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I'm trying to make compile a c++ code into an extension for Python but i'm having some troubles assigning values into the C++ code and also reading them into Python.

I have created methods to assign and read values that looks like this:

import hello

if __name__ == "__main__":

    # GET THE VALUE OF 'a'
    a = hello.get_a()
    print('the value of \'a\' is:',a)   

    # SET A VALUE FOR 'a'
    hello.set_a(5.0)

    # GET THE VALUE OF 'a'
    a = hello.get_a()
    print('the value of \'a\' is:',a)

that generates:

the value of 'a' is: 1.0
the value of 'a' is: 5.0

What I want to do is do it in a simpler way, like binding an attribute of Python with the value of the C++ variable:

import hello

if __name__ == "__main__":

    # GET THE VALUE OF 'a'
    print('the value of \'a\' is:',hello.a) 

    # SET A VALUE FOR 'a'
    hello.a = 5.0

    # GET THE VALUE OF 'a'
    print('the value of \'a\' is:',hello.a)

An example of my wrapper: 

#include <Python.h>

/* EXAMPLE OF A CLASS WITH VALUE */
class example {
    public:
        float a;
        example(float a_init);    
};

example::example(float a_init){
    this->a = a_init;
}

example ex(1.0);

/* METHODS CALLS */
static PyObject* get_a(PyObject *self, PyObject *args);
static PyObject* set_a(PyObject *self, PyObject *args);

static PyMethodDef hello_methods[] = {
        {"get_a", get_a, METH_VARARGS,"numpy function tester",},
        {"set_a", set_a, METH_VARARGS,"numpy function tester",},
        {NULL, NULL, 0, NULL}
};

/* MODULE */
static struct PyModuleDef hello_definition = {
        PyModuleDef_HEAD_INIT,
        "hello",
        "A Python module that prints 'hello world' from C code.",
        -1,
        hello_methods
};

/* INIT MODULE */
PyMODINIT_FUNC PyInit_hello(void) {
    Py_Initialize();
    return PyModule_Create(&hello_definition);
}

// METHODS
static PyObject* get_a(PyObject *self, PyObject *args) {
    /* RETURNING VALUE OF 'a' */
    return Py_BuildValue("f", ex.a);;
}

static PyObject* set_a(PyObject *self, PyObject *args) {

    float value;
    if (!PyArg_ParseTuple(args, "f", &value))
        return NULL;

    /* ASSIGNING VALUE OF 'a' */
    ex.a = value;
    Py_RETURN_NONE;
}

Is it possible to do it?

Daniel Vieira
  • 1
  • It looks like you're trying to bind a C++ global variable to be used in Python. Can you try this? https://stackoverflow.com/questions/3001239/define-a-global-in-a-python-module-from-a-c-api – John Zwinck Sep 29 '19 at 11:03
  • that indeed creates an attribute for the module, but how do I link a value of a C++ variable to the attribute of the module? – Daniel Vieira Sep 29 '19 at 12:26

0 Answers0