python - C++ to numpy and back to C++, tuple PyObject -

i implementing visual c++ code (ms vstudio 2013) acquires image camera, processes image, sends pre-processed data (an array 2 or more doubles) python through pyobject, , gets numpy-processed results python (a tuple of 3 doubles in py, becomes array of 3 doubles in c++). tested py function standalone script. however, keep getting null return pyobject_callobject(). python , vc++ project reside in same directory.

i have set log file record in py function document calls c, know in cases c++ calls not reach py function, , in other cases py function called, but, perhaps, returned values cannot processed. specifically, found can call py function (positive log record) when tuple contains 1 object, cannot call (no log file records) when tuple contains 2 or more objects. in addition, if py function needs no arguments, , not need return anything, calls successful.

i have googled , read having using tuples communicate between c++ , py, still did not make work. have read documentation , examples, still no luck.

i have stripped code bare minimum below , reproduced problem. below example when pass 2 zeros input, , expect 3 zeros back. grateful leads on this...

//c++ part     py_initialize();         pyname = pyimport_importmodulenoblock(_t("find_center_def"));     if (pyname == null) {                return false;     }     pyfunc = pyobject_getattrstring(pyname, _t("find_center"));     if (pyfunc == null) {         return false;     }     py_decref(pyname);     if (pycallable_check(pyfunc) == 0) {         return false;     }     pyobject * arg = pytuple_new(2);     pyobject * datum0 = pyfloat_fromdouble(0.0);     pyobject * datum1 = pyfloat_fromdouble(0.0);     if (pytuple_setitem(arg, 0, datum0) != 0) {         return false;         }     if (pytuple_setitem(arg, 1, datum1) != 0) {                  return false;         }     pyobject * result = pyobject_callobject(pyfunc, arg);     if (result == null) {         return false;     }     outputdebugstring(_t("\nsuccess calling find_center function!"));  #python part, inside of script called find_center_def.py def find_center(args):     import numpy             fo=open('foo.txt' ,'w')     fo.write('find_center called c++!')     fo.close()     return (0,0,0)  #python output: python 3.4.3 (v3.4.3:9b73f1c3e601, feb 24 2015, 22:44:40) [msc v.1600 64 bit (amd64)] on win32 >>> import importlib >>> test = importlib.import_module("find_center_def") >>> test.find_center((0,0)) (0, 0, 0) >>> 

you're able add single value tuple because

pyobject* pyobject_callobject(pyobject *callable_object, pyobject *args) call callable python object callable_object, with arguments given tuple args

(emphasis mine). in other words, second argument pyobject_callobject tuple of arguments. function takes single argument, length of tuple has 1.

since you're trying pass tuple 1 argument, need two-element tuple first element of single-element tuple pass second argument pyobject_callobject.

another option use pyobject_callfunction instead, , let build argument tuple.