2. Tutoriel : définir des types dans des extensions¶
Python permet à l'auteur d'un module d'extension C de définir de nouveaux types qui peuvent être manipulés depuis du code Python, à la manière des types natifs str
et list
. Les implémentations de tous les types d'extension ont des similarités, mais quelques subtilités doivent être abordées avant de commencer.
2.1. Les bases¶
The CPython runtime sees all Python objects as variables of type
PyObject*
, which serves as a "base type" for all Python objects.
The PyObject
structure itself only contains the object's
reference count 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".
Donc, pour définir un nouveau type dans une extension, vous devez créer un nouvel objet type.
Ce genre de chose ne s'explique correctement qu'avec des exemples, voici donc un module minimaliste mais suffisant qui définit un nouveau type nommé Custom
dans le module d'extension custom
:
Note
Ce qui est montré ici est la manière traditionnelle de définir des types d'extension statiques, et cela convient dans la majorité des cas. L'API C permet aussi de définir des types alloués sur le tas, via la fonction PyType_FromSpec()
, mais ce n'est pas couvert par ce tutoriel.
#define PY_SSIZE_T_CLEAN
#include <Python.h>
typedef struct {
PyObject_HEAD
/* Type-specific fields go here. */
} CustomObject;
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom.Custom",
.tp_doc = PyDoc_STR("Custom objects"),
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
.tp_flags = Py_TPFLAGS_DEFAULT,
.tp_new = PyType_GenericNew,
};
static PyModuleDef custommodule = {
PyModuleDef_HEAD_INIT,
.m_name = "custom",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_custom(void)
{
PyObject *m;
if (PyType_Ready(&CustomType) < 0)
return NULL;
m = PyModule_Create(&custommodule);
if (m == NULL)
return NULL;
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
return m;
}
C'est un peu long, mais vous devez déjà reconnaître quelques morceaux expliqués au chapitre précédent. Ce fichier définit trois choses :
Ce qu'un objet
Custom
contient : c'est la structureCustomObject
, qui est allouée une fois pour chaque instance deCustom
.Comment le type
Custom
se comporte : c'est la structureCustomType
, qui définit l'ensemble des options et pointeurs de fonction utilisés par l'interpréteur.Comment initialiser le module
custom
: c'est la fonctionPyInit_custom
et la structure associéecustommodule
.
Commençons par :
typedef struct {
PyObject_HEAD
} CustomObject;
This is what a Custom object will contain. PyObject_HEAD
is mandatory
at the start of each object struct and defines a field called ob_base
of type PyObject
, containing a pointer to a type object and a
reference count (these can be accessed using the macros Py_TYPE
and Py_REFCNT
respectively). The reason for the macro is to
abstract away the layout and to enable additional fields in debug builds.
Note
Il n'y a pas de point-virgule après la macro PyObject_HEAD
. Attention à ne pas l'ajouter par accident : certains compilateurs pourraient s'en plaindre.
Bien sûr, les objets ajoutent généralement des données supplémentaires après l'entête standard PyObject_HEAD
. Par exemple voici la définition du type standard Python float
:
typedef struct {
PyObject_HEAD
double ob_fval;
} PyFloatObject;
La deuxième partie est la définition de l'objet type :
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom.Custom",
.tp_doc = PyDoc_STR("Custom objects"),
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
.tp_flags = Py_TPFLAGS_DEFAULT,
.tp_new = PyType_GenericNew,
};
Note
Nous recommandons d'utiliser la syntaxe d'initialisation nommée (C99) pour remplir la structure, comme ci-dessus, afin d'éviter d'avoir à lister les champs de PyTypeObject
dont vous n'avez pas besoin, et de ne pas vous soucier de leur ordre.
La définition de PyTypeObject
dans object.h
contient en fait bien plus de champs que la définition ci-dessus. Les champs restants sont mis à zéro par le compilateur C, et c'est une pratique répandue de ne pas spécifier les champs dont vous n'avez pas besoin.
Regardons les champs de cette structure, un par un :
PyVarObject_HEAD_INIT(NULL, 0)
Cette ligne, obligatoire, initialise le champ ob_base
mentionné précédemment.
