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 <div class="section" id="defining-extension-types-assorted-topics">
<h1>3. Defining Extension Types: Assorted Topics<a class="headerlink" href="#defining-extension-types-assorted-topics" title="Permalink to this headline">¶</a></h1>
This section aims to give a quick fly-by on the various type methods you can
implement and what they do.
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">PyTypeObject</code></a>, with some fields only used in
debug builds omitted:
<div class="highlight-c notranslate"><div class="highlight"><pre>typedef struct _typeobject {
 PyObject_VAR_HEAD
 const char *tp_name; /* For printing, in format &quot;&lt;module&gt;.&lt;name&gt;&quot; */
 Py_ssize_t tp_basicsize, tp_itemsize; /* For allocation */

/* Methods to implement standard operations */

destructor tp_dealloc;
 printfunc tp_print;
 getattrfunc tp_getattr;
 setattrfunc tp_setattr;
 PyAsyncMethods *tp_as_async; /* formerly known as tp_compare (Python 2)
 or tp_reserved (Python 3) */
 reprfunc tp_repr;

/* Method suites for standard classes */

PyNumberMethods *tp_as_number;
 PySequenceMethods *tp_as_sequence;
 PyMappingMethods *tp_as_mapping;

/* More standard operations (here for binary compatibility) */

hashfunc tp_hash;
 ternaryfunc tp_call;
 reprfunc tp_str;
 getattrofunc tp_getattro;
 setattrofunc tp_setattro;

/* Functions to access object as input/output buffer */
 PyBufferProcs *tp_as_buffer;

/* Flags to define presence of optional/expanded features */
 unsigned long tp_flags;

const char *tp_doc; /* Documentation string */

/* call function for all accessible objects */
 traverseproc tp_traverse;

/* delete references to contained objects */
 inquiry tp_clear;

/* rich comparisons */
 richcmpfunc tp_richcompare;

/* weak reference enabler */
 Py_ssize_t tp_weaklistoffset;

/* Iterators */
 getiterfunc tp_iter;
 iternextfunc tp_iternext;

/* Attribute descriptor and subclassing stuff */
 struct PyMethodDef *tp_methods;
 struct PyMemberDef *tp_members;
 struct PyGetSetDef *tp_getset;
 struct _typeobject *tp_base;
 PyObject *tp_dict;
 descrgetfunc tp_descr_get;
 descrsetfunc tp_descr_set;
 Py_ssize_t tp_dictoffset;
 initproc tp_init;
 allocfunc tp_alloc;
 newfunc tp_new;
 freefunc tp_free; /* Low-level free-memory routine */
 inquiry tp_is_gc; /* For PyObject_IS_GC */
 PyObject *tp_bases;
 PyObject *tp_mro; /* method resolution order */
 PyObject *tp_cache;
 PyObject *tp_subclasses;
 PyObject *tp_weaklist;
 destructor tp_del;

/* Type attribute cache version tag. Added in version 2.6 */
 unsigned int tp_version_tag;

destructor tp_finalize;

} PyTypeObject;
</pre></div>
</div>
Now that’s a lot 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.
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.
<div class="highlight-c notranslate"><div class="highlight"><pre>const char *tp_name; /* For printing */
</pre></div>
</div>
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!
<div class="highlight-c notranslate"><div class="highlight"><pre>Py_ssize_t tp_basicsize, tp_itemsize; /* For allocation */
</pre></div>
</div>
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">tp_itemsize</code></a> field
comes in. This will be dealt with later.
<div class="highlight-c notranslate"><div class="highlight"><pre>const char *tp_doc;
</pre></div>
</div>
Here you can put a string (or its address) that you want returned when the
Python script references <code class="docutils literal notranslate">obj.__doc__</code> to retrieve the doc string.
Now we come to the basic type methods – the ones most extension types will
implement.
<div class="section" id="finalization-and-de-allocation">
<h2>3.1. 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>destructor tp_dealloc;
</pre></div>
</div>
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:
<div class="highlight-c notranslate"><div class="highlight"><pre>static void
newdatatype_dealloc(newdatatypeobject *obj)
{
 free(obj-&gt;obj_UnderlyingDatatypePtr);
 Py_TYPE(obj)-&gt;tp_free(obj);
}
</pre></div>
</div>
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">PyErr_Fetch()</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">PyErr_Restore()</code></a> functions:
<div class="highlight-c notranslate"><div class="highlight"><pre>static void
my_dealloc(PyObject *obj)
{
 MyObject *self = (MyObject *) obj;
 PyObject *cbresult;

if (self-&gt;my_callback != NULL) {
 PyObject *err_type, *err_value, *err_traceback;

