从扩展模块中定义和导出C的API

/* Destructor function for points */ static void del_Point(PyObject *obj) {

free(PyCapsule_GetPointer(obj,"Point")); }

/* Utility functions */ static Point *PyPoint_AsPoint(PyObject *obj) { return (Point *) PyCapsule_GetPointer(obj, "Point"); }

static PyObject *PyPoint_FromPoint(Point *p, int must_free) { return PyCapsule_New(p, "Point", must_free ? del_Point : NULL); }

本节主要问题是如何处理15.4小节中提到的Point对象。仔细回一下，在C代码中包含了如下这些工具函数：

/* Destructor function for points */
static void del_Point(PyObject *obj) {

  free(PyCapsule_GetPointer(obj,"Point"));
}

/* Utility functions */
static Point *PyPoint_AsPoint(PyObject *obj) {
  return (Point *) PyCapsule_GetPointer(obj, "Point");
}

static PyObject *PyPoint_FromPoint(Point *p, int must_free) {
  return PyCapsule_New(p, "Point", must_free ? del_Point : NULL);
}
现在的问题是怎样将 PyPoint_AsPoint() 和 Point_FromPoint() 函数作为API导出， 这样其他扩展模块能使用并链接它们，比如如果你有其他扩展也想使用包装的Point对象。

要解决这个问题，首先要为 sample 扩展写个新的头文件名叫 pysample.h ，如下：

/* pysample.h */
#include "Python.h"
#include "sample.h"
#ifdef __cplusplus
extern "C" {
#endif

/* Public API Table */
typedef struct {
  Point *(*aspoint)(PyObject *);
  PyObject *(*frompoint)(Point *, int);
} _PointAPIMethods;

#ifndef PYSAMPLE_MODULE
/* Method table in external module */
static _PointAPIMethods *_point_api = 0;

/* Import the API table from sample */
static int import_sample(void) {
  _point_api = (_PointAPIMethods *) PyCapsule_Import("sample._point_api",0);
  return (_point_api != NULL) ? 1 : 0;
}

/* Macros to implement the programming interface */
#define PyPoint_AsPoint(obj) (_point_api->aspoint)(obj)
#define PyPoint_FromPoint(obj) (_point_api->frompoint)(obj)
#endif

#ifdef __cplusplus
}
#endif
这里最重要的部分是函数指针表 _PointAPIMethods . 它会在导出模块时被初始化，然后导入模块时被查找到。 修改原始的扩展模块来填充表格并将它像下面这样导出：

/* pysample.c */

#include "Python.h"
#define PYSAMPLE_MODULE
#include "pysample.h"

...
/* Destructor function for points */
static void del_Point(PyObject *obj) {
  printf("Deleting point\n");
  free(PyCapsule_GetPointer(obj,"Point"));
}

/* Utility functions */
static Point *PyPoint_AsPoint(PyObject *obj) {
  return (Point *) PyCapsule_GetPointer(obj, "Point");
}

static PyObject *PyPoint_FromPoint(Point *p, int free) {
  return PyCapsule_New(p, "Point", free ? del_Point : NULL);
}

static _PointAPIMethods _point_api = {
  PyPoint_AsPoint,
  PyPoint_FromPoint
};
...

/* Module initialization function */
PyMODINIT_FUNC
PyInit_sample(void) {
  PyObject *m;
  PyObject *py_point_api;

  m = PyModule_Create(&samplemodule);
  if (m == NULL)
    return NULL;

  /* Add the Point C API functions */
  py_point_api = PyCapsule_New((void *) &_point_api, "sample._point_api", NULL);
  if (py_point_api) {
    PyModule_AddObject(m, "_point_api", py_point_api);
  }
  return m;
}
最后，下面是一个新的扩展模块例子，用来加载并使用这些API函数：

/* ptexample.c */

/* Include the header associated with the other module */
#include "pysample.h"

/* An extension function that uses the exported API */
static PyObject *print_point(PyObject *self, PyObject *args) {
  PyObject *obj;
  Point *p;
  if (!PyArg_ParseTuple(args,"O", &obj)) {
    return NULL;
  }

  /* Note: This is defined in a different module */
  p = PyPoint_AsPoint(obj);
  if (!p) {
    return NULL;
  }
  printf("%f %f\n", p->x, p->y);
  return Py_BuildValue("");
}

static PyMethodDef PtExampleMethods[] = {
  {"print_point", print_point, METH_VARARGS, "output a point"},
  { NULL, NULL, 0, NULL}
};

static struct PyModuleDef ptexamplemodule = {
  PyModuleDef_HEAD_INIT,
  "ptexample",           /* name of module */
  "A module that imports an API",  /* Doc string (may be NULL) */
  -1,                 /* Size of per-interpreter state or -1 */
  PtExampleMethods       /* Method table */
};

/* Module initialization function */
PyMODINIT_FUNC
PyInit_ptexample(void) {
  PyObject *m;

  m = PyModule_Create(&ptexamplemodule);
  if (m == NULL)
    return NULL;

  /* Import sample, loading its API functions */
  if (!import_sample()) {
    return NULL;
  }

  return m;
}
编译这个新模块时，你甚至不需要去考虑怎样将函数库或代码跟其他模块链接起来。 例如，你可以像下面这样创建一个简单的 setup.py 文件：

# setup.py
from distutils.core import setup, Extension

setup(name='ptexample',
      ext_modules=[
        Extension('ptexample',
                  ['ptexample.c'],
                  include_dirs = [],  # May need pysample.h directory
                  )
        ]
)
如果一切正常，你会发现你的新扩展函数能和定义在其他模块中的C API函数一起运行的很好。

>>> import sample
>>> p1 = sample.Point(2,3)
>>> p1
<capsule object "Point *" at 0x1004ea330>
>>> import ptexample
>>> ptexample.print_point(p1)
2.000000 3.000000
>>>