.tp_name = "custom.Custom",
C'est le nom de notre type. Il apparaît dans la représentation textuelle par défaut de nos objets, ainsi que dans quelques messages d'erreur, par exemple :
>>> "" + custom.Custom()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "custom.Custom") to str
Notez que le nom comporte un point : il inclut le nom du module et le nom du type. Dans ce cas le module est custom
, et le type est Custom
, donc nous donnons comme nom custom.Custom
. Nommer correctement son type, avec le point, est important pour le rendre compatible avec pydoc
et pickle
.
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
C'est pour que Python sache combien de mémoire allouer à la création d'une nouvelle instance de Custom
. tp_itemsize
n'est utilisé que pour les objets de taille variable, sinon il doit rester à zéro.
Note
Si vous voulez qu'une classe en Python puisse hériter de votre type, et que votre type a le même tp_basicsize
que son parent, vous rencontrerez des problèmes avec l'héritage multiple. Une sous-classe Python de votre type devra lister votre type en premier dans son __bases__
, sans quoi elle ne sera pas capable d'appeler la méthode __new__()
de votre type sans erreur. Vous pouvez éviter ce problème en vous assurant que votre type a un tp_basicsize
plus grand que son parent. La plupart du temps ce sera vrai (soit son parent sera object
, soit vous ajouterez des attributs à votre type, augmentant ainsi sa taille).
On utilise la constante Py_TPFLAGS_DEFAULT
comme seule option de type.
.tp_flags = Py_TPFLAGS_DEFAULT,
Chaque type doit inclure cette constante dans ses options : elle active tous les membres définis jusqu'à au moins Python 3.3. Si vous avez besoin de plus de membres, vous pouvez la combiner à d'autres constantes avec un ou binaire.
On fournit une docstring pour ce type via le membre tp_doc
.
.tp_doc = PyDoc_STR("Custom objects"),
Pour permettre la création d'une instance, nous devons fournir un handler tp_new
, qui est l'équivalent de la méthode Python __new__()
, mais elle a besoin d'être spécifiée explicitement. Dans ce cas, on se contente de l'implémentation par défaut fournie par la fonction PyType_GenericNew()
de l'API.
.tp_new = PyType_GenericNew,
Le reste du fichier doit vous être familier, en dehors du code de PyInit_custom()
:
if (PyType_Ready(&CustomType) < 0)
return;
Il initialise le type Custom
, en assignant quelques membres à leurs valeurs par défaut, tel que ob_type
qui valait initialement NULL
.
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
Ici on ajoute le type au dictionnaire du module. Cela permet de créer une instance de Custom
en appelant la classe Custom
:
>>> import custom
>>> mycustom = custom.Custom()
C'est tout ! Il ne reste plus qu'à compiler, placez le code ci-dessus dans un fichier custom.c
et :
from distutils.core import setup, Extension
setup(name="custom", version="1.0",
ext_modules=[Extension("custom", ["custom.c"])])
in a file called setup.py
; then typing
$ python setup.py build
at a shell should produce a file custom.so
in a subdirectory; move to
that directory and fire up Python --- you should be able to import custom
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.
Note
While this documentation showcases the standard distutils
module
for building C extensions, it is recommended in real-world use cases to
use the newer and better-maintained setuptools
library. Documentation
on how to do this is out of scope for this document and can be found in
the Python Packaging User's Guide.
