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 = "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 :

1. Ce qu'un **objet** "Custom" contient : c'est la structure
   "CustomObject", qui est allouée une fois pour chaque instance de
   "Custom".

2. Comment le **type** "Custom" se comporte : c'est la structure
   "CustomType", qui définit l'ensemble des options et pointeurs de
   fonction utilisés par l'interpréteur.

3. Comment initialiser le module "custom" : c'est la fonction
   "PyInit_custom" et la structure associée "custommodule".

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_REFCNT" and "Py_TYPE" 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 = "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 = "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 = "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 = "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 = "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 = "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.
