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<!DOCTYPE html>

<head>

<title>2. Defining Extension Types: Tutorial — مستندات Python3.14.6</title><meta name="viewport" content="width=device-width, initial-scale=1.0">

title="جستجو در مستندات Python3.14.6"

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<h3><a href="../contents.html">فهرست عناوین</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>

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<h4>موضوع قبلی</h4>

<a href="extending.html"

title="فصل قبلی">1. Extending Python with C or C++</a>

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<h1>2. Defining Extension Types: Tutorial<a class="headerlink" href="#defining-extension-types-tutorial" title="Link to this heading">¶</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.

<h2>2.1. The Basics<a class="headerlink" href="#the-basics" title="Link to this heading">¶</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">PyObject</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>:

توجه

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>#define PY_SSIZE_T_CLEAN

#include <Python.h>

typedef struct {

PyObject_HEAD

/* Type-specific fields go here. */

} CustomObject;

static PyTypeObject CustomType = {

.ob_base = PyVarObject_HEAD_INIT(NULL, 0)

.tp_name = "custom.Custom",

.tp_doc = PyDoc_STR("Custom objects"),

.tp_basicsize = sizeof(CustomObject),

.tp_itemsize = 0,

.tp_new = PyType_GenericNew,

};

static int

custom_module_exec(PyObject *m)

{

if (PyType_Ready(&CustomType) < 0) {

return -1;

}

if (PyModule_AddObjectRef(m, "Custom", (PyObject *) &CustomType) < 0) {

return -1;

}

return 0;

}

static PyModuleDef_Slot custom_module_slots[] = {

{Py_mod_exec, custom_module_exec},

// Just use this while using static types

{Py_mod_multiple_interpreters, Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED},

{0, NULL}

};

static PyModuleDef custom_module = {

.m_base = PyModuleDef_HEAD_INIT,

.m_name = "custom",

.m_doc = "Example module that creates an extension type.",

.m_size = 0,

.m_slots = custom_module_slots,

};

PyMODINIT_FUNC

PyInit_custom(void)

{

return PyModuleDef_Init(&custom_module);

}

</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:

<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 define and execute 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">custom_module</code> struct for

defining the module, and the <code class="docutils literal notranslate">custom_module_exec</code> function to set up

a fresh module object.</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_TYPE" title="Py_TYPE"><code class="xref c c-macro docutils literal notranslate">Py_TYPE</code></a>

and <a class="reference internal" href="../c-api/refcounting.html#c.Py_REFCNT" title="Py_REFCNT"><code class="xref c c-macro docutils literal notranslate">Py_REFCNT</code></a> respectively). The reason for the macro is to

abstract away the layout and to enable additional fields in <a class="reference internal" href="../using/configure.html#debug-build">debug builds</a>.

توجه

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:

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 = {

};

</pre></div>

</div>

توجه

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>.ob_base = 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 = "custom.Custom",

</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>>>> "" + custom.Custom()

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: can only concatenate str (not "custom.Custom") 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.

توجه

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="../reference/datamodel.html#type.__bases__" title="type.__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 <code class="xref c c-member docutils literal notranslate">tp_basicsize</code> 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#c.Py_TPFLAGS_DEFAULT" title="Py_TPFLAGS_DEFAULT"><code class="xref c c-macro 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 = PyDoc_STR("Custom objects"),

</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">custom_module_exec()</code>:

return -1;

}

</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 <a class="reference internal" href="../c-api/structures.html#c.PyObject.ob_type" title="PyObject.ob_type"><code class="xref c c-member docutils literal notranslate">ob_type</code></a> that we initially

set to <code class="docutils literal notranslate">NULL</code>.

<div class="highlight-c notranslate"><div class="highlight"><pre>if (PyModule_AddObjectRef(m, "Custom", (PyObject *) &CustomType) < 0) {

return -1;

}

</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>>>> import custom

>>> 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>,

<div class="highlight-c notranslate"><div class="highlight"><pre>[build-system]

requires = ["setuptools"]

build-backend = "setuptools.build_meta"

[project]

name = "custom"

version = "1"

</pre></div>

</div>

in a file called <code class="file docutils literal notranslate">pyproject.toml</code>, and

<div class="highlight-python notranslate"><div class="highlight"><pre>from setuptools import Extension, setup

setup(ext_modules=[Extension("custom", ["custom.c"])])

</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 -m pip install .

