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?
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.
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.
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;
}
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
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.
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.
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();
}
来源：https://stackoverflow.com/questions/3286448/calling-a-python-method-from-c-c-and-extracting-its-return-value