Python Cookbook

Implementing a Simple Extension Type

Credit: Alex Martelli

Problem

You want to code and build a C extension type for Python with a minimal amount of hard work.

Solution

First of all, we need to create a setup.py using the distutils package (in Python 2.0 and later) to build and install our module:

from distutils.core import setup, Extension

setup(name = "elemlist",
      version = "1.0",
      maintainer = "Alex Martelli",
      maintainer_email = "[e-mail]",
      description = "Sample, simple Python extension module",

      ext_modules = [Extension('elemlist',sources=['elemlist.c'])]
)

Then we need an elemlist.c file with our module’s source code:

#include "Python.h" /* type-definition and utility-macros */ typedef struct { PyObject_HEAD PyObject *car, *cdr; } cons_cell; staticforward PyTypeObject cons_type; /* a type-testing macro (we don't actually use it here) */ #define is_cons(v) ((v)->ob_type == &cons_type) /* utility macros to access car and cdr, both as lvalues and rvalues */ #define carof(v) (((cons_cell*)(v))->car) #define cdrof(v) (((cons_cell*)(v))->cdr) /* ctor ("internal" factory function) and dtor */ static cons_cell* cons_new(PyObject *car, PyObject *cdr) { cons_cell *cons = PyObject_NEW(cons_cell, &cons_type); if(cons) { cons->car = car; Py_INCREF(car); /* INCREF when holding a PyObject */ cons->cdr = cdr; Py_INCREF(cdr); /* ditto */ } return cons; } static void cons_dealloc(cons_cell* cons) { /* DECREF when releasing previously held PyObject*'s */ Py_DECREF(carof(cons)); Py_DECREF(cdrof(cons)); ...