2.2. Adding data and methods to the Basic example¶
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, custom2
that
adds these capabilities:
#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
static void
Custom_dealloc(CustomObject *self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject *) self);
}
static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
CustomObject *self;
self = (CustomObject *) type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyUnicode_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyUnicode_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *) self;
}
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_XDECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_XDECREF(tmp);
}
return 0;
}
static PyMemberDef Custom_members[] = {
{"first", T_OBJECT_EX, offsetof(CustomObject, first), 0,
"first name"},
{"last", T_OBJECT_EX, offsetof(CustomObject, last), 0,
"last name"},
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
if (self->first == NULL) {
PyErr_SetString(PyExc_AttributeError, "first");
return NULL;
}
if (self->last == NULL) {
PyErr_SetString(PyExc_AttributeError, "last");
return NULL;
}
return PyUnicode_FromFormat("%S %S", self->first, self->last);
}
static PyMethodDef Custom_methods[] = {
{"name", (PyCFunction) Custom_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom2.Custom",
.tp_doc = PyDoc_STR("Custom objects"),
.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 = "custom2",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_custom2(void)
{
PyObject *m;
if (PyType_Ready(&CustomType) < 0)
return NULL;
m = PyModule_Create(&custommodule);
if (m == NULL)
return NULL;
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
return m;
}
This version of the module has a number of changes.
We've added an extra include:
#include <structmember.h>
This include provides declarations that we use to handle attributes, as described a bit later.
The Custom
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:
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
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:
static void
Custom_dealloc(CustomObject *self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject *) self);
}
which is assigned to the tp_dealloc
member:
.tp_dealloc = (destructor) Custom_dealloc,
This method first clears the reference counts of the two Python attributes.
Py_XDECREF()
correctly handles the case where its argument is
NULL
(which might happen here if tp_new
failed midway). It then
calls the tp_free
member of the object's type
(computed by Py_TYPE(self)
) to free the object's memory. Note that
the object's type might not be CustomType
, because the object may
be an instance of a subclass.
Note
The explicit cast to destructor
above is needed because we defined
Custom_dealloc
to take a CustomObject *
argument, but the tp_dealloc
function pointer expects to receive a PyObject *
argument. Otherwise,
the compiler will emit a warning. This is object-oriented polymorphism,
in C!
We want to make sure that the first and last names are initialized to empty
strings, so we provide a tp_new
implementation:
static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
CustomObject *self;
self = (CustomObject *) type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyUnicode_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyUnicode_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *) self;
}
and install it in the tp_new
member:
.tp_new = Custom_new,
The tp_new
handler is responsible for creating (as opposed to initializing)
objects of the type. It is exposed in Python as the __new__()
method.
It is not required to define a tp_new
member, and indeed many extension
types will simply reuse PyType_GenericNew()
as done in the first
version of the Custom
type above. In this case, we use the tp_new
handler to initialize the first
and last
attributes to non-NULL
default values.
tp_new
is passed the type being instantiated (not necessarily CustomType
,
if a subclass is instantiated) and any arguments passed when the type was
called, and is expected to return the instance created. tp_new
handlers
always accept positional and keyword arguments, but they often ignore the
arguments, leaving the argument handling to initializer (a.k.a. tp_init
in C or __init__
in Python) methods.
Note
tp_new
shouldn't call tp_init
explicitly, as the interpreter
will do it itself.
The tp_new
implementation calls the tp_alloc
slot to allocate memory:
self = (CustomObject *) type->tp_alloc(type, 0);
Since memory allocation may fail, we must check the tp_alloc
result against NULL
before proceeding.
Note
We didn't fill the tp_alloc
slot ourselves. Rather
PyType_Ready()
fills it for us by inheriting it from our base class,
which is object
by default. Most types use the default allocation
strategy.
Note
If you are creating a co-operative tp_new
(one
that calls a base type's tp_new
or __new__()
),
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 tp_new
directly, or via
type->tp_base->tp_new
. 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 TypeError
.)
We also define an initialization function which accepts arguments to provide initial values for our instance:
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_XDECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_XDECREF(tmp);
}
return 0;
}
by filling the tp_init
slot.
.tp_init = (initproc) Custom_init,
The tp_init
slot is exposed in Python as the
__init__()
method. It is used to initialize an object after it's
created. Initializers always accept positional and keyword arguments,
and they should return either 0
on success or -1
on error.
