Calling a python method from C/C++, and extracting its return value

Question

I'd like to call a custom function that is defined in a Python module from C. I have some preliminary code to do that, but it just prints the output to stdout.

mytest.py

import math

def myabs(x):
    return math.fabs(x)

test.cpp

#include <Python.h>

int main() {
    Py_Initialize();
    PyRun_SimpleString("import sys; sys.path.append('.')");
    PyRun_SimpleString("import mytest;");
    PyRun_SimpleString("print mytest.myabs(2.0)");
    Py_Finalize();

    return 0;
}

How can I extract the return value into a C double and use it in C?

Answer 1

As explained before, using PyRun_SimpleString seems to be a bad idea.

You should definitely use the methods provided by the C-API (http://docs.python.org/c-api/).

Reading the introduction is the first thing to do to understand the way it works.

First, you have to learn about PyObject that is the basic object for the C API. It can represent any kind of python basic types (string, float, int,...).

Many functions exist to convert for example python string to char* or PyFloat to double.

First, import your module :

PyObject* myModuleString = PyString_FromString((char*)"mytest");
PyObject* myModule = PyImport_Import(myModuleString);

Then getting a reference to your function :

PyObject* myFunction = PyObject_GetAttrString(myModule,(char*)"myabs");
PyObject* args = PyTuple_Pack(1,PyFloat_FromDouble(2.0));

Then getting your result :

PyObject* myResult = PyObject_CallObject(myFunction, args)

And getting back to a double :

double result = PyFloat_AsDouble(myResult);

You should obviously check the errors (cf. link given by Mark Tolonen).

If you have any question, don't hesitate. Good luck.

Answer 2

Here is a sample code I wrote (with the help of various online sources) to send a string to a Python code, then return a value.

Here is the C code call_function.c:

#include <Python.h>
#include <stdlib.h>
int main()
{
   // Set PYTHONPATH TO working directory
   setenv("PYTHONPATH",".",1);

   PyObject *pName, *pModule, *pDict, *pFunc, *pValue, *presult;


   // Initialize the Python Interpreter
   Py_Initialize();


   // Build the name object
   pName = PyString_FromString((char*)"arbName");

   // Load the module object
   pModule = PyImport_Import(pName);


   // pDict is a borrowed reference 
   pDict = PyModule_GetDict(pModule);


   // pFunc is also a borrowed reference 
   pFunc = PyDict_GetItemString(pDict, (char*)"someFunction");

   if (PyCallable_Check(pFunc))
   {
       pValue=Py_BuildValue("(z)",(char*)"something");
       PyErr_Print();
       printf("Let's give this a shot!\n");
       presult=PyObject_CallObject(pFunc,pValue);
       PyErr_Print();
   } else 
   {
       PyErr_Print();
   }
   printf("Result is %d\n",PyInt_AsLong(presult));
   Py_DECREF(pValue);

   // Clean up
   Py_DECREF(pModule);
   Py_DECREF(pName);

   // Finish the Python Interpreter
   Py_Finalize();


    return 0;
}

Here is the Python code, in file arbName.py:

 def someFunction(text):
    print 'You passed this Python program '+text+' from C! Congratulations!'
    return 12345

I use the command gcc call_function.c -I/usr/include/python2.6 -lpython2.6 ; ./a.out to run this process. I'm on redhat. I recommend using PyErr_Print(); for error checking.

Answer 3

A complete example of calling a Python function and retrieving the result is located at http://docs.python.org/release/2.6.5/extending/embedding.html#pure-embedding:

#include <Python.h>

int
main(int argc, char *argv[])
{
    PyObject *pName, *pModule, *pDict, *pFunc;
    PyObject *pArgs, *pValue;
    int i;

    if (argc < 3) {
        fprintf(stderr,"Usage: call pythonfile funcname [args]\n");
        return 1;
    }

    Py_Initialize();
    pName = PyString_FromString(argv[1]);
    /* Error checking of pName left out */

