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python - 如何从 C 模块中实例化自定义对象？

问题描述

我在创建从 C 模块中编写的类（类型）的实例时遇到问题。我写了一个最小的、独立的例子来说明这一点。只需将 spam.c、spamtest.py 和 setup.py 这三个文件复制粘贴到一个目录并运行

$ python setup.py develop && python spamtest.py

我不明白为什么在实例化垃圾邮件实例时不调用基本函数 new() 和 init() 。不用说，这会在实际应用程序中导致大量时间段错误，其中这些函数为新创建的对象分配动态内存。

以下是在 Python 的调试版本下运行 spamtest.py 时发生的情况。注意 new() 和 init() 在从 Python 解释器中实例化垃圾邮件对象时被调用，而不是从 C 中。

(pyenv36d) $ python spamtest.py 
---------
From Python
New Spam at 0x7fa7a5ca9280
Init Spam at 0x7fa7a5ca9280
Finalize Spam at 0x7fa7a5ca9280
---------
From C
Finalize Spam at 0x7fa7a5ca92c0
---------
* ob
object  : <refcnt 0 at 0x7fa7a5bb6670>
type    : bytes
refcount: 0
address : 0x7fa7a5bb6670
* op->_ob_prev->_ob_next
<NULL object>
* op->_ob_next->_ob_prev
object  : <refcnt 0 at 0x7fa7a5bb6670>
type    : bytes
refcount: 0
address : 0x7fa7a5bb6670
Fatal Python error: UNREF invalid object

Current thread 0x00007fa7a5ff8080 (most recent call first):
Aborted
(pyenv36d) $

模块：

#include <Python.h>

typedef struct {
     PyObject_HEAD
} Spam;

static PyObject *new(PyTypeObject *type,
         PyObject *args, PyObject *kw) {
     Spam *self;

     self = (Spam *) type->tp_alloc(type, 0);
     fprintf(stderr, "New Spam at %p\n", self);
     return (PyObject*)self;
}

static int init(Spam *self, PyObject *args, PyObject *kw) {
     fprintf(stderr, "Init Spam at %p\n", self);
     return 0;
}

static void finalize(PyObject *self) {
     fprintf(stderr, "Finalize Spam at %p\n", self);
}

static PyMethodDef spam_methods[] = {
     {NULL, NULL, 0, NULL},
};

static PyTypeObject spam_type = {
     PyVarObject_HEAD_INIT(NULL, 0)
     .tp_name = "Spam",
     .tp_basicsize = sizeof(Spam),
     .tp_flags = 0
         | Py_TPFLAGS_DEFAULT
         | Py_TPFLAGS_BASETYPE,
     .tp_doc = "Spam object",
     .tp_methods = spam_methods,
     .tp_new = new,
     .tp_init = (initproc) init,
     .tp_dealloc = finalize,
};

/* To create a new Spam object directly from C */
PyObject *make_spam() {
     Spam *spam;
     if (PyType_Ready(&spam_type) != 0) {
         Py_RETURN_NONE;
     }
     spam = PyObject_New(Spam, &spam_type);
     PyObject_Init((PyObject *)spam, &spam_type);
     return (PyObject *) spam;
}

static PyMethodDef module_methods[] = {
     {"make_spam",  (PyCFunction)make_spam, METH_NOARGS,
      "Instantiate and return a new Spam object."},
     {NULL, NULL, 0, NULL}
};

static PyModuleDef spam_module = {
     PyModuleDef_HEAD_INIT,
     "spam",
     "Defines the Spam (time, value) class"
     ,
     -1,
     module_methods
};

PyMODINIT_FUNC PyInit_spam(void) {
     PyObject *m;
     m = PyModule_Create(&spam_module);
     if (PyType_Ready(&spam_type) < 0) {
         return NULL;
     }
     PyModule_AddObject(m, "Spam", (PyObject*)&spam_type);
     return m;
}

安装程序.py

from setuptools import setup, Extension

spam = Extension('spam', sources=['spam.c'])

setup (
    name = 'spam',
    version = '0.1',
    description = 'Trying to instantiate an object from C',
    ext_modules = [spam],
    packages = [],
)

垃圾邮件测试.py：

from spam import Spam, make_spam

print("---------\nFrom Python")
s1 = Spam()
del s1

print("---------\nFrom C")
s2 = make_spam()
del s2
print("---------")

标签： pythonpython-3.xpython-extensions

解决方案

一个答案是通过调用类型对象来构造你的新对象，就像你从 Python 本身一样。

也就是说，更改make_spam()为执行以下操作：

/* To create a new Spam object directly from C */
PyObject *make_spam() {
     Spam *spam;
     if (PyType_Ready(&spam_type) != 0) {
         Py_RETURN_NONE;
     }
     spam = (Spam *)PyObject_CallObject((PyObject *)&spam_type, Py_BuildValue(""));
     /* your other init here, assuming you have some */
     return (PyObject *) spam;
}

这也让我很困惑。我想知道为什么您的原始代码不起作用，以及是否有更好的方法可以“正确”地执行此操作，但这种方法对我有用。

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