Unlike the tp_new
handler, there is no guarantee that tp_init
is called at all (for example, the pickle
module by default
doesn't call __init__()
on unpickled instances). It can also be
called multiple times. Anyone can call the __init__()
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
first
member like this:
if (first) {
Py_XDECREF(self->first);
Py_INCREF(first);
self->first = first;
}
But this would be risky. Our type doesn't restrict the type of the
first
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
first
member; or that destructor could release the
Global interpreter Lock 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?
when we absolutely know that the reference count is greater than 1;
when we know that deallocation of the object 1 will neither release the GIL nor cause any calls back into our type's code;
when decrementing a reference count in a
tp_dealloc
handler on a type which doesn't support cyclic garbage collection 2.
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:
static PyMemberDef Custom_members[] = {
{"first", T_OBJECT_EX, offsetof(CustomObject, first), 0,
"first name"},
{"last", T_OBJECT_EX, offsetof(CustomObject, last), 0,
"last name"},
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
and put the definitions in the tp_members
slot:
.tp_members = Custom_members,
Each member definition has a member name, type, offset, access flags and documentation string. See the Gestion des attributs génériques 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, Custom.name()
, that outputs the objects name as the
concatenation of the first and last names.
static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
if (self->first == NULL) {
PyErr_SetString(PyExc_AttributeError, "first");
return NULL;
}
if (self->last == NULL) {
PyErr_SetString(PyExc_AttributeError, "last");
return NULL;
}
return PyUnicode_FromFormat("%S %S", self->first, self->last);
}
The method is implemented as a C function that takes a Custom
(or
Custom
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:
def name(self):
return "%s %s" % (self.first, self.last)
Note that we have to check for the possibility that our first
and
last
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:
static PyMethodDef Custom_methods[] = {
{"name", (PyCFunction) Custom_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
(note that we used the METH_NOARGS
flag to indicate that the method
is expecting no arguments other than self)
and assign it to the tp_methods
slot:
.tp_methods = Custom_methods,
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 Py_TPFLAGS_BASETYPE
to our class flag definition:
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
We rename PyInit_custom()
to PyInit_custom2()
, update the
module name in the PyModuleDef
struct, and update the full class
name in the PyTypeObject
struct.
Finally, we update our setup.py
file to build the new module:
from distutils.core import setup, Extension
setup(name="custom", version="1.0",
ext_modules=[
Extension("custom", ["custom.c"]),
Extension("custom2", ["custom2.c"]),
])
2.3. Providing finer control over data attributes¶
In this section, we'll provide finer control over how the first
and
last
attributes are set in the Custom
example. In the previous
version of our module, the instance variables first
and last
could be set to non-string values or even deleted. We want to make sure that
these attributes always contain strings.
#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
static void
Custom_dealloc(CustomObject *self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject *) self);
}
static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
CustomObject *self;
self = (CustomObject *) type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyUnicode_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyUnicode_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *) self;
}
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_DECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_DECREF(tmp);
}
return 0;
}
static PyMemberDef Custom_members[] = {
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
Py_INCREF(self->first);
return self->first;
}
static int
Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
{
PyObject *tmp;
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The first attribute value must be a string");
return -1;
}
tmp = self->first;
Py_INCREF(value);
self->first = value;
Py_DECREF(tmp);
return 0;
}
static PyObject *
Custom_getlast(CustomObject *self, void *closure)
{
Py_INCREF(self->last);
return self->last;
}
static int
Custom_setlast(CustomObject *self, PyObject *value, void *closure)
{
PyObject *tmp;
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the last attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The last attribute value must be a string");
return -1;
}
tmp = self->last;
Py_INCREF(value);
self->last = value;
Py_DECREF(tmp);
return 0;
}
static PyGetSetDef Custom_getsetters[] = {
{"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
"first name", NULL},
{"last", (getter) Custom_getlast, (setter) Custom_setlast,
"last name", NULL},
{NULL} /* Sentinel */
};
static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
return PyUnicode_FromFormat("%S %S", self->first, self->last);
}
static PyMethodDef Custom_methods[] = {
{"name", (PyCFunction) Custom_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom3.Custom",
.tp_doc = PyDoc_STR("Custom objects"),
.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 = "custom3",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_custom3(void)
{
PyObject *m;
if (PyType_Ready(&CustomType) < 0)
return NULL;
m = PyModule_Create(&custommodule);
if (m == NULL)
return NULL;
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
return m;
}
To provide greater control, over the first
and last
attributes,
we'll use custom getter and setter functions. Here are the functions for
getting and setting the first
attribute:
static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
Py_INCREF(self->first);
return self->first;
}
static int
Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
{
PyObject *tmp;
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The first attribute value must be a string");
return -1;
}
tmp = self->first;
Py_INCREF(value);
self->first = value;
Py_DECREF(tmp);
return 0;
}
The getter function is passed a Custom
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 Custom
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 PyGetSetDef
structures:
static PyGetSetDef Custom_getsetters[] = {
{"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
"first name", NULL},
{"last", (getter) Custom_getlast, (setter) Custom_setlast,
"last name", NULL},
{NULL} /* Sentinel */
};
and register it in the tp_getset
slot:
.tp_getset = Custom_getsetters,
The last item in a PyGetSetDef
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:
static PyMemberDef Custom_members[] = {
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
We also need to update the tp_init
handler to only
allow strings 3 to be passed:
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_DECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_DECREF(tmp);
}
return 0;
}
With these changes, we can assure that the first
and last
members are
never NULL
so we can remove checks for NULL
values in almost all cases.
