C 语言中的 Python 扩展 - 元类

问题描述

我有以下 python 代码:

I have the following python code:

class Meta(type):
    def __call__(cls, *args, **kwargs):
        obj = type.__call__(cls, *args, **kwargs)
        # Only do checks for subclasses
        if cls.__name__ == 'Parent':
            return obj
        required_attrs = ['x']
        for ra in required_attrs:
            if ra not in dir(obj):
                fmt = 'Subclasses of Parent must define the %s attribute'
                raise NotImplementedError(fmt % ra)
        return obj

class Parent(metaclass=Meta):
    pass

class Child(Parent):
    def __init__(self):
        self.x = True

Meta 仅用于要求 Child 定义某些属性.这个类结构必须保持原样，因为这就是我的项目的结构.Parent 实际上被称为 DefaultConfig 而 Child 实际上是一个从 DefaultConfig 派生的用户定义类.

Meta is used only to require that Child defines certain attributes. This class structure must remain as is because this is how my project is structured. Parent is actually called DefaultConfig and Child is actually a user-defined class derived from DefaultConfig.

我正在将 Meta 和 Parent 翻译成 C 扩展.这是模块:

I'm working on translating Meta and Parent into a C extension. This is the module:

#include <Python.h>
#include <structmember.h>

#define ARRLEN(x) sizeof(x)/sizeof(x[0])


typedef struct {
    PyObject_HEAD
} MetaObject;

typedef struct {
    PyObject_HEAD
} ParentObject;


static PyObject *Meta_call(MetaObject *type, PyObject *args, PyObject *kwargs) {
    PyObject *obj = PyType_GenericNew((PyTypeObject *) type, args, kwargs);

    // Only do checks for subclasses of Parent
    if (strcmp(obj->ob_type->tp_name, "Parent") == 0)
        return obj;

    // Get obj's attributes
    PyObject *obj_dir = PyObject_Dir(obj);
    if (obj_dir == NULL)
        return NULL;

    char *required_attrs[] = {"x"};

    // Raise an exception of obj doesn't define all required_attrs
    PyObject *attr_obj;
    int has_attr;
    for (int i=0; i<ARRLEN(required_attrs); i++) {
        attr_obj = PyUnicode_FromString(required_attrs[i]);
        has_attr = PySequence_Contains(obj_dir, attr_obj);
        if (has_attr == 0) {
            printf("Subclasses of Parent must define %s\n", required_attrs[i]);
            // raise NotImplementedError
            return NULL;
        } else if (has_attr == -1) {
            return NULL;
        }
    }

    return obj;
}


static PyTypeObject MetaType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Meta",
    .tp_basicsize = sizeof(MetaObject),
    .tp_itemsize = 0,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
    .tp_new = PyType_GenericNew,
    .tp_call = (ternaryfunc) Meta_call,
};

static PyTypeObject ParentType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Parent",
    .tp_basicsize = sizeof(ParentObject),
    .tp_itemsize = 0,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
    .tp_new = PyType_GenericNew,
};


static PyModuleDef custommodule = {
    PyModuleDef_HEAD_INIT,
    .m_name = "custom",
    .m_size = -1,
};


PyMODINIT_FUNC PyInit_custom(void) {
    PyObject *module = PyModule_Create(&custommodule);
    if (module == NULL)
        return NULL;

    // Should Parent inherit from Meta?
    ParentType.tp_base = &MetaType;

    if (PyType_Ready(&MetaType) < 0)
        return NULL;
    Py_INCREF(&MetaType);
    PyModule_AddObject(module, "Meta", (PyObject *) &MetaType);

    if (PyType_Ready(&ParentType) < 0)
        return NULL;
    Py_INCREF(&ParentType);
    PyModule_AddObject(module, "Parent", (PyObject *) &ParentType);

    return module;
}

这是用于测试模块custom的python代码:

This is the python code used to test module custom:

import custom

class Child(custom.Parent):
    def __init__(self):
        self.x = True

if __name__ == '__main__':
    c = Child()

不幸的是，PyTypeObject 结构体中没有.tp_meta 成员，那么如何指定Meta 作为 的元类父?

