Develop Reference

How can I choose which Python interpreter is being used when embedding it in a C program?

For a project of mine, I need to embed the Python interpreter in a wider C application, using the interpreter of an Anaconda environment. How can I do so ? I've looked online, but nothing mentions it. I'm guessing I should change something near Py_Initialize(), but what ?
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#ifndef PY_SSIZE_T_CLEAN
#define PY_SSIZE_T_CLEAN
#endif
#include <Python.h>
int main()
{
Py_Initialize(); // Here ?
// Maybe something like Py_InitializeInterpreter("whatever/path") ?
PyObject* pName = PyUnicode_DecodeFSDefault("script");
PyObject* pModule = PyImport_Import(pName);
Py_DecRef(pName);
bool running = false;
while(running)
{
//whatever I need to do
}
Py_DecRef(pModule);
Py_Finalize();
return EXIT_SUCCESS;
}
EDIT : I'd just like to point out one thing : I'm not a Python dev by any means, I just need to call Python functions from my C application. The Python itself is coded by someone else, so I don't have much knowledge at all about how it works.

Unable to import Python module written in C

I have been trying to work out how to make a .pyd (Python Extension Module) file from a C script (without swig or anything else except MinGW) and have successfully built it into a .pyd.
The problem however occurs when I try and import the module.
If I run it the module runs successfully (as far as I can see) and then an error appears saying Python Has Stopped Working and it closes without executing the rest of the program.
Here is my C script (test.c):
#include <python.h>
int main()
{
PyInit_test();
return 0;
}
int PyInit_test()
{
printf("hello world");
}
And Python Script (file.py):
import test
print('Run From Python Extension')
I compiled the script with:
gcc -c file.py
gcc -shared -o test.pyd test.c
I can't find any errors when compiling in command prompt and am using python 3.6 (running on Windows 10).
I can't find much on the subject and would prefer to keep away from Cython (I already know C) and Swig.
Any help to tell me what is wrong would be fantastic.

Creating a Python extension is completely different than writing regular C code. What you have done is simply creating a valid C program but that doesn't make sense for Python.
That's how your program should look like (it's just a skeleton, not the proper, working code):
#include <Python.h>
#include <stdlib.h>
static PyObject* test(PyObject* self, PyObject* args)
{
printf("hello world");
return NULL;
}
static PyMethodDef test_methods[] = {
{"test", test, METH_VARARGS, "My test method."},
{NULL, NULL, 0, NULL} /* Sentinel */
};
PyMODINIT_FUNC init_test_methods() {
Py_InitModule("test", test_methods);
}
int main(int argc, char** argv)
{
/* Pass argv[0] to the Python interpreter */
Py_SetProgramName(argv[0]);
/* Initialize the Python interpreter. Required. */
Py_Initialize();
/* Add a static module */
init_test_methods();
}
I recommend you read more about this at the following link: http://dan.iel.fm/posts/python-c-extensions/ as well as in the official docs.

Embedding python 3.4 into C++ Qt Application?

I'm making an Qt Quick GUI application(for windows), which uses OpenGL and C++ for some computationally intensive stuff. I want to embed python code into the app, for doing some stuff which is comparatively easier in python.
Basically, I just want the c++ code to call a function in a python script and let the script do the job, then store the returned data in a variable(string, or float etc.) for further use. I'm using Qt creator, and I got python3 lib for MinGW compiler. I tried some code, but its looks like python lib is not quite compatible with Qt creator. IS using pyqt for this will be a good idea? What will be the best and easiest way to do this ?
EDIT: This is the basic code I tried, first it gave me an error saying, cannot find pyconfig.h. Then I added an INCUDEPATH to my python34 include directory.
#include "mainwindow.h"
#include <QApplication>
#include <boost/python.hpp>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
using namespace boost::python;
PyObject *pName, *pModule, *pDict, *pFunc, *pValue;
Py_Initialize();
pName = PyString_FromString(argv[1]);
pModule = PyImport_Import(pName);
pDict = PyModule_GetDict(pModule);
pFunc = PyDict_GetItemString(pDict, argv[2]);
if (PyCallable_Check(pFunc))
{
PyObject_CallObject(pFunc, NULL);
} else
{
PyErr_Print();
}
// Clean up
Py_DECREF(pModule);
Py_DECREF(pName);
Py_Finalize();
return a.exec();
}
My .pro file:
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = TestWidgetApp
TEMPLATE = app
INCLUDEPATH += C:/boost_1_57_0
INCLUDEPATH += C:/Python34/include
SOURCES += main.cpp\
mainwindow.cpp
HEADERS += mainwindow.h
FORMS += mainwindow.ui
OTHER_FILES +=
Then the following errors:
C:\Python34\include\object.h:435: error: C2059: syntax error : ';'
C:\Python34\include\object.h:435: error: C2238: unexpected token(s) preceding ';'
C:\Users\Amol\Desktop\TestWidgetApp\main.cpp:19: error: C3861: 'PyString_FromString': identifier not found

The problem here is that Python 3.4 has a struct member called "slots", (file object.h, in the typedef for PyType_Spec), which Qt defines out from under you so that you can say things like:
public slots:
in your code. The solution is to add:
#undef slots
just before you include Python.h, and to redefine it before you include anything that uses "slots" in the way that Qt does:
#undef slots
#include <Python.h>
#define slots
#include "myinclude.h"
#include <QString>
A bit of a hack (because you're depending on a particular definition of slots in Qt), but it should get you going.

