We Keep Coding

Embedding Python into C - can't import method from python module

I'm building C application which will be using Python plugins. When trying to call the method from another Python module, the function PyImport_ImportModule() seems to imports the module properly, then i try to get the function from this module using PyObject_GetAttrString() and all that I get is null.
I already tried using PyModule_GetDict() and PyDict_GetItemString() to get the method from the module, but the effect was the same.
main.c:
#include <stdio.h>
#include <python3.6/Python.h>
int main()
{
PyObject *arg, *pModule, *ret, *pFunc, *pValue, *pMethod, *pDict;
Py_Initialize();
PyObject *sys = PyImport_ImportModule("sys");
PyObject *path = PyObject_GetAttrString(sys, "path");
PyList_Append(path, PyUnicode_FromString("."));
pModule = PyImport_ImportModule("test");
if(pModule == NULL)
{
perror("Can't open module");
}
pMethod = PyObject_GetAttrString(pModule, "myfun");
if(pMethod == NULL)
{
perror("Can't find method");
}
ret = PyEval_CallObject(pMethod, NULL);
if(ret == NULL)
{
perror("Couldn't call method");
}
PyArg_Parse(ret, "&d", pValue);
printf("&d \n", pValue);
Py_Finalize();
return 0;
}
test.py:
def myfun():
c = 123 + 123
print('the result is: ', c)
myfun()
The result i got is:
Can't find method: Success
Segmentation fault (core dumped)
When I used the gdb debugger the output was:
pModule = (PyObject *) 0x7ffff5a96f48
pMethod = (PyObject *) 0x0

Your program is not wroking because the module being imported is the test built-in module, rather than your test.py script. This is because you are appending the current directory to sys.path, so it is checked after every other already existing path in the list. You should insert it at the beginning of the list instead, so that it is checked first.
This will work:
PyObject *sys = PyImport_ImportModule("sys");
PyObject *path = PyObject_GetAttrString(sys, "path");
PyList_Insert(path, 0, PyUnicode_FromString("."));
By the way, you should #include the Python header before anything else, as stated in the documentation:
Note: Since Python may define some pre-processor definitions which affect the standard headers on some systems, you must include Python.h before any standard headers are included.

C++ application crashes with embedded Python code

I'm trying to run a Python code within a C++ application, based of example from this tutorial. Everything works fine with just running a script:
char filename[] = "D:\\Docs\\Embedding\\bin\\Debug\\load_continiuous_snl_data.py";
FILE* fp;
Py_Initialize();
fp = _Py_fopen(filename, "r");
PyRun_SimpleFile(fp, filename);
Py_Finalize();
But when I try to import script as a module, application crashes
std::vector<int> ExampleData(5);
for(int i=0;i<5;i++)
{
ExampleData[i] = i+1;
}
PyObject *pName,*pModule,*pFunc,*pname;
PyObject *pValue,*pArgs,*pDict;
Py_Initialize();
pName = PyUnicode_FromString("D:\\Docs\\Embedding\\bin\\Debug\\load_continiuous_snl_data.py");
//pModule = PyImport_Import(pname); //running without this line doesn't lead to crashing
Py_Finalize();
I'm using Python 3.7 32bit on Windows. Thanks in advance for any suggestion.

Embedding python in c++: Segmentation fault [duplicate]

This question already has an answer here:
Embedded Python 2.7.2 Importing a module from a user-defined directory
(1 answer)
Closed 5 years ago.
I am remote Debugging a c++ application with Visual studio on a linux device (raspberry pi/raspbian). In this c++ application I embedded a simple Python script by loading the function using the Python/c api. This is my c++ Code:
#include <Python.h>
int main(int argc, char* argv[])
{
PyObject *pName, *pModule, *pDict, *pFunc, *pValue;
// Initialize the Python Interpreter
Py_Initialize();
// Build the name object
pName = PyUnicode_FromString("//home//pi//projects//InfoBeam//WebScraperPython.txt");
// 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, "pyMain");
if (PyCallable_Check(pFunc))
{
PyObject_CallObject(pFunc, NULL);
}
else
{
PyErr_Print();
}
// Clean up
Py_DECREF(pModule);
Py_DECREF(pName);
// Finish the Python Interpreter
Py_Finalize();
return 0;
}
The Problem is, that I get a segmantation fault while running the function PyModule_GetDict(pModule). What am I doing wrong? This is the error message:
Program received signal SIGSEGV, Segmentation fault.
0x76bfdd28 in PyModule_GetDict () from /usr/lib/arm-linux-gnueabihf/libpython3.5m.so.1.0
Segmentation fault
EDIT: Okay, pModule was indeed NULL, probably because PyUnicode_FromString failed. Since PyImport_Import is failing: Where do I Need to save my Script, or: how do I pass the api the info where it is?

Before calling PyModule_GetDict, check that pModule isn't NULL. It would mean that PyImport_Import failed to import the module (problem with path, etc...)