/* This saves the current exception state */
 PyErr_Fetch(&amp;err_type, &amp;err_value, &amp;err_traceback);

cbresult = PyObject_CallObject(self-&gt;my_callback, NULL);
 if (cbresult == NULL)
 PyErr_WriteUnraisable(self-&gt;my_callback);
 else
 Py_DECREF(cbresult);

/* This restores the saved exception state */
 PyErr_Restore(err_type, err_value, err_traceback);

Py_DECREF(self-&gt;my_callback);
 }
 Py_TYPE(obj)-&gt;tp_free((PyObject*)self);
}
</pre></div>
</div>
<div class="admonition note">
Note
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">tp_traverse</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">tp_clear</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">tp_dealloc</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">tp_dealloc</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">tp_dealloc</code></a> again, causing a
double free and a crash.
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">tp_dealloc</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">tp_finalize</code></a> type method.
<div class="last admonition seealso">
See also
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0442">PEP 442</a> explains the new finalization scheme.
</div>
</div>
</div>
<div class="section" id="object-presentation">
<h2>3.2. Object Presentation<a class="headerlink" href="#object-presentation" title="Permalink to this headline">¶</a></h2>
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">repr()</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">str()</code></a> function. (The <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</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">str()</code></a>.) These handlers are both optional.
<div class="highlight-c notranslate"><div class="highlight"><pre>reprfunc tp_repr;
reprfunc tp_str;
</pre></div>
</div>
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">tp_repr</code></a> handler should return a string object containing a
representation of the instance for which it is called. Here is a simple
example:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyObject *
newdatatype_repr(newdatatypeobject * obj)
{
 return PyUnicode_FromFormat(&quot;Repr-ified_newdatatype{{size:%d}}&quot;,
 obj-&gt;obj_UnderlyingDatatypePtr-&gt;size);
}
</pre></div>
</div>
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">tp_repr</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">tp_name</code></a> and a uniquely-identifying
value for the object.
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">tp_str</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">str()</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">tp_repr</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">repr()</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">str()</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">tp_repr</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">tp_str</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">tp_repr</code></a> handler is
used instead.
Here is a simple example:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyObject *
newdatatype_str(newdatatypeobject * obj)
{
 return PyUnicode_FromFormat(&quot;Stringified_newdatatype{{size:%d}}&quot;,
 obj-&gt;obj_UnderlyingDatatypePtr-&gt;size);
}
</pre></div>
</div>
</div>
<div class="section" id="attribute-management">
<h2>3.3. Attribute Management<a class="headerlink" href="#attribute-management" title="Permalink to this headline">¶</a></h2>
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 NULL.
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">char*</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">PyObject*</code></a>. Each type can use whichever pair makes more
sense for the implementation’s convenience.
<div class="highlight-c notranslate"><div class="highlight"><pre>getattrfunc tp_getattr; /* char * version */
setattrfunc tp_setattr;
/* ... */
getattrofunc tp_getattro; /* PyObject * version */
setattrofunc tp_setattro;
</pre></div>
</div>
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">PyObject*</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.
<div class="section" id="generic-attribute-management">
<h3>3.3.1. Generic Attribute Management<a class="headerlink" href="#generic-attribute-management" title="Permalink to this headline">¶</a></h3>
Most extension types only use simple attributes. So, what makes the
attributes simple? There are only a couple of conditions that must be met:
<ol class="arabic simple">
<li>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">PyType_Ready()</code></a> is
called.</li>
<li>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.</li>
</ol>
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.
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">PyType_Ready()</code></a> is called, it uses three tables referenced by the
type object to create <a class="reference internal" href="../glossary.html#term-descriptor">descriptor</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 NULL, 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">tp_getattro</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">tp_setattro</code></a> fields NULL as
well, allowing the base type to handle attributes.
The tables are declared as three fields of the type object:
<div class="highlight-c notranslate"><div class="highlight"><pre>struct PyMethodDef *tp_methods;
struct PyMemberDef *tp_members;
struct PyGetSetDef *tp_getset;
</pre></div>
</div>
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">tp_methods</code></a> is not NULL, 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">PyMethodDef</code></a> structures. Each entry in the table is an instance of this
structure:
<div class="highlight-c notranslate"><div class="highlight"><pre>typedef struct PyMethodDef {
 const char *ml_name; /* method name */
 PyCFunction ml_meth; /* implementation function */
 int ml_flags; /* flags */
 const char *ml_doc; /* docstring */
} PyMethodDef;
</pre></div>
</div>
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">ml_name</code> field of the sentinel must be NULL.
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:
<div class="highlight-c notranslate"><div class="highlight"><pre>typedef struct PyMemberDef {
 char *name;
 int type;
 int offset;
 int flags;
 char *doc;
} PyMemberDef;
</pre></div>
</div>
For each entry in the table, a <a class="reference internal" href="../glossary.html#term-descriptor">descriptor</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">type</code></a> field should contain one of the type codes defined in the
<code class="file docutils literal notranslate">structmember.h</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">flags</code> field is used to
store flags which control how the attribute can be accessed.
The following flag constants are defined in <code class="file docutils literal notranslate">structmember.h</code>; they may be
combined using bitwise-OR.
<table border="1" class="docutils">
<colgroup>
<col width="37%" />
<col width="63%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">READONLY</code></td>
<td>Never writable.</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">READ_RESTRICTED</code></td>
<td>Not readable in restricted mode.</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">WRITE_RESTRICTED</code></td>
<td>Not writable in restricted mode.</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">RESTRICTED</code></td>
<td>Not readable or writable in restricted mode.</td>
</tr>
</tbody>
</table>
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">tp_members</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">__doc__</code> attribute.
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">tp_methods</code></a> table, a sentinel entry with a <code class="xref py py-attr docutils literal notranslate">name</code> value
of NULL is required.
</div>
<div class="section" id="type-specific-attribute-management">
<h3>3.3.2. Type-specific Attribute Management<a class="headerlink" href="#type-specific-attribute-management" title="Permalink to this headline">¶</a></h3>
For simplicity, only the <code class="xref c c-type docutils literal notranslate">char*</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">char*</code>
and <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> 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.
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">tp_getattr</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">__getattr__()</code></a>
method of a class would be called.
Here is an example:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyObject *
newdatatype_getattr(newdatatypeobject *obj, char *name)
{
 if (strcmp(name, &quot;data&quot;) == 0)
 {
 return PyLong_FromLong(obj-&gt;data);
 }

