Programming Python, 3rd Edition

A Simple C Extension Module

At least that’s the short story; we need to turn to some code to make this more concrete. C types generally export a C module with a constructor function. Because of that, and because they are simpler, let’s start off by studying the basics of C module coding with a quick example.

As mentioned, when you add new or existing C components to Python, you need to code an interface (“glue”) logic layer in C that handles cross-language dispatching and data translation. The C source file in Example 22-1 shows how to code one by hand. It implements a simple C extension module named hello for use in Python scripts, with a function named message that simply returns its input string argument with extra text prepended. Python scripts will call this function as usual, but this one is coded in C, not in Python.

Example 22-1. PP3E\Integrate\Extend\Hello\hello.c

/******************************************************************** * A simple C extension module for Python, called "hello"; compile * this into a ".so" on python path, import and call hello.message; ********************************************************************/ #include <Python.h> #include <string.h> /* module functions */ static PyObject * /* returns object */ message(PyObject *self, PyObject *args) /* self unused in modules */ { /* args from Python call */ char *fromPython, result[64]; if (! PyArg_Parse(args, "(s)", &fromPython)) /* convert Python -> C */ return NULL; /* null=raise exception */ else ...