C 中的 Python 扩展 - 元类

Question

我有以下 python 代码:

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 实际上称为 DefaultConfig，而 Child 实际上是从 DefaultConfig 派生的用户定义类。

我正在将 Meta 和 Parent 转换为 C 扩展。这是模块:

#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:

import custom

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

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

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

<小时/>

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;
}

Answer 1

元类不过是用作类(类型)的类型( ob_type !)的类型。 ..(很清楚，不是吗)... ParentType不继承自MetaType但是是`MetaType 的一个实例。

因此，&MetaType的地方应该去(如果有的话)，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 - 如果是 NULL ，需要ob_type .tp_base的;但如果ob_type已设置，保持原样。

其实可以在 ParentType 中设置初始化器:

PyVarObject_HEAD_INIT(&MetaType, 0)

第一个参数指向 ob_type字段。