This means that most of the Py_XDECREF()
calls can be converted to
Py_DECREF()
calls. The only place we can't change these calls is in
the tp_dealloc
implementation, where there is the possibility that the
initialization of these members failed in tp_new
.
We also rename the module initialization function and module name in the
initialization function, as we did before, and we add an extra definition to the
setup.py
file.
2.4. Supporting cyclic garbage collection¶
Python has a cyclic garbage collector (GC) that can identify unneeded objects even when their reference counts are not zero. This can happen when objects are involved in cycles. For example, consider:
>>> l = []
>>> l.append(l)
>>> del l
In this example, we create a list that contains itself. When we delete it, it still has a reference from itself. Its reference count doesn't drop to zero. Fortunately, Python's cyclic garbage collector will eventually figure out that the list is garbage and free it.
In the second version of the Custom
example, we allowed any kind of
object to be stored in the first
or last
attributes 4.
Besides, in the second and third versions, we allowed subclassing
Custom
, and subclasses may add arbitrary attributes. For any of
those two reasons, Custom
objects can participate in cycles:
>>> import custom3
>>> class Derived(custom3.Custom): pass
...
>>> n = Derived()
>>> n.some_attribute = n
To allow a Custom
instance participating in a reference cycle to
be properly detected and collected by the cyclic GC, our Custom
type
needs to fill two additional slots and to enable a flag that enables these slots:
#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include "structmember.h"
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
Py_VISIT(self->first);
Py_VISIT(self->last);
return 0;
}
static int
Custom_clear(CustomObject *self)
{
Py_CLEAR(self->first);
Py_CLEAR(self->last);
return 0;
}
static void
Custom_dealloc(CustomObject *self)
{
PyObject_GC_UnTrack(self);
Custom_clear(self);
Py_TYPE(self)->tp_free((PyObject *) self);
}
static PyObject *
Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
CustomObject *self;
self = (CustomObject *) type->tp_alloc(type, 0);
if (self != NULL) {
self->first = PyUnicode_FromString("");
if (self->first == NULL) {
Py_DECREF(self);
return NULL;
}
self->last = PyUnicode_FromString("");
if (self->last == NULL) {
Py_DECREF(self);
return NULL;
}
self->number = 0;
}
return (PyObject *) self;
}
static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"first", "last", "number", NULL};
PyObject *first = NULL, *last = NULL, *tmp;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
&first, &last,
&self->number))
return -1;
if (first) {
tmp = self->first;
Py_INCREF(first);
self->first = first;
Py_DECREF(tmp);
}
if (last) {
tmp = self->last;
Py_INCREF(last);
self->last = last;
Py_DECREF(tmp);
}
return 0;
}
static PyMemberDef Custom_members[] = {
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
Py_INCREF(self->first);
return self->first;
}
static int
Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
{
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The first attribute value must be a string");
return -1;
}
Py_INCREF(value);
Py_CLEAR(self->first);
self->first = value;
return 0;
}
static PyObject *
Custom_getlast(CustomObject *self, void *closure)
{
Py_INCREF(self->last);
return self->last;
}
static int
Custom_setlast(CustomObject *self, PyObject *value, void *closure)
{
if (value == NULL) {
PyErr_SetString(PyExc_TypeError, "Cannot delete the last attribute");
return -1;
}
if (!PyUnicode_Check(value)) {
