE" title="Py_TPFLAGS_BASETYPE"><code class="xref py py-const docutils literal notranslate">Py_TPFLAGS_BASETYPE</code></a> to our class flag definition:
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
</pre></div>
</div>
We rename <code class="xref c c-func docutils literal notranslate">PyInit_custom()</code> to <code class="xref c c-func docutils literal notranslate">PyInit_custom2()</code>, update the
module name in the <a class="reference internal" href="../c-api/module.html#c.PyModuleDef" title="PyModuleDef"><code class="xref c c-type docutils literal notranslate">PyModuleDef</code></a> struct, and update the full class
name in the <a class="reference internal" href="../c-api/type.html#c.PyTypeObject" title="PyTypeObject"><code class="xref c c-type docutils literal notranslate">PyTypeObject</code></a> struct.
Finally, we update our <code class="file docutils literal notranslate">setup.py</code> file to build the new module:
<div class="highlight-python notranslate"><div class="highlight"><pre>from distutils.core import setup, Extension
setup(name=&quot;custom&quot;, version=&quot;1.0&quot;,
 ext_modules=[
 Extension(&quot;custom&quot;, [&quot;custom.c&quot;]),
 Extension(&quot;custom2&quot;, [&quot;custom2.c&quot;]),
 ])
</pre></div>
</div>
</div>
<div class="section" id="providing-finer-control-over-data-attributes">
<h2>2.3. Providing finer control over data attributes<a class="headerlink" href="#providing-finer-control-over-data-attributes" title="Permalink to this headline">¶</a></h2>
In this section, we’ll provide finer control over how the <code class="xref py py-attr docutils literal notranslate">first</code> and
<code class="xref py py-attr docutils literal notranslate">last</code> attributes are set in the <code class="xref py py-class docutils literal notranslate">Custom</code> example. In the previous
version of our module, the instance variables <code class="xref py py-attr docutils literal notranslate">first</code> and <code class="xref py py-attr docutils literal notranslate">last</code>
could be set to non-string values or even deleted. We want to make sure that
these attributes always contain strings.
<div class="highlight-c notranslate"><div class="highlight"><pre>#include &lt;Python.h&gt;
#include &quot;structmember.h&quot;

if (!PyArg_ParseTupleAndKeywords(args, kwds, &quot;|UUi&quot;, kwlist,
 &amp;first, &amp;last,
 &amp;self-&gt;number))
 return -1;

static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
 Py_INCREF(self-&gt;first);
 return self-&gt;first;
}

static PyObject *
Custom_getlast(CustomObject *self, void *closure)
{
 Py_INCREF(self-&gt;last);
 return self-&gt;last;
}

static int
Custom_setlast(CustomObject *self, PyObject *value, void *closure)
{
 PyObject *tmp;
 if (value == NULL) {
 PyErr_SetString(PyExc_TypeError, &quot;Cannot delete the last attribute&quot;);
 return -1;
 }
 if (!PyUnicode_Check(value)) {
 PyErr_SetString(PyExc_TypeError,
 &quot;The last attribute value must be a string&quot;);
 return -1;
 }
 tmp = self-&gt;last;
 Py_INCREF(value);
 self-&gt;last = value;
 Py_DECREF(tmp);
 return 0;
}

static PyGetSetDef Custom_getsetters[] = {
 {&quot;first&quot;, (getter) Custom_getfirst, (setter) Custom_setfirst,
 &quot;first name&quot;, NULL},
 {&quot;last&quot;, (getter) Custom_getlast, (setter) Custom_setlast,
 &quot;last name&quot;, NULL},
 {NULL} /* Sentinel */
};

static PyTypeObject CustomType = {
 PyVarObject_HEAD_INIT(NULL, 0)
 .tp_name = &quot;custom3.Custom&quot;,
 .tp_doc = &quot;Custom objects&quot;,
 .tp_basicsize = sizeof(CustomObject),
 .tp_itemsize = 0,
 .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
 .tp_new = Custom_new,
 .tp_init = (initproc) Custom_init,
 .tp_dealloc = (destructor) Custom_dealloc,
 .tp_members = Custom_members,
 .tp_methods = Custom_methods,
 .tp_getset = Custom_getsetters,
};

static PyModuleDef custommodule = {
 PyModuleDef_HEAD_INIT,
 .m_name = &quot;custom3&quot;,
 .m_doc = &quot;Example module that creates an extension type.&quot;,
 .m_size = -1,
};