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

    if (pModule != NULL) {
        pFunc = PyObject_GetAttrString(pModule, argv[2]);
        /* pFunc is a new reference */

        if (pFunc && PyCallable_Check(pFunc)) {
            pArgs = PyTuple_New(argc - 3);
            for (i = 0; i < argc - 3; ++i) {
                pValue = PyInt_FromLong(atoi(argv[i + 3]));
                if (!pValue) {
                    Py_DECREF(pArgs);
                    Py_DECREF(pModule);
                    fprintf(stderr, "Cannot convert argument\n");
                    return 1;
                }
                /* pValue reference stolen here: */
                PyTuple_SetItem(pArgs, i, pValue);
            }
            pValue = PyObject_CallObject(pFunc, pArgs);
            Py_DECREF(pArgs);
            if (pValue != NULL) {
                printf("Result of call: %ld\n", PyInt_AsLong(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", argv[2]);
        }
        Py_XDECREF(pFunc);
        Py_DECREF(pModule);
    }
    else {
        PyErr_Print();
        fprintf(stderr, "Failed to load \"%s\"\n", argv[1]);
        return 1;
    }
    Py_Finalize();
    return 0;
}

Answer 4

To prevent the extra .py file as in the other answers, you can just retrieve the __main__ module, which is created by the first call to PyRun_SimpleString:

PyObject *moduleMainString = PyString_FromString("__main__");
PyObject *moduleMain = PyImport_Import(moduleMainString);

PyRun_SimpleString(
    "def mul(a, b):                                 \n"\
    "   return a * b                                \n"\
);

PyObject *func = PyObject_GetAttrString(moduleMain, "mul");
PyObject *args = PyTuple_Pack(2, PyFloat_FromDouble(3.0), PyFloat_FromDouble(4.0));

PyObject *result = PyObject_CallObject(func, args);

printf("mul(3,4): %.2f\n", PyFloat_AsDouble(result)); // 12

Answer 5

As others have already mentioned, this is answered at the Python docs. However, since I come from Python and don't have that much experience using C/C++, I had some issues running it with Python 3. So here is my full working example to get the Python docs running after spending some time on other posts of stackoverflow:

File c_function.c

#define PY_SSIZE_T_CLEAN
#include <Python.h>

int main(int argc, char *argv[])
{
    PyObject *pName, *pModule, *pFunc;
    PyObject *pArgs, *pValue;
    int i;

    if (argc < 3) {
        fprintf(stderr,"Usage: call pythonfile funcname [args]\n");
        return 1;
    }

    Py_Initialize();

    // I had to add the following two lines to make it work
    PyRun_SimpleString("import sys");
    PyRun_SimpleString("sys.path.append(\".\")");

    pName = PyUnicode_DecodeFSDefault(argv[1]);
    /* Error checking of pName left out */

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

    if (pModule != NULL) {
        pFunc = PyObject_GetAttrString(pModule, argv[2]);
        /* pFunc is a new reference */

        if (pFunc && PyCallable_Check(pFunc)) {
            pArgs = PyTuple_New(argc - 3);
            for (i = 0; i < argc - 3; ++i) {
                pValue = PyLong_FromLong(atoi(argv[i + 3]));
                if (!pValue) {
                    Py_DECREF(pArgs);
                    Py_DECREF(pModule);
                    fprintf(stderr, "Cannot convert argument\n");
                    return 1;
                }
                /* pValue reference stolen here: */
                PyTuple_SetItem(pArgs, i, pValue);
            }
            pValue = PyObject_CallObject(pFunc, pArgs);
            Py_DECREF(pArgs);
            if (pValue != NULL) {
                printf("Result of call: %ld\n", PyLong_AsLong(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", argv[2]);
        }
        Py_XDECREF(pFunc);
        Py_DECREF(pModule);
    }
    else {
        PyErr_Print();
        fprintf(stderr, "Failed to load \"%s\"\n", argv[1]);
        return 1;
    }
    if (Py_FinalizeEx() < 0) {
        return 120;
    }
    return 0;
}

File multiply.py

def multiply(a,b):
    print("Will compute", a, "times", b)
    c = 0
    for i in range(0, a):
        c = c + b
    return c