Unfortunately, there is no .tp_meta member in the PyTypeObject struct, so how do I specify Meta as the metaclass of Parent?

编辑:

修改后的 C 代码:

#include <Python.h>
#include <structmember.h>

#define ARRLEN(x) sizeof(x)/sizeof(x[0])


typedef struct {
    PyObject_HEAD
    PyTypeObject base;
} MetaObject;

typedef struct {
    PyObject_HEAD
} ParentObject;


static PyObject *Meta_call(MetaObject *type, PyObject *args, PyObject *kwargs) {
    PyObject *obj = PyType_GenericNew((PyTypeObject *) type, args, kwargs);

    // Only do checks for subclasses of Parent
    if (strcmp(obj->ob_type->tp_name, "Parent") == 0)
        return obj;

    // Get obj's attributes
    PyObject *obj_dir = PyObject_Dir(obj);
    if (obj_dir == NULL)
        return NULL;

    char *required_attrs[] = {"x"};

    // Raise an exception of obj doesn't define all required_attrs
    PyObject *attr_obj;
    int has_attr;
    for (int i=0; i<ARRLEN(required_attrs); i++) {
        attr_obj = PyUnicode_FromString(required_attrs[i]);
        has_attr = PySequence_Contains(obj_dir, attr_obj);
        if (has_attr == 0) {
            printf("Subclasses of Parent must define %s\n", required_attrs[i]);
            // raise NotImplementedError
            return NULL;
        } else if (has_attr == -1) {
            return NULL;
        }
    }

    return obj;
}


static PyTypeObject MetaType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "custom.Meta",
    .tp_basicsize = sizeof(MetaObject),
    .tp_itemsize = 0,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
    .tp_new = PyType_GenericNew,
    .tp_call = (ternaryfunc) Meta_call,
};

static PyTypeObject ParentType = {
    PyVarObject_HEAD_INIT(&MetaType, 0)
    .tp_name = "custom.Parent",
    .tp_basicsize = sizeof(ParentObject),
    .tp_itemsize = 0,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
    .tp_new = PyType_GenericNew,
};


static PyModuleDef custommodule = {
    PyModuleDef_HEAD_INIT,
    .m_name = "custom",
    .m_size = -1,
};


PyMODINIT_FUNC PyInit_custom(void) {
    PyObject *module = PyModule_Create(&custommodule);
    if (module == NULL)
        return NULL;

    MetaType.tp_base = &PyType_Type;
    if (PyType_Ready(&MetaType) < 0)
        return NULL;
    Py_INCREF(&MetaType);
    PyModule_AddObject(module, "Meta", (PyObject *) &MetaType);

    if (PyType_Ready(&ParentType) < 0)
        return NULL;
    Py_INCREF(&ParentType);
    PyModule_AddObject(module, "Parent", (PyObject *) &ParentType);

    return module;
}

推荐答案

元类不过是一种类型，它被用作类型(ob_type!) 的类(类型)...(清楚，不是吗)... ParentType 不是从 MetaType 但它是 `MetaType 的一个实例.

The metaclass is nothing but a type that is used as the type (ob_type!) of the class (type)... (clear, isn't it)... ParentType does not inherit from MetaType but is an instance of `MetaType.

因此，&MetaType应该去的地方是ParentType.ob_type:

PyModule_AddObject(module, "Meta", (PyObject *) &MetaType);

ParentType.ob_type = &MetaType;

if (PyType_Ready(&ParentType) < 0)

PyType_Ready 检查 ob_type 字段 - 如果它是 NULL，则取 ob_type 的 >.tp_base;但是如果 ob_type 已经设置，它会保持原样.

PyType_Ready checks the ob_type field - if it is NULL, it takes the ob_type of the .tp_base; but if ob_type is set already, it is left as is.

其实你可以在ParentType初始化器中设置:

Actually you can set it in the ParentType initializer:

PyVarObject_HEAD_INIT(&MetaType, 0)

第一个参数进入 ob_type 字段.

The first argument goes to the ob_type field.

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