PyMODINIT_FUNC
PyInit_custom3(void)
{
 PyObject *m;
 if (PyType_Ready(&amp;CustomType) &lt; 0)
 return NULL;

static int
Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
{
 PyObject *tmp;
 if (value == NULL) {
 PyErr_SetString(PyExc_TypeError, &quot;Cannot delete the first attribute&quot;);
 return -1;
 }
 if (!PyUnicode_Check(value)) {
 PyErr_SetString(PyExc_TypeError,
 &quot;The first attribute value must be a string&quot;);
 return -1;
 }
 tmp = self-&gt;first;
 Py_INCREF(value);
 self-&gt;first = value;
 Py_DECREF(tmp);
 return 0;
}
</pre></div>
</div>
The getter function is passed a <code class="xref py py-class docutils literal notranslate">Custom</code> object and a “closure”, which is
a void pointer. In this case, the closure is ignored. (The closure supports an
advanced usage in which definition data is passed to the getter and setter. This
could, for example, be used to allow a single set of getter and setter functions
that decide the attribute to get or set based on data in the closure.)
The setter function is passed the <code class="xref py py-class docutils literal notranslate">Custom</code> object, the new value, and the
closure. The new value may be NULL, in which case the attribute is being
deleted. In our setter, we raise an error if the attribute is deleted or if its
new value is not a string.
We create an array of <a class="reference internal" href="../c-api/structures.html#c.PyGetSetDef" title="PyGetSetDef"><code class="xref c c-type docutils literal notranslate">PyGetSetDef</code></a> structures:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyGetSetDef Custom_getsetters[] = {
 {&quot;first&quot;, (getter) Custom_getfirst, (setter) Custom_setfirst,
 &quot;first name&quot;, NULL},
 {&quot;last&quot;, (getter) Custom_getlast, (setter) Custom_setlast,
 &quot;last name&quot;, NULL},
 {NULL} /* Sentinel */
};
</pre></div>
</div>
and register it in the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_getset" title="PyTypeObject.tp_getset"><code class="xref c c-member docutils literal notranslate">tp_getset</code></a> slot:
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_getset = Custom_getsetters,
</pre></div>
</div>
The last item in a <a class="reference internal" href="../c-api/structures.html#c.PyGetSetDef" title="PyGetSetDef"><code class="xref c c-type docutils literal notranslate">PyGetSetDef</code></a> structure is the “closure” mentioned
above. In this case, we aren’t using a closure, so we just pass NULL.
We also remove the member definitions for these attributes:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyMemberDef Custom_members[] = {
 {&quot;number&quot;, T_INT, offsetof(CustomObject, number), 0,
 &quot;custom number&quot;},
 {NULL} /* Sentinel */
};
</pre></div>
</div>
We also need to update the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_init" title="PyTypeObject.tp_init"><code class="xref c c-member docutils literal notranslate">tp_init</code></a> handler to only
allow strings <a class="footnote-reference" href="#id7" id="id3">[3]</a> to be passed:
<div class="highlight-c notranslate"><div class="highlight"><pre>static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
 static char *kwlist[] = {&quot;first&quot;, &quot;last&quot;, &quot;number&quot;, NULL};
 PyObject *first = NULL, *last = NULL, *tmp;

if (!PyArg_ParseTupleAndKeywords(args, kwds, &quot;|UUi&quot;, kwlist,
 &amp;first, &amp;last,
 &amp;self-&gt;number))
 return -1;

static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
 Py_VISIT(self-&gt;first);
 Py_VISIT(self-&gt;last);
 return 0;
}

static int
Custom_clear(CustomObject *self)
{
 Py_CLEAR(self-&gt;first);
 Py_CLEAR(self-&gt;last);
 return 0;
}

static void
Custom_dealloc(CustomObject *self)
{
 PyObject_GC_UnTrack(self);
 Custom_clear(self);
 Py_TYPE(self)-&gt;tp_free((PyObject *) self);
}

if (!PyArg_ParseTupleAndKeywords(args, kwds, &quot;|UUi&quot;, kwlist,
 &amp;first, &amp;last,
 &amp;self-&gt;number))
 return -1;

static int
Custom_setlast(CustomObject *self, PyObject *value, void *closure)
{
 if (value == NULL) {
 PyErr_SetString(PyExc_TypeError, &quot;Cannot delete the last attribute&quot;);
 return -1;
 }
 if (!PyUnicode_Check(value)) {
 PyErr_SetString(PyExc_TypeError,
 &quot;The last attribute value must be a string&quot;);
 return -1;
 }
 Py_INCREF(value);
 Py_CLEAR(self-&gt;last);
 self-&gt;last = value;
 return 0;
}