We need to compile and link it. This can be done with the command:

gcc c_function.c -c $(python3.6-config --cflags) -fPIC

followed by

gcc c_function.o $(python3.6-config --ldflags) -o call

for the example of Python 3.6. Afterwards, the example from the Python docs can just be executed by

./call multiply multiply 3 2

Answer 6

I have done it using BOOST to embedded Python to C++ [This working C module should help]

#include <boost/python.hpp>

void main()
{
using namespace boost::python;
 Py_Initialize();
 PyObject* filename = PyString_FromString((char*)"memory_leak_test");
     PyObject* imp = PyImport_Import(filename);
     PyObject* func = PyObject_GetAttrString(imp,(char*)"begin");
     PyObject* args = PyTuple_Pack(1,PyString_FromString("CacheSetup"));
     PyObject* retured_value = PyObject_CallObject(func, args); // if you have arg
     double retured_value = PyFloat_AsDouble(myResult);
 std::cout << result << std::endl;
 Py_Finalize();
}

Answer 7

You have to extract the python method somehow and run it with PyObject_CallObject(). To do that, you can provide Python a way to set the function, as the Extending and Embedding Python Tutorial example does.

Answer 8

If you assign the return value to a variable, then you can use something like PyEval_GetGlobals() and PyDict_GetItemString() to get the PyObject. From there, PyNumber_Float can get you the value you want.

I suggest browsing through the whole API - certain things become obvious when you see the different methods that are available to you, and there may well be a better method than the one I've described.

Answer 9

Here is a simple and direct answer to your question:

    #include <iostream>
    #include <Python.h>
    using namespace std;
    int main()
    {
    const char *scriptDirectoryName = "/yourDir";
    Py_Initialize();
    PyObject *sysPath = PySys_GetObject("path");
    PyObject *path = PyString_FromString(scriptDirectoryName);
    int result = PyList_Insert(sysPath, 0, path);
    PyObject *pModule = PyImport_ImportModule("mytest");

    PyObject* myFunction = PyObject_GetAttrString(pModule,(char*)"myabs");
    PyObject* args = PyTuple_Pack(1,PyFloat_FromDouble(-2.0));


    PyObject* myResult = PyObject_CallObject(myFunction, args);
    double getResult = PyFloat_AsDouble(myResult);
    return 0;
    }

Answer 10

Here's a minimal executable version that also works with Python 3 (tested with Python 2.7 and 3.9).

Links into the docs are included in the comments, but all are accessible under https://docs.python.org/3/c-api/

#include <Python.h>
#include <stdio.h>

int main()
{
    // Initialize the Python Interpreter
    Py_Initialize();

    // see https://docs.python.org/3/c-api/structures.html
    // NULL objects are special and Py_CLEAR knows this
    PyObject *module = NULL, *result = NULL;

    // https://docs.python.org/3/c-api/import.html
    module = PyImport_ImportModule("mytest");
    if (!module) {
        // Python generally uses exceptions to indicate an error state which
        // gets flagged in the C-API (a NULL pointer in this case) indicating
        // "something" failed. the PyErr_* API should be used to get more
        // details
        goto done;
    }

    // see https://docs.python.org/3/c-api/call.html#c.PyObject_CallMethod
    // and https://docs.python.org/3/c-api/arg.html#building-values
    result = PyObject_CallMethod(module, "myabs", "f", 3.14);
    if (!result) {
        goto done;
    }

    // make sure we got our number back
    if (PyFloat_Check(result)) {
        printf("Successfully got a float: %f\n", PyFloat_AsDouble(result));
    } else {
        printf("Successfully got something unexpected!\n");
    }

  done:
    // see https://docs.python.org/3/c-api/exceptions.html
    PyErr_Print();

    // see https://docs.python.org/3/c-api/refcounting.html
    Py_CLEAR(result);
    Py_CLEAR(module);

    // Optionally release Python Interpreter
    Py_Finalize();

    return 0;
}

This uses the OP's Python code mytest.py, or this one-line equivalent:

from math import fabs as myabs

Building is going to be OS/Python version specific, but the following works for me:

cc -o test -I/usr/include/python3.9 /usr/lib/libpython3.9.so test.c