Calling sympy using C++ embedding

Question

I am trying to use sympy with my c++ program and am using the following embedding code to do that.

int sym_ex(const char * input_expression, const char * output_expression){

    PyObject *pName, *pModule, *pDict, *pFunc;
    PyObject *pArgs, *pValue;
    int i;

    const char * file = "sympy";
    const char * function = "simplify";


    Py_Initialize();
    //PyRun_SimpleString("from sympy import *\n");
    pName = PyString_FromString(file);
    /* Error checking of pName left out */

    pModule = PyImport_Import(pName);
    Py_DECREF(pName);

    if (pModule != NULL) {
        pFunc = PyObject_GetAttrString(pModule, function);
        /* pFunc is a new reference */

        if (pFunc && PyCallable_Check(pFunc)) {

            pArgs = PyTuple_New(1);
            pValue = PyString_FromString(input_expression);
            printf("the string passed %s\n", input_expression);
            if (!pValue) {
                Py_DECREF(pArgs);
                Py_DECREF(pModule);
                fprintf(stderr, "Cannot convert argument\n");
                return 1;
            }
            /* pValue reference stolen here: */
            PyTuple_SetItem(pArgs, 0, pValue);

            pValue = PyObject_CallObject(pFunc, pArgs);
            Py_DECREF(pArgs);
            if (pValue != NULL) {
                printf("Result of call: %s\n", PyString_AsString(pValue));
                Py_DECREF(pValue);
            }
            else {
                Py_DECREF(pFunc);
                Py_DECREF(pModule);
                PyErr_Print();
                fprintf(stderr, "Call failed\n");
                return 1;
            }
        }
        else {
            if (PyErr_Occurred())
                PyErr_Print();
            fprintf(stderr, "Cannot find function \"%s\"\n", function);
        }
        Py_XDECREF(pFunc);
        Py_DECREF(pModule);
    }
    else {
        PyErr_Print();
        fprintf(stderr, "Failed to load \"%s\"\n", file);
        return 1;
    }
    Py_Finalize();
    return 0;

}

But, when I write a small program using this function and run it,

#include <stdio>
#include <iostream>

int main(){

char * input_expression = "2 + 2";
char * output_expression = new char[250];

sym_ex(input_expression, output_expression);

return 0;
}

It gives out the following type error. I am running on windows in Visual C++, Python 2.7.8

 Exception TypeError: 'expected string or Unicode object, Integer found' in <module 'threading' from 'C:\Python278\Lib\threading.pyc'> ignored

Can somebody help me? Thanks

What is the greater goal? It's unclear to me why you would need to do this. Is it for speed? Then CSymPy might be what you want... — Bjoern Dahlgren, Oct 03 '14 at 21:37
Does CSymPy compile under Visual C++ (microsoft's compiler)? — Charith, Oct 05 '14 at 13:28
I don't know. It looks like they use CMake so chances are fairly good it does: https://github.com/sympy/csympy — Bjoern Dahlgren, Oct 06 '14 at 10:56

score 0 · Answer 1 · answered Oct 26 '14 at 09:51

The problem here is the call to String_AsString(pValue)

printf("Result of call: %s\n", PyString_AsString(pValue));

The result of sympy.simplify("2+2") is 4, which is an integer and not a string. The result is that Python raises an TypeError exception.

If you want a string as the result you should probably first call PyObject_Str(pValue) and then convert it into a C string.

Calling sympy using C++ embedding

1 Answers1