PyErr_SetString(PyExc_TypeError,
"The last attribute value must be a string");
return -1;
}
Py_INCREF(value);
Py_CLEAR(self->last);
self->last = value;
return 0;
}
static PyGetSetDef Custom_getsetters[] = {
{"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
"first name", NULL},
{"last", (getter) Custom_getlast, (setter) Custom_setlast,
"last name", NULL},
{NULL} /* Sentinel */
};
static PyObject *
Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
{
return PyUnicode_FromFormat("%S %S", self->first, self->last);
}
static PyMethodDef Custom_methods[] = {
{"name", (PyCFunction) Custom_name, METH_NOARGS,
"Return the name, combining the first and last name"
},
{NULL} /* Sentinel */
};
static PyTypeObject CustomType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "custom4.Custom",
.tp_doc = PyDoc_STR("Custom objects"),
.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 = "custom4",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_custom4(void)
{
PyObject *m;
if (PyType_Ready(&CustomType) < 0)
return NULL;
m = PyModule_Create(&custommodule);
if (m == NULL)
return NULL;
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
return m;
}
First, the traversal method lets the cyclic GC know about subobjects that could participate in cycles:
static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
int vret;
if (self->first) {
vret = visit(self->first, arg);
if (vret != 0)
return vret;
}
if (self->last) {
vret = visit(self->last, arg);
if (vret != 0)
return vret;
}
return 0;
}
For each subobject that can participate in cycles, we need to call the
visit()
function, which is passed to the traversal method. The
visit()
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 Py_VISIT()
macro that automates calling visit
functions. With Py_VISIT()
, we can minimize the amount of boilerplate
in Custom_traverse
:
static int
Custom_traverse(CustomObject *self, visitproc visit, void *arg)
{
Py_VISIT(self->first);
Py_VISIT(self->last);
return 0;
}
Note
The tp_traverse
implementation must name its
arguments exactly visit and arg in order to use Py_VISIT()
.
Second, we need to provide a method for clearing any subobjects that can participate in cycles:
static int
Custom_clear(CustomObject *self)
{
Py_CLEAR(self->first);
Py_CLEAR(self->last);
return 0;
}
Notice the use of the Py_CLEAR()
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 Py_XDECREF()
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).
Note
You could emulate Py_CLEAR()
by writing:
PyObject *tmp;
tmp = self->first;
self->first = NULL;
Py_XDECREF(tmp);
Nevertheless, it is much easier and less error-prone to always
use Py_CLEAR()
when deleting an attribute. Don't
try to micro-optimize at the expense of robustness!
The deallocator Custom_dealloc
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 PyObject_GC_UnTrack()
before clearing members.
Here is our reimplemented deallocator using PyObject_GC_UnTrack()
and Custom_clear
:
static void
Custom_dealloc(CustomObject *self)
{
PyObject_GC_UnTrack(self);
Custom_clear(self);
Py_TYPE(self)->tp_free((PyObject *) self);
}
Finally, we add the Py_TPFLAGS_HAVE_GC
flag to the class flags:
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
That's pretty much it. If we had written custom tp_alloc
or
tp_free
handlers, we'd need to modify them for cyclic
garbage collection. Most extensions will use the versions automatically provided.
2.5. Subclassing other types¶
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 PyTypeObject
it needs. It can be difficult to share
these PyTypeObject
structures between extension modules.