static PyTypeObject CustomType = {
 PyVarObject_HEAD_INIT(NULL, 0)
 .tp_name = &quot;custom4.Custom&quot;,
 .tp_doc = &quot;Custom objects&quot;,
 .tp_basicsize = sizeof(CustomObject),
 .tp_itemsize = 0,
 .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
 .tp_new = Custom_new,
 .tp_init = (initproc) Custom_init,
 .tp_dealloc = (destructor) Custom_dealloc,
 .tp_traverse = (traverseproc) Custom_traverse,
 .tp_clear = (inquiry) Custom_clear,
 .tp_members = Custom_members,
 .tp_methods = Custom_methods,
 .tp_getset = Custom_getsetters,
};

static PyModuleDef custommodule = {
 PyModuleDef_HEAD_INIT,
 .m_name = &quot;custom4&quot;,
 .m_doc = &quot;Example module that creates an extension type.&quot;,
 .m_size = -1,
};

PyMODINIT_FUNC
PyInit_custom4(void)
{
 PyObject *m;
 if (PyType_Ready(&amp;CustomType) &lt; 0)
 return NULL;

Py_INCREF(&amp;CustomType);
 PyModule_AddObject(m, &quot;Custom&quot;, (PyObject *) &amp;CustomType);
 return m;
}
</pre></div>
</div>
First, the traversal method lets the cyclic GC know about subobjects that could
participate in cycles:
<div class="highlight-c notranslate"><div class="highlight"><pre>static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
 int vret;
 if (self-&gt;first) {
 vret = visit(self-&gt;first, arg);
 if (vret != 0)
 return vret;
 }
 if (self-&gt;last) {
 vret = visit(self-&gt;last, arg);
 if (vret != 0)
 return vret;
 }
 return 0;
}
</pre></div>
</div>
For each subobject that can participate in cycles, we need to call the
<code class="xref c c-func docutils literal notranslate">visit()</code> function, which is passed to the traversal method. The
<code class="xref c c-func docutils literal notranslate">visit()</code> function takes as arguments the subobject and the extra argument
arg passed to the traversal method. It returns an integer value that must be
returned if it is non-zero.
Python provides a <a class="reference internal" href="../c-api/gcsupport.html#c.Py_VISIT" title="Py_VISIT"><code class="xref c c-func docutils literal notranslate">Py_VISIT()</code></a> macro that automates calling visit
functions. With <a class="reference internal" href="../c-api/gcsupport.html#c.Py_VISIT" title="Py_VISIT"><code class="xref c c-func docutils literal notranslate">Py_VISIT()</code></a>, we can minimize the amount of boilerplate
in <code class="docutils literal notranslate">Custom_traverse</code>:
<div class="highlight-c notranslate"><div class="highlight"><pre>static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
 Py_VISIT(self-&gt;first);
 Py_VISIT(self-&gt;last);
 return 0;
}
</pre></div>
</div>
<div class="admonition note">
Note
The <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">tp_traverse</code></a> implementation must name its
arguments exactly visit and arg in order to use <a class="reference internal" href="../c-api/gcsupport.html#c.Py_VISIT" title="Py_VISIT"><code class="xref c c-func docutils literal notranslate">Py_VISIT()</code></a>.
</div>
Second, we need to provide a method for clearing any subobjects that can
participate in cycles:
<div class="highlight-c notranslate"><div class="highlight"><pre>static int
Custom_clear(CustomObject *self)
{
 Py_CLEAR(self-&gt;first);
 Py_CLEAR(self-&gt;last);
 return 0;
}
</pre></div>
</div>
Notice the use of the <a class="reference internal" href="../c-api/refcounting.html#c.Py_CLEAR" title="Py_CLEAR"><code class="xref c c-func docutils literal notranslate">Py_CLEAR()</code></a> macro. It is the recommended and safe
way to clear data attributes of arbitrary types while decrementing
their reference counts. If you were to call <a class="reference internal" href="../c-api/refcounting.html#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate">Py_XDECREF()</code></a> instead
on the attribute before setting it to NULL, there is a possibility
that the attribute’s destructor would call back into code that reads the
attribute again (especially if there is a reference cycle).
<div class="admonition note">
Note
You could emulate <a class="reference internal" href="../c-api/refcounting.html#c.Py_CLEAR" title="Py_CLEAR"><code class="xref c c-func docutils literal notranslate">Py_CLEAR()</code></a> by writing:
<div class="highlight-c notranslate"><div class="highlight"><pre>PyObject *tmp;
tmp = self-&gt;first;
self-&gt;first = NULL;
Py_XDECREF(tmp);
</pre></div>
</div>
Nevertheless, it is much easier and less error-prone to always
use <a class="reference internal" href="../c-api/refcounting.html#c.Py_CLEAR" title="Py_CLEAR"><code class="xref c c-func docutils literal notranslate">Py_CLEAR()</code></a> when deleting an attribute. Don’t