PyErr_Format(PyExc_AttributeError,
 &quot;&#39;%.50s&#39; object has no attribute &#39;%.400s&#39;&quot;,
 tp-&gt;tp_name, name);
 return NULL;
}
</pre></div>
</div>
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">tp_setattr</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">__setattr__()</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">__delattr__()</code></a> method of a class instance would be called. When an
attribute should be deleted, the third parameter will be NULL. 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">tp_setattr</code></a> handler should be set to NULL.
<div class="highlight-c notranslate"><div class="highlight"><pre>static int
newdatatype_setattr(newdatatypeobject *obj, char *name, PyObject *v)
{
 PyErr_Format(PyExc_RuntimeError, &quot;Read-only attribute: %s&quot;, name);
 return -1;
}
</pre></div>
</div>
</div>
</div>
<div class="section" id="object-comparison">
<h2>3.4. Object Comparison<a class="headerlink" href="#object-comparison" title="Permalink to this headline">¶</a></h2>
<div class="highlight-c notranslate"><div class="highlight"><pre>richcmpfunc tp_richcompare;
</pre></div>
</div>
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">tp_richcompare</code></a> handler is called when comparisons are needed. It is
analogous to the <a class="reference internal" href="../reference/datamodel.html#richcmpfuncs">rich comparison methods</a>, like
<a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate">__lt__()</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">PyObject_RichCompare()</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">PyObject_RichCompareBool()</code></a>.
This function is called with two Python objects and the operator as arguments,
where the operator is one of <code class="docutils literal notranslate">Py_EQ</code>, <code class="docutils literal notranslate">Py_NE</code>, <code class="docutils literal notranslate">Py_LE</code>, <code class="docutils literal notranslate">Py_GT</code>,
<code class="docutils literal notranslate">Py_LT</code> or <code class="docutils literal notranslate">Py_GT</code>. It should compare the two objects with respect to the
specified operator and return <code class="docutils literal notranslate">Py_True</code> or <code class="docutils literal notranslate">Py_False</code> if the comparison is
successful, <code class="docutils literal notranslate">Py_NotImplemented</code> to indicate that comparison is not
implemented and the other object’s comparison method should be tried, or NULL
if an exception was set.
Here is a sample implementation, for a datatype that is considered equal if the
size of an internal pointer is equal:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyObject *
newdatatype_richcmp(PyObject *obj1, PyObject *obj2, int op)
{
 PyObject *result;
 int c, size1, size2;

