calling a method from a python interpreter

Question

per Voo's advice on this thread:

How can I "hook into" Python from C++ when it executes a function? My goal is to profile

I have opened up a new thread for a new question, which is, in C++, how can I initialize a PythonInterpreter and then call a method from it. Specifically, I'd like to be able to call cProfile's methods and also get data from it.

Answer 1

Okay that'll be a bit longer. Note that I pretty much ignore all "usual" error checking - pretty much any python method may return NULL in which case you should handle that gracefully. I show the "unusual" part checking if the given object is callable. Note that PyDECREF fails if the object pointer is NULL, Py_XDECREF does not. And now to the code - there may be a better way to solve all this, but this works fine for me and sadly the documentation is extremely lacking.

C++ code:

#include <Python.h>

static PyThreadState *mainstate;

void initPython(){
    PyEval_InitThreads();
    Py_Initialize();
    mainstate = PyThreadState_Swap(NULL);
    PyEval_ReleaseLock();
}

void exitPython(){
    PyEval_AcquireLock();
    PyThreadState_Swap(mainstate);
    Py_Finalize(); 
}

void callScript() {
    PyGILState_STATE gstate = PyGILState_Ensure();
    PyObject *pName = PyUnicode_FromString("Startup"); 
    PyObject *pModule = PyImport_Import(pName);
    Py_DECREF(pName);
    PyObject *pFunc = PyObject_GetAttrString(pModule, "startup");
    if (pFunc && PyCallable_Check(pFunc)) {
        PyObject *arglist = Py_BuildValue("(u)", "TestScript");
        PyObject *result = PyObject_CallObject(pFunc, arglist);
        Py_DECREF(arglist);
        // Now you have the returned object of the function - do something with it.
        // In our case that's None, but you should extend the python scrip to return
        // whatever you need - see the profiler API.
        Py_DECREF(result);          
    }
    Py_XDECREF(pFunc);  // XDECREF does not fail if pointer is NULL.
    Py_DECREF(pModule);
    PyGILState_Release(gstate); 
}

int main() {
    printf("Start.\n");
    initPython();
    callScript();
    exitPython();
    printf("Exit.\n");
    return 0;
}

Your specific script that is always called, change this so that you return all the data you want in a useful manner - at the moment we just use cProfile.run() which just prints some information:

Startup.py
import cProfile

def startup(module_name):
    print("Start script")
    cProfile.run("import " + module_name)
    print("Finished script")

Finally the trivial script that is executed:

TestScript.py
sum = 0
for i in range(10000):
    sum += i