2. 拡張の型の定義: チュートリアル¶
Python では、組み込みの str
型や list
型のような Python コードから操作できる新しい型を C 拡張モジュールの作者が定義できます。
全ての拡張の型のコードはあるパターンに従うのですが、書き始める前に理解しておくべき細かいことがあります。
このドキュメントはその話題についてのやさしい入門です。
2.1. 基本的なこと¶
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".
それなので、新しい拡張の型を定義したいときは、新しい型オブジェクトを作成すればよいわけです。
This sort of thing can only be explained by example, so here's a minimal, but
complete, module that defines a new type named Custom
inside a C
extension module custom
:
注釈
ここで紹介している例は、 静的な 拡張の型を定義する伝統的な実装方法です。
これはほとんどの場面で十分なものなのです。
C API では、 PyType_FromSpec()
関数を使い、ヒープ上に配置された拡張の型も定義できますが、これについてはこのチュートリアルでは扱いません。
#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;
}
一度に把握するにはちょっと量が多いですが、前の章よりはとっつきやすくなっていることと思います。このファイルでは、3つの要素が定義されています:
What a
Custom
object contains: this is theCustomObject
struct, which is allocated once for eachCustom
instance.How the
Custom
type behaves: this is theCustomType
struct, which defines a set of flags and function pointers that the interpreter inspects when specific operations are requested.How to initialize the
custom
module: this is thePyInit_custom
function and the associatedcustommodule
struct.
まず最初はこれです:
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.
注釈
上の例では PyObject_HEAD
マクロの後にセミコロンはありません。
うっかりセミコロンを追加しないように気を付けてください: これを警告するコンパイラもあります。
もちろん、一般的にはオブジェクトは標準的な PyObject_HEAD
ボイラープレートの他にもデータを保持しています; 例えば、これは Python 標準の浮動小数点数の定義です:
typedef struct {
PyObject_HEAD
double ob_fval;
} PyFloatObject;
2つ目は型オブジェクトの定義です。
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,
};
注釈
上にあるように C99 スタイルの指示付き初期化子を使って、 PyTypeObject
の特に関心の無いフィールドまで全て並べたり、フィールドを宣言する順序に気を使ったりせずに済ませるのをお薦めします。
object.h
にある実際の PyTypeObject
の定義には上の定義にあるよりももっと多くの フィールド があります。
ここに出てきていないフィールドは C コンパイラによってゼロで埋められるので、必要でない限り明示的には値の指定をしないのが一般的な作法になっています。
一度に1つずつフィールドを取り上げていきましょう:
PyVarObject_HEAD_INIT(NULL, 0)
この行は、上で触れた ob_base
フィールドの初期化に必須のボイラープレートです。
.tp_name = "custom.Custom",
実装している型の名前です。 これは、オブジェクトのデフォルトの文字列表現やエラーメッセージに現れます。例えば次の通りです:
>>> "" + custom.Custom()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "custom.Custom") to str
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 custom
and
the type is Custom
, so we set the type name to custom.Custom
.
Using the real dotted import path is important to make your type compatible
with the pydoc
and pickle
modules.
.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,
This is so that Python knows how much memory to allocate when creating
new Custom
instances. tp_itemsize
is
only used for variable-sized objects and should otherwise be zero.
注釈
If you want your type to be subclassable from Python, and your type has the same
tp_basicsize
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 __bases__
, or else it will not be able to call your type's
__new__()
method without getting an error. You can avoid this problem by
ensuring that your type has a larger value for tp_basicsize
than its
base type does. Most of the time, this will be true anyway, because either your
base type will be object
, or else you will be adding data members to
your base type, and therefore increasing its size.
We set the class flags to Py_TPFLAGS_DEFAULT
.
.tp_flags = Py_TPFLAGS_DEFAULT,
すべての型はフラグにこの定数を含めておく必要があります。これは最低でも Python 3.3 までに定義されているすべてのメンバを許可します。それ以上のメンバが必要なら、対応するフラグの OR をとる必要があります。
この型の docstring は tp_doc
に入れます。
.tp_doc = PyDoc_STR("Custom objects"),
To enable object creation, we have to provide a tp_new
handler. This is the equivalent of the Python method __new__()
, but
has to be specified explicitly. In this case, we can just use the default
implementation provided by the API function PyType_GenericNew()
.
.tp_new = PyType_GenericNew,
Everything else in the file should be familiar, except for some code in
PyInit_custom()
:
if (PyType_Ready(&CustomType) < 0)
return;
This initializes the Custom
type, filling in a number of members
to the appropriate default values, including ob_type
that we initially
set to NULL
.
Py_INCREF(&CustomType);
if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
Py_DECREF(&CustomType);
Py_DECREF(m);
return NULL;
}
This adds the type to the module dictionary. This allows us to create
Custom
instances by calling the Custom
class:
>>> import custom
>>> mycustom = custom.Custom()
That's it! All that remains is to build it; put the above code in a file called
custom.c
and:
from distutils.core import setup, Extension
setup(name="custom", version="1.0",
ext_modules=[Extension("custom", ["custom.c"])])
そして、シェルから以下のように入力します
$ 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.
