C++ Python import class; call methods

Question

I am using Python 2.7. It is not clear to me how to embed Python inside C++ as found here: http://docs.python.org/2.7/extending/embedding.html.

I have a simple python example here in a file named test.py:

class math:
    #def __init__(self):
    def add(self, num1, num2):
        return num1 + num2

    def subtract(self, num1, num2):
        return num1 - num2

From python, I would do something like this:

>>> from test import math
>>> m = math()
>>> a = m.add(1, 2)
>>> s = m.subtract(1, 2)

I have the beginning of some C++ code to do this:

PyObject *pName, *pModule;
Py_Initialize();
pName = PyString_FromString("test");
pModule = PyImport_Import(pName);

That seems to work just fine. But, that seems to be the equivalent of doing this in Python:

import test

How do I import the Python class math?

Thanks

Answer 1

Here's a quick n' dirty example in C which does the equivalent of...

>>> import mymath
>>> m = mymath.math()
>>> print '1 + 2 = %d' % m.add(1, 2)

Note that I've renamed the module from test to mymath because there's a module called test in the standard Python library.

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <python2.7/Python.h>

int main()
{
    setenv("PYTHONPATH", ".", 1);

    Py_Initialize();

    PyObject* module = PyImport_ImportModule("mymath");
    assert(module != NULL);

    PyObject* klass = PyObject_GetAttrString(module, "math");
    assert(klass != NULL);

    PyObject* instance = PyInstance_New(klass, NULL, NULL);
    assert(instance != NULL);

    PyObject* result = PyObject_CallMethod(instance, "add", "(ii)", 1, 2);
    assert(result != NULL);

    printf("1 + 2 = %ld\n", PyInt_AsLong(result));

    Py_Finalize();

    return 0;
}

...which outputs...

$ gcc foo.c -lpython2.7 && ./a.out
1 + 2 = 3

However, if you do any major amount of work with the Python/C API between Py_Initialize and Py_Finalize, you'll have to watch your reference counts, and use Py_INCREF and Py_DECREF when appropriate.

Answer 2

You can't import a class, but you can import a file or library (as is true with most languages?) - which is just what you did here in the case of test.py.