</pre></div>

</div>

in a shell should produce a file <code class="file docutils literal notranslate">custom.so</code> in a subdirectory

and install it; now fire up Python --- you should be able to <code class="docutils literal notranslate">import custom</code>

and play around with <code class="docutils literal notranslate">Custom</code> 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.

</section>

<h2>2.2. Adding data and methods to the Basic example<a class="headerlink" href="#adding-data-and-methods-to-the-basic-example" title="Link to this heading">¶</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>#define PY_SSIZE_T_CLEAN

#include <Python.h>

#include <stddef.h> /* for offsetof() */

typedef struct {

PyObject_HEAD

PyObject *first; /* first name */

PyObject *last; /* last name */

int number;

} CustomObject;

static void

Custom_dealloc(PyObject *op)

{

CustomObject *self = (CustomObject *) op;

Py_XDECREF(self->first);

Py_XDECREF(self->last);

}

static PyObject *

Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)

{

CustomObject *self;

self = (CustomObject *) type->tp_alloc(type, 0);

if (self != NULL) {

self->first = Py_GetConstant(Py_CONSTANT_EMPTY_STR);

if (self->first == NULL) {

Py_DECREF(self);

return NULL;

}

self->last = Py_GetConstant(Py_CONSTANT_EMPTY_STR);

if (self->last == NULL) {

Py_DECREF(self);

return NULL;

}

self->number = 0;

}

return (PyObject *) self;

}

static int

Custom_init(PyObject *op, PyObject *args, PyObject *kwds)

{

static char *kwlist[] = {"first", "last", "number", NULL};

PyObject *first = NULL, *last = NULL;

if (!PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,

&first, &last,

&self->number))

return -1;

if (first) {

Py_XSETREF(self->first, Py_NewRef(first));

}

if (last) {

Py_XSETREF(self->last, Py_NewRef(last));

}

return 0;

}

static PyMemberDef Custom_members[] = {

{"first", Py_T_OBJECT_EX, offsetof(CustomObject, first), 0,

"first name"},

{"last", Py_T_OBJECT_EX, offsetof(CustomObject, last), 0,

"last name"},

{"number", Py_T_INT, offsetof(CustomObject, number), 0,

"custom number"},

{NULL} /* Sentinel */

};

static PyObject *

Custom_name(PyObject *op, PyObject *Py_UNUSED(dummy))

{

PyErr_SetString(PyExc_AttributeError, "first");

return NULL;

}

if (self->last == NULL) {

return NULL;

}

return PyUnicode_FromFormat("%S %S", self->first, self->last);

}

static PyMethodDef Custom_methods[] = {

{"name", Custom_name, METH_NOARGS,

"Return the name, combining the first and last name"

},

{NULL} /* Sentinel */

};

.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,

.tp_init = Custom_init,

.tp_dealloc = Custom_dealloc,

.tp_members = Custom_members,

.tp_methods = Custom_methods,

};

static int

custom_module_exec(PyObject *m)

{

return -1;

}

return -1;

}

return 0;

}

{Py_mod_exec, custom_module_exec},

{0, NULL}

};

.m_size = 0,

};

PyMODINIT_FUNC

PyInit_custom2(void)

{

}

</pre></div>

</div>

This version of the module has a number of changes.

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:

PyObject_HEAD

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(PyObject *op)

{

Py_XDECREF(self->first);

Py_XDECREF(self->last);

}

</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 = 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

<code class="docutils literal notranslate">NULL</code> (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.

توجه

The explicit cast to <code class="docutils literal notranslate">CustomObject *</code> above is needed because we defined

<code class="docutils literal notranslate">Custom_dealloc</code> to take a <code class="docutils literal notranslate">PyObject *</code> argument, as the <code class="docutils literal notranslate">tp_dealloc</code>

function pointer expects to receive a <code class="docutils literal notranslate">PyObject *</code> argument.

By assigning to the <code class="docutils literal notranslate">tp_dealloc</code> slot of a type, we declare

that it can only be called with instances of our <code class="docutils literal notranslate">CustomObject</code>

class, so the cast to <code class="docutils literal notranslate">(CustomObject *)</code> is safe.

This is object-oriented polymorphism, in C!

