5.4 Extending Embedded Python

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5.4 Extending Embedded Python

Until now, the embedded Python interpreter had no access to functionality from the application itself. The Python API allows this by extending the embedded interpreter. That is, the embedded interpreter gets extended with routines provided by the application. While it sounds complex, it is not so bad. Simply forget for a while that the application starts the Python interpreter. Instead, consider the application to be a set of subroutines, and write some glue code that gives Python access to those routines, just like you would write a normal Python extension. For example:

    static int numargs=0;

    /* Return the number of arguments of the application command line */
    static PyObject*
    emb_numargs(PyObject *self, PyObject *args)
    {
        if(!PyArg_ParseTuple(args, ":numargs"))
            return NULL;
        return Py_BuildValue("i", numargs);
    }

    static PyMethodDef EmbMethods[] = {
        {"numargs", emb_numargs, METH_VARARGS,
         "Return the number of arguments received by the process."},
        {NULL, NULL, 0, NULL}
    };

Insert the above code just above the `main()` function. Also, insert the following two statements directly after `Py_Initialize()`:

        numargs = argc;
        Py_InitModule("emb", EmbMethods);

These two lines initialize the `numargs` variable, and make the `emb.numargs()` function accessible to the embedded Python interpreter. With these extensions, the Python script can do things like

    import emb
    print "Number of arguments", emb.numargs()

In a real application, the methods will expose an API of the application to Python.