try to micro-optimize at the expense of robustness!
</div>
The deallocator <code class="docutils literal notranslate">Custom_dealloc</code> may call arbitrary code when clearing
attributes. It means the circular GC can be triggered inside the function.
Since the GC assumes reference count is not zero, we need to untrack the object
from the GC by calling <a class="reference internal" href="../c-api/gcsupport.html#c.PyObject_GC_UnTrack" title="PyObject_GC_UnTrack"><code class="xref c c-func docutils literal notranslate">PyObject_GC_UnTrack()</code></a> before clearing members.
Here is our reimplemented deallocator using <a class="reference internal" href="../c-api/gcsupport.html#c.PyObject_GC_UnTrack" title="PyObject_GC_UnTrack"><code class="xref c c-func docutils literal notranslate">PyObject_GC_UnTrack()</code></a>
and <code class="docutils literal notranslate">Custom_clear</code>:
<div class="highlight-c notranslate"><div class="highlight"><pre>static void
Custom_dealloc(CustomObject *self)
{
 PyObject_GC_UnTrack(self);
 Custom_clear(self);
 Py_TYPE(self)-&gt;tp_free((PyObject *) self);
}
</pre></div>
</div>
Finally, we add the <a class="reference internal" href="../c-api/typeobj.html#Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref py py-const docutils literal notranslate">Py_TPFLAGS_HAVE_GC</code></a> flag to the class flags:
<div class="highlight-c notranslate"><div class="highlight"><pre>.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
</pre></div>
</div>
That’s pretty much it. If we had written custom <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">tp_alloc</code></a> or
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_free" title="PyTypeObject.tp_free"><code class="xref c c-member docutils literal notranslate">tp_free</code></a> handlers, we’d need to modify them for cyclic
garbage collection. Most extensions will use the versions automatically provided.
</div>
<div class="section" id="subclassing-other-types">
<h2>2.5. Subclassing other types<a class="headerlink" href="#subclassing-other-types" title="Permalink to this headline">¶</a></h2>
It is possible to create new extension types that are derived from existing
types. It is easiest to inherit from the built in types, since an extension can
easily use the <a class="reference internal" href="../c-api/type.html#c.PyTypeObject" title="PyTypeObject"><code class="xref c c-type docutils literal notranslate">PyTypeObject</code></a> it needs. It can be difficult to share
these <a class="reference internal" href="../c-api/type.html#c.PyTypeObject" title="PyTypeObject"><code class="xref c c-type docutils literal notranslate">PyTypeObject</code></a> structures between extension modules.
In this example we will create a <code class="xref py py-class docutils literal notranslate">SubList</code> type that inherits from the
built-in <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a> type. The new type will be completely compatible with
regular lists, but will have an additional <code class="xref py py-meth docutils literal notranslate">increment()</code> method that
increases an internal counter:
<div class="highlight-pycon notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import sublist
&gt;&gt;&gt; s = sublist.SubList(range(3))
&gt;&gt;&gt; s.extend(s)
&gt;&gt;&gt; print(len(s))
6
&gt;&gt;&gt; print(s.increment())
1
&gt;&gt;&gt; print(s.increment())
2
</pre></div>
</div>
<div class="highlight-c notranslate"><div class="highlight"><pre>#include &lt;Python.h&gt;

typedef struct {
 PyListObject list;
 int state;
} SubListObject;

static PyObject *
SubList_increment(SubListObject *self, PyObject *unused)
{
 self-&gt;state++;
 return PyLong_FromLong(self-&gt;state);
}

static PyMethodDef SubList_methods[] = {
 {&quot;increment&quot;, (PyCFunction) SubList_increment, METH_NOARGS,
 PyDoc_STR(&quot;increment state counter&quot;)},
 {NULL},
};

static int
SubList_init(SubListObject *self, PyObject *args, PyObject *kwds)
{
 if (PyList_Type.tp_init((PyObject *) self, args, kwds) &lt; 0)
 return -1;
 self-&gt;state = 0;
 return 0;
}

static PyTypeObject SubListType = {
 PyVarObject_HEAD_INIT(NULL, 0)
 .tp_name = &quot;sublist.SubList&quot;,
 .tp_doc = &quot;SubList objects&quot;,
 .tp_basicsize = sizeof(SubListObject),
 .tp_itemsize = 0,
 .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
 .tp_init = (initproc) SubList_init,
 .tp_methods = SubList_methods,
};

static PyModuleDef sublistmodule = {
 PyModuleDef_HEAD_INIT,
 .m_name = &quot;sublist&quot;,
 .m_doc = &quot;Example module that creates an extension type.&quot;,
 .m_size = -1,
};