In this example we will create a SubList
type that inherits from the
built-in list
type. The new type will be completely compatible with
regular lists, but will have an additional increment()
method that
increases an internal counter:
>>> import sublist
>>> s = sublist.SubList(range(3))
>>> s.extend(s)
>>> print(len(s))
6
>>> print(s.increment())
1
>>> print(s.increment())
2
#define PY_SSIZE_T_CLEAN
#include <Python.h>
typedef struct {
PyListObject list;
int state;
} SubListObject;
static PyObject *
SubList_increment(SubListObject *self, PyObject *unused)
{
self->state++;
return PyLong_FromLong(self->state);
}
static PyMethodDef SubList_methods[] = {
{"increment", (PyCFunction) SubList_increment, METH_NOARGS,
PyDoc_STR("increment state counter")},
{NULL},
};
static int
SubList_init(SubListObject *self, PyObject *args, PyObject *kwds)
{
if (PyList_Type.tp_init((PyObject *) self, args, kwds) < 0)
return -1;
self->state = 0;
return 0;
}
static PyTypeObject SubListType = {
PyVarObject_HEAD_INIT(NULL, 0)
.tp_name = "sublist.SubList",
.tp_doc = PyDoc_STR("SubList objects"),
.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 = "sublist",
.m_doc = "Example module that creates an extension type.",
.m_size = -1,
};
PyMODINIT_FUNC
PyInit_sublist(void)
{
PyObject *m;
SubListType.tp_base = &PyList_Type;
if (PyType_Ready(&SubListType) < 0)
return NULL;
m = PyModule_Create(&sublistmodule);
if (m == NULL)
return NULL;
Py_INCREF(&SubListType);
if (PyModule_AddObject(m, "SubList", (PyObject *) &SubListType) < 0) {
Py_DECREF(&SubListType);
Py_DECREF(m);
return NULL;
}
return m;
}
As you can see, the source code closely resembles the Custom
examples in
previous sections. We will break down the main differences between them.
typedef struct {
PyListObject list;
int state;
} SubListObject;
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 PyObject_HEAD()
at the beginning of its structure.
When a Python object is a SubList
instance, its PyObject *
pointer
can be safely cast to both PyListObject *
and SubListObject *
:
static int
SubList_init(SubListObject *self, PyObject *args, PyObject *kwds)
{
if (PyList_Type.tp_init((PyObject *) self, args, kwds) < 0)
return -1;
self->state = 0;
return 0;
}
We see above how to call through to the __init__
method of the base
type.
This pattern is important when writing a type with custom
tp_new
and tp_dealloc
members. The tp_new
handler should not actually
create the memory for the object with its tp_alloc
,
but let the base class handle it by calling its own tp_new
.
The PyTypeObject
struct supports a tp_base
specifying the type's concrete base class. Due to cross-platform compiler
issues, you can't fill that field directly with a reference to
PyList_Type
; it should be done later in the module initialization
function:
PyMODINIT_FUNC
PyInit_sublist(void)
{
PyObject* m;
SubListType.tp_base = &PyList_Type;
if (PyType_Ready(&SubListType) < 0)
return NULL;
m = PyModule_Create(&sublistmodule);
if (m == NULL)
return NULL;
Py_INCREF(&SubListType);
if (PyModule_AddObject(m, "SubList", (PyObject *) &SubListType) < 0) {
Py_DECREF(&SubListType);
Py_DECREF(m);
return NULL;
}
return m;
}
Before calling PyType_Ready()
, the type structure must have the
tp_base
slot filled in. When we are deriving an
existing type, it is not necessary to fill out the tp_alloc
slot with PyType_GenericNew()
-- the allocation function from the base
type will be inherited.
After that, calling PyType_Ready()
and adding the type object to the
module is the same as with the basic Custom
examples.
Notes
- 1
This is true when we know that the object is a basic type, like a string or a float.
- 2
We relied on this in the
tp_dealloc
handler in this example, because our type doesn't support garbage collection.- 3
We now know that the first and last members are strings, so perhaps we could be less careful about decrementing their reference counts, however, we accept instances of string subclasses. Even though deallocating normal strings won't call back into our objects, we can't guarantee that deallocating an instance of a string subclass won't call back into our objects.
- 4
Also, even with our attributes restricted to strings instances, the user could pass arbitrary
str
subclasses and therefore still create reference cycles.