そんなにむずかしくありません、よね?
もちろん、現在の Custom 型は面白みに欠けています。何もデータを持っていないし、何もできません。継承してサブクラスを作ることさえできないのです。
注釈
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. 基本のサンプルにデータとメソッドを追加する¶
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;
}
このバージョンでは、いくつもの変更をおこないます。
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.
これにしたがうと、オブジェクトの構造体は次のようになります:
typedef struct {
PyObject_HEAD
PyObject *first; /* first name */
PyObject *last; /* last name */
int number;
} CustomObject;
いまや管理すべきデータができたので、オブジェクトの割り当てと解放に際してはより慎重になる必要があります。最低限、オブジェクトの解放メソッドが必要です:
static void
Custom_dealloc(CustomObject *self)
{
Py_XDECREF(self->first);
Py_XDECREF(self->last);
Py_TYPE(self)->tp_free((PyObject *) self);
}
この関数は tp_dealloc
メンバに代入されます。
.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.
注釈
上の destructor
への明示的な型変換は必要です。なぜなら、 Custom_dealloc
が CustomObject *
引数をとると定義しましたが、 tp_dealloc
関数のポインタは PyObject *
引数を受け取ることになっているからです。もし明示的に型変換をしなければ、コンパイラが警告を発するでしょう。これは、Cにおけるオブジェクト指向のポリモーフィズムです!
ファーストネームとラストネームを空文字列に初期化しておきたいので、tp_new
の実装を追加することにしましょう:
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;
}
そしてこれを tp_new
メンバとしてインストールします:
.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.
注釈
tp_new
は明示的に tp_init
を呼び出してはいけません、これはインタープリタが自分で行うためです。
この tp_new
の実装は、tp_alloc
スロットを呼び出してメモリを割り当てます:
self = (CustomObject *) type->tp_alloc(type, 0);
Since memory allocation may fail, we must check the tp_alloc
result against NULL
before proceeding.
注釈
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.
注釈
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;
}
これは tp_init
メンバに代入されます。
.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?
その参照カウントが 1 より大きいと確信できる場合
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.
ここではインスタンス変数を属性として見えるようにしたいのですが、これにはいくつもの方法があります。もっとも簡単な方法は、メンバの定義を与えることです:
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 */
};
そして、この定義を tp_members
スロットに入れましょう:
.tp_members = Custom_members,
Each member definition has a member name, type, offset, access flags and documentation string. See the 総称的な属性を管理する 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.
さて、メソッドを定義したので、ここでメソッド定義用の配列を作成する必要があります:
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. データ属性をこまかく制御する¶
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.
ここでは PyGetSetDef
構造体の配列をつくります:
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 */
};
そしてこれを tp_getset
スロットに登録します:
.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
.
また、メンバ定義からはこれらの属性を除いておきましょう:
static PyMemberDef Custom_members[] = {
{"number", T_INT, offsetof(CustomObject, number), 0,
"custom number"},
{NULL} /* Sentinel */
};
また、ここでは tp_init
ハンドラも渡されるものとして文字列のみを許可するように修正する必要があります 3:
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
.
さて、先ほどもしたように、このモジュール初期化関数と初期化関数内にあるモジュール名を変更しましょう。そして setup.py
ファイルに追加の定義をくわえます。
2.4. 循環ガベージコレクションをサポートする¶
Python は 循環ガベージコレクタ (GC) 機能 をもっており、これは不要なオブジェクトを、たとえ参照カウントがゼロでなくても発見することができます。そのような状況はオブジェクトの参照が循環しているときに起こりえます。たとえば以下の例を考えてください:
>>> l = []
>>> l.append(l)
>>> del l
この例では、自分自身をふくむリストを作りました。たとえこのリストを 削除しても、それは自分自身への参照をまだ持ちつづけますから、参照カウントはゼロにはなりません。嬉しいことに Python には循環ガベージコレクタは最終的にはこのリストが不要であることを検出し、解放できます。
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;
}
注釈
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).
注釈
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,
これで完了です。 tp_alloc
スロットまたは tp_free
ハンドラが書かれていれば、それらを循環ガベージコレクションに使えるよう修正すればよいのです。ほとんどの拡張機能は自動的に提供されるバージョンを使うでしょう。
2.5. 他の型のサブクラスを作る¶
既存の型を継承した新しい拡張型を作成することができます。組み込み型から継承するのは特に簡単です。必要な PyTypeObject
を簡単に利用できるからです。それに比べて、 PyTypeObject
構造体を拡張モジュール間で共有するのは難しいです。
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;
}
PyType_Read()
を呼ぶ前に、型の構造体の tp_base
スロットは埋められていなければなりません。既存の型を継承する際には、 tp_alloc
スロットを PyType_GenericNew()
で埋める必要はありません。 -- 基底型のアロケーション関数が継承されます。
After that, calling PyType_Ready()
and adding the type object to the
module is the same as with the basic Custom
examples.
脚注
- 1
これはそのオブジェクトが文字列や実数などの基本タイプであるような時に成り立ちます。
- 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.