In existing code, or in previous versions of this tutorial,

you might see similar functions take a pointer to the subtype

object structure (<code class="docutils literal notranslate">CustomObject*</code>) directly, like this:

<div class="highlight-c notranslate"><div class="highlight"><pre>Custom_dealloc(CustomObject *self)

{

Py_XDECREF(self->first);

Py_XDECREF(self->last);

Py_TYPE(self)->tp_free((PyObject *) self);

}

...

.tp_dealloc = (destructor) Custom_dealloc,

</pre></div>

</div>

This does the same thing on all architectures that CPython

supports, but according to the C standard, it invokes

undefined behavior.

</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 *

{

CustomObject *self;

self->first = PyUnicode_FromString("");

Py_DECREF(self);

return NULL;

}

self->last = PyUnicode_FromString("");

if (self->last == NULL) {

Py_DECREF(self);

return NULL;

}

}

</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:

</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="xref py py-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-<code class="docutils literal notranslate">NULL</code>

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.

توجه

<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->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 <code class="docutils literal notranslate">NULL</code> before proceeding.

توجه

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>

توجه

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 <code class="xref c c-member docutils literal notranslate">tp_new</code> 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 <code class="xref c c-member docutils literal notranslate">tp_new</code> directly, or via

<code class="docutils literal notranslate">type->tp_base->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

{

&first, &last,

&self->number))

return -1;

if (first) {

tmp = self->first;

Py_INCREF(first);

Py_XDECREF(tmp);

}

if (last) {

Py_INCREF(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 = 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 <code class="xref py py-meth docutils literal notranslate">__init__()</code> 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:

Py_XDECREF(self->first);

Py_INCREF(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 detach the

<a class="reference internal" href="../glossary.html#term-attached-thread-state">thread state</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?

<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 brackets" href="#id5" id="id1" role="doc-noteref">[1]</a> will neither detach

the <a class="reference internal" href="../glossary.html#term-attached-thread-state">thread state</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 brackets" href="#id6" id="id2" role="doc-noteref">[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[] = {

"first name"},

"last name"},

"custom number"},

{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 <code class="docutils literal notranslate">NULL</code>. Even

though we can make sure the members are initialized to non-<code class="docutils literal notranslate">NULL</code> values, the

members can be set to <code class="docutils literal notranslate">NULL</code> 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.

{

return NULL;

}

if (self->last == NULL) {

return NULL;

}

}

</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 "%s %s" % (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 <code class="docutils literal notranslate">NULL</code>. This is because they can be deleted, in which

case they are set to <code class="docutils literal notranslate">NULL</code>. 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[] = {

"Return the name, combining the first and last name"

},

{NULL} /* Sentinel */

};

</pre></div>

</div>

(note that we used the <a class="reference internal" href="../c-api/structures.html#c.METH_NOARGS" title="METH_NOARGS"><code class="xref c c-macro 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#c.Py_TPFLAGS_BASETYPE" title="Py_TPFLAGS_BASETYPE"><code class="xref c c-macro docutils literal notranslate">Py_TPFLAGS_BASETYPE</code></a> to our class flag definition:

</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 include the new module,

setup(ext_modules=[

Extension("custom", ["custom.c"]),

Extension("custom2", ["custom2.c"]),

])

</pre></div>

</div>

and then we re-install so that we can <code class="docutils literal notranslate">import custom2</code>:

</pre></div>

</div>

</section>

<h2>2.3. Providing finer control over data attributes<a class="headerlink" href="#providing-finer-control-over-data-attributes" title="Link to this heading">¶</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>#define PY_SSIZE_T_CLEAN

#include <Python.h>

#include <stddef.h> /* for offsetof() */

typedef struct {

PyObject_HEAD

int number;

} CustomObject;

static void

Custom_dealloc(PyObject *op)

{

Py_XDECREF(self->first);

Py_XDECREF(self->last);

}

static PyObject *

{

CustomObject *self;

Py_DECREF(self);

return NULL;

}

if (self->last == NULL) {

Py_DECREF(self);

return NULL;

}

}

static int

{

if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,

&first, &last,

&self->number))

return -1;

if (first) {

Py_SETREF(self->first, Py_NewRef(first));

}

if (last) {

Py_SETREF(self->last, Py_NewRef(last));

}

return 0;

}

"custom number"},

{NULL} /* Sentinel */

};

static PyObject *

Custom_getfirst(PyObject *op, void *closure)

{

return Py_NewRef(self->first);

}

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