2. 定义扩展类型：教程 ¶

Python 允许 C 扩展模块作者定义可以从 Python 代码操纵的新类型，非常像内置 str and list 类型。所有扩展类型代码遵循模式，但开始之前需要了解一些细节。此文档是话题的温柔介绍。

2.1. 基础 ¶

The CPython 运行时将所有 Python 对象视为变量类型 PyObject * ，充当所有 Python 对象的 "基类型"。 PyObject 结构本身仅包含对象的引用计数和指向对象 "类型对象" 的指针。这是行动所在；类型对象确定解释器调用哪些 C 函数，例如，查找对象属性、调用方法、或乘以另一对象。这些 C 函数称为 "类型方法"。

所以，若想要定义新扩展类型，需要创建新类型对象。

这种事情只能通过范例解释，所以这里最小但完整的模块，定义新类型名称 Custom 在 C 扩展名模块 custom :

注意

这里展示以传统方式定义的 static 扩展类型。它应该足以满足大多数用法。 C API 还允许定义堆分配扩展类型使用 PyType_FromSpec() 函数，此教程未涵盖。

#define PY_SSIZE_T_CLEAN
#include <Python.h>
typedef struct {
    PyObject_HEAD
    /* Type-specific fields go here. */
} CustomObject;
static PyTypeObject CustomType = {
    .ob_base = PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Custom",
    .tp_doc = PyDoc_STR("Custom objects"),
    .tp_basicsize = sizeof(CustomObject),
    .tp_itemsize = 0,
    .tp_flags = Py_TPFLAGS_DEFAULT,
    .tp_new = PyType_GenericNew,
};
static PyModuleDef custommodule = {
    .m_base = 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;
    if (PyModule_AddObjectRef(m, "Custom", (PyObject *) &CustomType) < 0) {
        Py_DECREF(m);
        return NULL;
    }
    return m;
}

Now that’s quite a bit to take in at once, but hopefully bits will seem familiar from the previous chapter. This file defines three things:

为什么 Custom 对象包含：这是 CustomObject struct, which is allocated once for each Custom 实例。
如何 Custom type 行为：这是 CustomType struct, which defines a set of flags and function pointers that the interpreter inspects when specific operations are requested.
如何初始化 custom 模块：这是 PyInit_custom 函数且关联 custommodule struct.

The first bit is:

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 类型 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 .

注意

There is no semicolon above after the PyObject_HEAD macro. Be wary of adding one by accident: some compilers will complain.

Of course, objects generally store additional data besides the standard PyObject_HEAD boilerplate; for example, here is the definition for standard Python floats:

typedef struct {
    PyObject_HEAD
    double ob_fval;
} PyFloatObject;

The second bit is the definition of the type object.

static PyTypeObject CustomType = {
    .ob_base = PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Custom",
    .tp_doc = PyDoc_STR("Custom objects"),
    .tp_basicsize = sizeof(CustomObject),
    .tp_itemsize = 0,
    .tp_flags = Py_TPFLAGS_DEFAULT,
    .tp_new = PyType_GenericNew,
};

注意

We recommend using C99-style designated initializers as above, to avoid listing all the PyTypeObject fields that you don’t care about and also to avoid caring about the fields’ declaration order.

The actual definition of PyTypeObject in object.h has many more 字段 than the definition above. The remaining fields will be filled with zeros by the C compiler, and it’s common practice to not specify them explicitly unless you need them.

We’re going to pick it apart, one field at a time:

.ob_base = PyVarObject_HEAD_INIT(NULL, 0)

This line is mandatory boilerplate to initialize the ob_base field mentioned above.

.tp_name = "custom.Custom",

The name of our type. This will appear in the default textual representation of our objects and in some error messages, for example:

>>> "" + 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 模块。

.tp_basicsize = sizeof(CustomObject),
.tp_itemsize = 0,

This is so that Python knows how much memory to allocate when creating new Custom 实例。 tp_itemsize is only used for variable-sized objects and should otherwise be zero.

注意

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.

将类标志设为 Py_TPFLAGS_DEFAULT .

.tp_flags = Py_TPFLAGS_DEFAULT,

All types should include this constant in their flags. It enables all of the members defined until at least Python 3.3. If you need further members, you will need to OR the corresponding flags.

We provide a doc string for the type in tp_doc .

.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;

这初始化 Custom type, filling in a number of members to the appropriate default values, including ob_type that we initially set to NULL .

if (PyModule_AddObjectRef(m, "Custom", (PyObject *) &CustomType) < 0) {
    Py_DECREF(m);
    return NULL;
}

This adds the type to the module dictionary. This allows us to create Custom 实例通过调用 Custom 类：

>>> 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 ,

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"
[project]
name = "custom"
version = "1"

in a file called pyproject.toml ，和

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

in a file called setup.py ; then typing

$ python -m pip install .

in a shell should produce a file custom.so in a subdirectory and install it; now fire up Python — you should be able to import custom and play around with Custom 对象。

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.