/* code to make sure that both arguments are of type
 newdatatype omitted */

size1 = obj1-&gt;obj_UnderlyingDatatypePtr-&gt;size;
 size2 = obj2-&gt;obj_UnderlyingDatatypePtr-&gt;size;

switch (op) {
 case Py_LT: c = size1 &lt; size2; break;
 case Py_LE: c = size1 &lt;= size2; break;
 case Py_EQ: c = size1 == size2; break;
 case Py_NE: c = size1 != size2; break;
 case Py_GT: c = size1 &gt; size2; break;
 case Py_GE: c = size1 &gt;= size2; break;
 }
 result = c ? Py_True : Py_False;
 Py_INCREF(result);
 return result;
 }
</pre></div>
</div>
</div>
<div class="section" id="abstract-protocol-support">
<h2>3.5. Abstract Protocol Support<a class="headerlink" href="#abstract-protocol-support" title="Permalink to this headline">¶</a></h2>
Python supports a variety of abstract ‘protocols;’ the specific interfaces
provided to use these interfaces are documented in <a class="reference internal" href="../c-api/abstract.html#abstract">Abstract Objects Layer</a>.
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-NULL. The flag may be set
to indicate the presence of a slot, but a slot may still be unfilled.)
<div class="highlight-c notranslate"><div class="highlight"><pre>PyNumberMethods *tp_as_number;
PySequenceMethods *tp_as_sequence;
PyMappingMethods *tp_as_mapping;
</pre></div>
</div>
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">PyNumberMethods</code></a>, <a class="reference internal" href="../c-api/typeobj.html#c.PySequenceMethods" title="PySequenceMethods"><code class="xref c c-type docutils literal notranslate">PySequenceMethods</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">PyMappingMethods</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">Objects</code> directory of the Python source distribution.
<div class="highlight-c notranslate"><div class="highlight"><pre>hashfunc tp_hash;
</pre></div>
</div>
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:
<div class="highlight-c notranslate"><div class="highlight"><pre>static Py_hash_t
newdatatype_hash(newdatatypeobject *obj)
{
 Py_hash_t result;
 result = obj-&gt;some_size + 32767 * obj-&gt;some_number;
 if (result == -1)
 result = -2;
 return result;
}
</pre></div>
</div>
<code class="xref c c-type docutils literal notranslate">Py_hash_t</code> is a signed integer type with a platform-varying width.
Returning <code class="docutils literal notranslate">-1</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">tp_hash</code></a> indicates an error,
which is why you should be careful to avoid returning it when hash computation
is successful, as seen above.
<div class="highlight-c notranslate"><div class="highlight"><pre>ternaryfunc tp_call;
</pre></div>
</div>
This function is called when an instance of your data type is “called”, for
example, if <code class="docutils literal notranslate">obj1</code> is an instance of your data type and the Python script
contains <code class="docutils literal notranslate">obj1('hello')</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">tp_call</code></a> handler is invoked.
This function takes three arguments:
<ol class="arabic simple">
<li>self is the instance of the data type which is the subject of the call.
If the call is <code class="docutils literal notranslate">obj1('hello')</code>, then self is <code class="docutils literal notranslate">obj1</code>.</li>
<li>args 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">PyArg_ParseTuple()</code></a> to extract the arguments.</li>
<li>kwds is a dictionary of keyword arguments that were passed. If this is
non-NULL 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">PyArg_ParseTupleAndKeywords()</code></a> to extract the arguments. If you
do not want to support keyword arguments and this is non-NULL, raise a
<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> with a message saying that keyword arguments are not supported.</li>
</ol>
Here is a toy <code class="docutils literal notranslate">tp_call</code> implementation:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyObject *
newdatatype_call(newdatatypeobject *self, PyObject *args, PyObject *kwds)
{
 PyObject *result;
 char *arg1;
 char *arg2;
 char *arg3;

