Programming Python, 3rd Edition

Wrapping C Environment Calls

Let’s move on to a more useful application of C extension modules. The handcoded C file in Example 22-9 integrates the standard C library’s getenv and putenv shell environment variable calls for use in Python scripts.

Example 22-9. PP3E\Integrate\Extend\CEnviron\cenviron.c

/****************************************************************** * A C extension module for Python, called "cenviron". Wraps the * C library's getenv/putenv routines for use in Python programs. ******************************************************************/ #include <Python.h> #include <stdlib.h> #include <string.h> /***********************/ /* 1) module functions */ /***********************/ static PyObject * /* returns object */ wrap_getenv(PyObject *self, PyObject *args) /* self not used */ { /* args from python */ char *varName, *varValue; PyObject *returnObj = NULL; /* null=exception */ if (PyArg_Parse(args, "s", &varName)) { /* Python -> C */ varValue = getenv(varName); /* call C getenv */ if (varValue != NULL) returnObj = Py_BuildValue("s", varValue); /* C -> Python */ else PyErr_SetString(PyExc_SystemError, "Error calling getenv"); } return returnObj; } static PyObject * wrap_putenv(PyObject *self, PyObject *args) { char *varName, *varValue, *varAssign; PyObject *returnObj = NULL; if (PyArg_Parse(args, "(ss)", &varName, &varValue)) { varAssign = malloc(strlen(varName) + strlen(varValue) + 2); sprintf(varAssign, "%s=%s", varName, varValue); if (putenv(varAssign) == 0) ...