I have removed all the Qt code from your example and then I tried to compile it (Qt has nothing to do with your compile error). And it compiles for me. The difference was I used the include files from Python 2.7.
So I did a little search for the string PyString_FromString in the folders: C:\Python33\includes (I noted you use python 3.4 and not 3.3 but I suspect this is a 3.x thing) and C:\Python27\includes.
Results:
Python 3.3
Python 2.7
So, apparently, Python 3.4 is not supported by your BoostPython version.

Python3 has no PyString_FromString function. Python3 str type internally is unicode objects with complex structure.
Use PyUnicode_FromString or PyUnicode_FromStringAndSize for constructing str object from UTF-8 encoded C string (char*).

Move your
#include "boost/python.hpp"
...to be before your other includes and it will resolve your problem.
The actual issue is as Scott Deerwester described in his answer.

PyObject_GetAttrString C++ function returning NULL: Unable to call Python functions from C++

I've just started working with Python with C++ and I'm a bit confused on why I'm unable to call functions in Python from C++.
Here is my current test code in C++:
#include <iostream>
#include <Python.h>
using namespace std;
int main()
{
Py_Initialize();
PyObject* myModuleString = PyString_FromString("test");
PyObject* myModule = PyImport_Import(myModuleString);
if( myModule )
{
PyObject* myFunction = PyObject_GetAttrString(myModule, "Hello");
if( myFunction )
{
PyEval_CallObject( myFunction, NULL );
}
else
{
fprintf( stderr, "myFunction is NULL" );
}
}
else
{
fprintf( stderr, "myModule is NULL" );
}
Py_Finalize();
cin.get();
return 0;
}
Here is my test.py Python code:
import sys
def Hello():
print "Hello, world!"
Before I had a more complicated test, but I ran into an issue where PyObject_GetAttrString passed back NULL, so I wanted to make a simpler test and I still received NULL after calling PyObject_GetAttrString. From my understanding PyObject_GetAttrString gets you the PyObject* to the function and then I call it after, so receiving NULL there basically means I can't call the function.
Also yes I have looked at https://docs.python.org/2.7/ and even tested the example given in step 5.3 of https://docs.python.org/release/2.6.5/extending/embedding.html#pure-embedding (I'm using 2.7.7 because I'm planning to integrate with 3ds Max ). It still runs into the same issues with PyObject_GetAttrString.
I'm assuming it's a simple mistake or step I'm missing in the C++ or Python code.

Changing the Python script name from test.py to anything else worked for me.

I was having a similar problem. I found that the whitespace in the string I was passing to python was way off. From what I could tell, your print statement has 6 spaces in lieu of 4. Back it up and see if everything doesn't clear up.

I had the same problem. Although test.py and mycode.c were in the same folder (/home/user/python/example), I had to add a reference to the path after Py_Initialize(); as shown in following line
PyRun_SimpleString ("import sys; sys.path.insert(0, '/home/user/python/spp')");*
Replace /home... to your path.
My previous code did not have that line of code but PyImport_Importwas working and PyObject_GetAttrString wasn't. Makes no sense to me, "don't ask me, I don't know - Ozzy".

Embedding Python into C

Win7 x64, Python3.3 32bit, Visual Studio 2010/2012 (same behavior). The following code compiles and runs just fine (i.e. prints current date):
extern "C"{ // not having it doesn't make any difference either
#include <Python.h>
}
int main() {
Py_Initialize();
PyRun_SimpleString("from time import time,ctime\n"
"print('Today is', ctime(time()))\n");
Py_Finalize();
return 0;
}
while this here fails with a MessageBox saying The application was unable to start correctly (0xc0000005). Click OK to close the application. before main executed (so no breakpoint possible).
extern "C"{ // not having it doesn't make any difference either
#include <Python.h>
}
int main() {
Py_Initialize();
PyObject *p = PyUnicode_FromString("test");
Py_Finalize();
return 0;
}

So the problem seems to have been the following: I was linking with python3.lib but since the string functions were completely overworked with Python3.3 there seemed to have been some problem with correctly linking them. (can't really explain why this would be so, since PyUnicode_FromString existed obviously in earlier versions of python3 too).
Why I couldn't get a useful error message about that fact is also beyond me, but there we go: linking against python33.lib solved the problem perfectly.

I think this could be for 2 reasons, but I'm pretty sure its this one:
http://docs.python.org/2/c-api/unicode.html
You need to null terminate y our constant string "test" by making it "test\0". If that doesn't work, it might have to do with the fact that c files are ansi and not utf8.

Your broken program is linking against a build of Python that uses UCS-2 as the internal unicode representation, but the installed Python uses UCS-4, and therefore the PyUnicodeUCS2_* imports can't be resolved. You'll need to link against the UCS-4 build instead.