2.2. 把数据和方法添加到基本范例 ¶

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 <stddef.h> /* for offsetof() */
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;
    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
                                     &first, &last,
                                     &self->number))
        return -1;
    if (first) {
        Py_XSETREF(self->first, Py_NewRef(first));
    }
    if (last) {
        Py_XSETREF(self->last, Py_NewRef(last));
    }
    return 0;
}
static PyMemberDef Custom_members[] = {
    {"first", Py_T_OBJECT_EX, offsetof(CustomObject, first), 0,
     "first name"},
    {"last", Py_T_OBJECT_EX, offsetof(CustomObject, last), 0,
     "last name"},
    {"number", Py_T_INT, offsetof(CustomObject, number), 0,
     "custom number"},
    {NULL}  /* Sentinel */
};
static PyObject *
Custom_name(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 = {
    .ob_base = 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 = {
    .m_base =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;
    if (PyModule_AddObjectRef(m, "Custom", (PyObject *) &CustomType) < 0) {
        Py_DECREF(m);
        return NULL;
    }
    return m;
}

This version of the module has a number of changes.

The Custom type now has three data attributes in its C struct, 第一 , last ，和 number 。第一 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 成员：

.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.

注意

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;
}

并将它安装在 tp_new 成员：

.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.

注意

tp_new shouldn’t call tp_init explicitly, as the interpreter will do it itself.

The tp_new 实现调用 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.

注意

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.

注意

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 槽。

.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 当成功时或 -1 当出错时。

不像 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", Py_T_OBJECT_EX, offsetof(CustomObject, first), 0,
     "first name"},
    {"last", Py_T_OBJECT_EX, offsetof(CustomObject, last), 0,
     "last name"},
    {"number", Py_T_INT, offsetof(CustomObject, number), 0,
     "custom number"},
    {NULL}  /* Sentinel */
};

and put the definitions in the tp_members 槽：

.tp_members = Custom_members,

Each member definition has a member name, type, offset, access flags and documentation string. See the Generic Attribute Management 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.

定义一个方法 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 (或 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 槽：

.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,

重命名 PyInit_custom() to PyInit_custom2() ，更新模块名称在 PyModuleDef struct, and update the full class name in the PyTypeObject struct.

最后，更新 setup.py file to include the new module,

from setuptools import Extension, setup
setup(ext_modules=[
    Extension("custom", ["custom.c"]),
    Extension("custom2", ["custom2.c"]),
])

and then we re-install so that we can import custom2 :

$ python -m pip install .

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 <stddef.h> /* for offsetof() */
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;
    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
                                     &first, &last,
                                     &self->number))
        return -1;
    if (first) {
        Py_SETREF(self->first, Py_NewRef(first));
    }
    if (last) {
        Py_SETREF(self->last, Py_NewRef(last));
    }
    return 0;
}
static PyMemberDef Custom_members[] = {
    {"number", Py_T_INT, offsetof(CustomObject, number), 0,
     "custom number"},
    {NULL}  /* Sentinel */
};
static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
    return Py_NewRef(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_SETREF(self->first, Py_NewRef(value));
    return 0;
}
static PyObject *
Custom_getlast(CustomObject *self, void *closure)
{
    return Py_NewRef(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_SETREF(self->last, Py_NewRef(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 = {
    .ob_base = 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 = {
    .m_base = 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;
    if (PyModule_AddObjectRef(m, "Custom", (PyObject *) &CustomType) < 0) {
        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 属性：

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 结构：

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 槽：

.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", Py_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 文件。

2.4. Supporting cyclic garbage collection ¶

Python 拥有 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 属性 [ 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 <stddef.h> /* for offsetof() */
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;
    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
                                     &first, &last,
                                     &self->number))
        return -1;
    if (first) {
        Py_SETREF(self->first, Py_NewRef(first));
    }
    if (last) {
        Py_SETREF(self->last, Py_NewRef(last));
    }
    return 0;
}
static PyMemberDef Custom_members[] = {
    {"number", Py_T_INT, offsetof(CustomObject, number), 0,
     "custom number"},
    {NULL}  /* Sentinel */
};
static PyObject *
Custom_getfirst(CustomObject *self, void *closure)
{
    return Py_NewRef(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_XSETREF(self->first, Py_NewRef(value));
    return 0;
}
static PyObject *
Custom_getlast(CustomObject *self, void *closure)
{
    return Py_NewRef(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_XSETREF(self->last, Py_NewRef(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 = {
    .ob_base = 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 = {
    .m_base = 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;
    if (PyModule_AddObjectRef(m, "Custom", (PyObject *) &CustomType) < 0) {
        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;
}

注意

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).

注意

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;
    if (PyModule_AddObjectRef(m, "SubList", (PyObject *) &SubListType) < 0) {
        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;
    if (PyModule_AddObjectRef(m, "SubList", (PyObject *) &SubListType) < 0) {
        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 范例。

脚注

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2. 定义扩展类型：教程 ¶

2.1. 基础 ¶

2.2. 把数据和方法添加到基本范例 ¶

2.3. Providing finer control over data attributes ¶

2.4. Supporting cyclic garbage collection ¶

2.5. Subclassing other types ¶

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