Undeclared identifier: PyUnicode_DecodeFSDefault

I try to compile the example code from Python website https://docs.python.org/3/extending/embedding.html. Everything works fine except the following line:
pName = PyUnicode_DecodeFSDefault(argv[1]);
I have Python 3.6 installed on my MacOS El Captain. My make file looks like the following:
call_function: call_function.o
gcc -o call_function call_function.o -export-dynamic -L/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/config-3.6m-darwin -lpython3.6m -lpthread -lm -ldl -lutil
call_function.o:call_function.cpp
gcc -c call_function.cpp -I/Library/Frameworks/Python.framework/Versions/3.6/include/python3.6m
When I compile the C++ code, I get the following error:
gcc -c call_function.cpp -I/Library/Frameworks/Python.framework/Versions/3.6/include/python3.6m
call_function.cpp:16:13: error: use of undeclared identifier 'PyUnicode_DecodeFSDefault'
pName = PyUnicode_DecodeFSDefault(argv[1]);
^
1 error generated.
Does anyone know how to fix the above error? I would greatly appreciated for your help.
Here is the complete example code:
#include <Python/Python.h>
#include <Python/unicodeobject.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 = PyUnicode_DecodeFSDefault(argv[1]);
// pName = PyUnicode_FromString(argv[1]); <-- also gives me an error
/* 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;
}
Py_Finalize();
return 0;
}

I figure out what the problem is. The include <Python/Python.h> statement is actually linked to /System/Library/Frameworks/Python.framework/Versions/Current which currently is Python2.7. I have installed Python 3.6 on my Mac and even I include /Library/Frameworks/Python.framework/Versions/3.6/include/python3.6m in my make file, it's still referred to the Python.h file in MacOS's Python 2.7 folder. I tried to change the symbolic link of the Current to the new 3.6 folder and it's not letting me make the changes. Does anybody know how to change the symbolic link of the Python2.7 current folder so my gcc would look into the Python 3.6 folder for Python.h instead?

It is been a long time since this question got posted, but the solution that I came up is as follows,
char * full_cls_name = argv[1];
Py_Initialize();
pName = PyString_FromString(full_cls_name);
You should also check whether argv[1] exists or not?

This is what worked for me.
gcc -o call $(python3-config --cflags --embed) multiply_embed.c $(python3-config --ldflags --embed)
I got the answer from this post - answer by Ciro Santilli Путлер.
I also got module not found error when running the code, which was overcome by setting the PYTHONPATH variable in the zshrc file as described here.

Calling a Python function from C++

I am trying to make a call to a python module function from my cpp file.
The call i have made is as follows:
#include <iostream>
#include "Python.h"
int
main(int argc, char** argv)
{
Py_Initialize();
PyObject *pName = PyString_FromString("tmpPyth");
PyObject *pModule = PyImport_Import(pName);
std::cout<< "Works fine till here";
PyObject *pDict = PyModule_GetDict(pModule);
if (pModule != NULL) {
PyObject *pFunc = PyObject_GetAttrString(pDict, "pyFunc");
if(pFunc != NULL){
PyObject_CallObject(pFunc, NULL);
}
}
else
std::cout << "Python Module not found";
return 0;
}
My python module is defined as follows:
import numpy
import scipy
import matplotlib
from scipy import stats
def blah():
baseline = [9.74219, 10.2226, 8.7469, 8.69791, 9.96442, 9.96472, 9.37913, 9.75004]
follow_up = [9.94227,9.46763,8.53081,9.43679,9.97695,10.4285,10.159,8.86134]
paired_sample = stats.ttest_rel(baseline , follow_up )
print "The t-statistic is %.3f and the p-value is %.3f." % paired_sample
The code in the cpp file runs fine till the 1st "std::cout" but then ends up giving me a "seg fault". Running the python code separately works fine and gives the desired output.
I cant figure out what is going wrong. Any help will be appreciated.
(Note the program is compiling correctly and running correctly till the 1st "cout")

So there are a couple of things that you were not doing right. See the comments inline. Assuming that both your CPP file and Python file lives at the following path: /home/shanil/project.
test.cpp:
#include <iostream>
#include "Python.h"
int
main(int argc, char** argv)
{
Py_Initialize();
// First set in path where to find your custom python module.
// You have to tell the path otherwise the next line will try to load
// your module from the path where Python's system modules/packages are
// found.
PyObject* sysPath = PySys_GetObject("path");
PyList_Append(sysPath, PyString_FromString("/home/shanil/project"));
// Load the module
PyObject *pName = PyString_FromString("my_mod");
PyObject *pModule = PyImport_Import(pName);
// Random use-less check
std::cout<< "Works fine till here\n";
if (pModule != NULL) {
std::cout << "Python module found\n";
// Load all module level attributes as a dictionary
PyObject *pDict = PyModule_GetDict(pModule);
// Remember that you are loading the module as a dictionary, the lookup you were
// doing on pDict would fail as you were trying to find something as an attribute
// which existed as a key in the dictionary
PyObject *pFunc = PyDict_GetItem(pDict, PyString_FromString("my_func"));
if(pFunc != NULL){
PyObject_CallObject(pFunc, NULL);
} else {
std::cout << "Couldn't find func\n";
}
}
else
std::cout << "Python Module not found\n";
return 0;
}
my_mod.py:
def my_func():
print 'got called'