if (!PyArg_ParseTuple(args, &quot;sss:call&quot;, &amp;arg1, &amp;arg2, &amp;arg3)) {
 return NULL;
 }
 result = PyUnicode_FromFormat(
 &quot;Returning -- value: [%d] arg1: [%s] arg2: [%s] arg3: [%s]\n&quot;,
 obj-&gt;obj_UnderlyingDatatypePtr-&gt;size,
 arg1, arg2, arg3);
 return result;
}
</pre></div>
</div>
<div class="highlight-c notranslate"><div class="highlight"><pre>/* Iterators */
getiterfunc tp_iter;
iternextfunc tp_iternext;
</pre></div>
</div>
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 NULL. <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">tp_iter</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">__iter__()</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">tp_iternext</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">__next__()</code></a> method.
Any <a class="reference internal" href="../glossary.html#term-iterable">iterable</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">tp_iter</code></a>
handler, which must return an <a class="reference internal" href="../glossary.html#term-iterator">iterator</a> object. Here the same guidelines
apply as for Python classes:
<ul class="simple">
<li>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">tp_iter</code></a>.</li>
<li>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">tp_iter</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">tp_iternext</code></a> handler.</li>
</ul>
Any <a class="reference internal" href="../glossary.html#term-iterator">iterator</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">tp_iter</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">tp_iternext</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">tp_iter</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">tp_iternext</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">tp_iternext</code></a>
may return NULL 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">StopIteration</code></a> in addition to returning NULL; 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">tp_iternext</code></a> should always set an exception
and return NULL.
</div>
<div class="section" id="weak-reference-support">
<h2>3.6. Weak Reference Support<a class="headerlink" href="#weak-reference-support" title="Permalink to this headline">¶</a></h2>
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).
<div class="admonition seealso">
See also
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">weakref</code></a> module.
</div>
For an object to be weakly referencable, the extension type must do two things:
<ol class="arabic simple">
<li>Include a <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> field in the C object structure dedicated to
the weak reference mechanism. The object’s constructor should leave it
NULL (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">tp_alloc</code></a>).</li>
<li>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">tp_weaklistoffset</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.</li>
</ol>
Concretely, here is how a trivial object structure would be augmented
with the required field:
<div class="highlight-c notranslate"><div class="highlight"><pre>typedef struct {
 PyObject_HEAD
 PyObject *weakreflist; /* List of weak references */
} TrivialObject;
</pre></div>
</div>
And the corresponding member in the statically-declared type object:
<div class="highlight-c notranslate"><div class="highlight"><pre>static PyTypeObject TrivialType = {
 PyVarObject_HEAD_INIT(NULL, 0)
 /* ... other members omitted for brevity ... */
 .tp_weaklistoffset = offsetof(TrivialObject, weakreflist),
};
</pre></div>
</div>
The only further addition is that <code class="docutils literal notranslate">tp_dealloc</code> needs to clear any weak
references (by calling <code class="xref c c-func docutils literal notranslate">PyObject_ClearWeakRefs()</code>) if the field is
non-NULL:
<div class="highlight-c notranslate"><div class="highlight"><pre>static void
Trivial_dealloc(TrivialObject *self)
{
 /* Clear weakrefs first before calling any destructors */
 if (self-&gt;weakreflist != NULL)
 PyObject_ClearWeakRefs((PyObject *) self);
 /* ... remainder of destruction code omitted for brevity ... */
 Py_TYPE(self)-&gt;tp_free((PyObject *) self);
}
</pre></div>
</div>
</div>
<div class="section" id="more-suggestions">
<h2>3.7. More Suggestions<a class="headerlink" href="#more-suggestions" title="Permalink to this headline">¶</a></h2>
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">CPython</a> source code. Go to the <code class="file docutils literal notranslate">Objects</code> directory,
then search the C source files for <code class="docutils literal notranslate">tp_</code> plus the function you want
(for example, <code class="docutils literal notranslate">tp_richcompare</code>). You will find examples of the function
you want to implement.
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">PyObject_TypeCheck()</code></a> function. A sample of
its use might be something like the following:
<div class="highlight-c notranslate"><div class="highlight"><pre>if (!PyObject_TypeCheck(some_object, &amp;MyType)) {
 PyErr_SetString(PyExc_TypeError, &quot;arg #1 not a mything&quot;);
 return NULL;
}
</pre></div>
</div>
<div class="admonition seealso">
See also
<dl class="last docutils">
<dt>Download CPython source releases.</dt>
<dd><a class="reference external" href="https://www.python.org/downloads/source/">https://www.python.org/downloads/source/</a></dd>
<dt>The CPython project on GitHub, where the CPython source code is developed.</dt>
<dd><a class="reference external" href="https://github.com/python/cpython">https://github.com/python/cpython</a></dd>
</dl>
</div>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">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>
</ul>
</li>
<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>
</ul>
</li>
</ul>

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