PyMODINIT_FUNC
PyInit_sublist(void)
{
 PyObject *m;
 SubListType.tp_base = &amp;PyList_Type;
 if (PyType_Ready(&amp;SubListType) &lt; 0)
 return NULL;

m = PyModule_Create(&amp;sublistmodule);
 if (m == NULL)
 return NULL;

Py_INCREF(&amp;SubListType);
 PyModule_AddObject(m, &quot;SubList&quot;, (PyObject *) &amp;SubListType);
 return m;
}
</pre></div>
</div>
As you can see, the source code closely resembles the <code class="xref py py-class docutils literal notranslate">Custom</code> examples in
previous sections. We will break down the main differences between them.
<div class="highlight-c notranslate"><div class="highlight"><pre>typedef struct {
 PyListObject list;
 int state;
} SubListObject;
</pre></div>
</div>
The primary difference for derived type objects is that the base type’s
object structure must be the first value. The base type will already include
the <a class="reference internal" href="../c-api/structures.html#c.PyObject_HEAD" title="PyObject_HEAD"><code class="xref c c-func docutils literal notranslate">PyObject_HEAD()</code></a> at the beginning of its structure.
When a Python object is a <code class="xref py py-class docutils literal notranslate">SubList</code> instance, its <code class="docutils literal notranslate">PyObject *</code> pointer
can be safely cast to both <code class="docutils literal notranslate">PyListObject *</code> and <code class="docutils literal notranslate">SubListObject *</code>:
<div class="highlight-c notranslate"><div class="highlight"><pre>static int
SubList_init(SubListObject *self, PyObject *args, PyObject *kwds)
{
 if (PyList_Type.tp_init((PyObject *) self, args, kwds) &lt; 0)
 return -1;
 self-&gt;state = 0;
 return 0;
}
</pre></div>
</div>
We see above how to call through to the <code class="xref py py-attr docutils literal notranslate">__init__</code> method of the base
type.
This pattern is important when writing a type with custom
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_new" title="PyTypeObject.tp_new"><code class="xref c c-member docutils literal notranslate">tp_new</code></a> and <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">tp_dealloc</code></a>
members. The <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_new" title="PyTypeObject.tp_new"><code class="xref c c-member docutils literal notranslate">tp_new</code></a> handler should not actually
create the memory for the object with its <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">tp_alloc</code></a>,
but let the base class handle it by calling its own <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_new" title="PyTypeObject.tp_new"><code class="xref c c-member docutils literal notranslate">tp_new</code></a>.
The <a class="reference internal" href="../c-api/type.html#c.PyTypeObject" title="PyTypeObject"><code class="xref c c-type docutils literal notranslate">PyTypeObject</code></a> struct supports a <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_base" title="PyTypeObject.tp_base"><code class="xref c c-member docutils literal notranslate">tp_base</code></a>
specifying the type’s concrete base class. Due to cross-platform compiler
issues, you can’t fill that field directly with a reference to
<a class="reference internal" href="../c-api/list.html#c.PyList_Type" title="PyList_Type"><code class="xref c c-type docutils literal notranslate">PyList_Type</code></a>; it should be done later in the module initialization
function:
<div class="highlight-c notranslate"><div class="highlight"><pre>PyMODINIT_FUNC
PyInit_sublist(void)
{
 PyObject* m;
 SubListType.tp_base = &amp;PyList_Type;
 if (PyType_Ready(&amp;SubListType) &lt; 0)
 return NULL;

</div>
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 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">2. Defining Extension Types: Tutorial</a><ul>
<li><a class="reference internal" href="#the-basics">2.1. The Basics</a></li>
<li><a class="reference internal" href="#adding-data-and-methods-to-the-basic-example">2.2. Adding data and methods to the Basic example</a></li>
<li><a class="reference internal" href="#providing-finer-control-over-data-attributes">2.3. Providing finer control over data attributes</a></li>
<li><a class="reference internal" href="#supporting-cyclic-garbage-collection">2.4. Supporting cyclic garbage collection</a></li>
<li><a class="reference internal" href="#subclassing-other-types">2.5. Subclassing other types</a></li>
</ul>
</li>
</ul>

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