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Run Python Code in C++

Posted on Edited on Disqus:
Symbols count in article: 527 Reading time ≈ 2 mins.
By Z.H. Fu
https://fuzihaofzh.github.io/blog/
## Goals We explore the method to embed python code into C++ . This is useful when we want to integrate some python code snippets into some C++ based systems. ## compile python Here, we compile python into dynamic link libraries. 1. Download python from python.org 
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wget https://www.python.org/ftp/python/2.7.13/Python-2.7.13.tgz
2. Unzip and build the python 
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tar xvf Python-2.7.13.tgz
cd Python-2.7.13
mkdir build
cd build
../configure --prefix=/home/me/pythonPKG --enable-unicode=ucs4 --disable-ipv6 --enable-shared
make -j20
make install
Here, we have almost gotten all of our necessary tools ready. However, some 3rd party library is useful when we are developing some programs such as numpy. Numpy is a powerful tool for algebra calculation in python. In order to add numpy support. We should firstly install pip and then use pip to install the numpy library.
  1. Add numpy support
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wget https://bootstrap.pypa.io/get-pip.py --no-check-certificate
/home/me/pythonPKG/bin/python get-pip.py
pip install numpy

Run Python in C++

In this charpter, we begin with a toy example to show how to run python code in C++.

Run Simple Code

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#include <python2.7/Python.h>
int main(){
    char pythonPath[] = "./pythonpkg";
    char pythonSysPath[] = "./pythonpkg/lib/python27.zip:./pythonpkg/lib/python2.7:./pythonpkg/lib\
/python2.7/plat-linux2:./pythonpkg/lib/python2.7/lib-tk:./pythonpkg/lib/python2.7/lib-old:./pythonpkg/lib/python2.7/lib-dynload:./pythonpkg/lib/python2.7/site-packages";
    Py_SetPythonHome(pythonPath);// optional
    Py_SetProgramName("myPythonProgram");
    Py_Initialize();
    PySys_SetPath();// optional
    PyRun_SimpleString("print 'Hello World.'");
    Py_Finalize();
}

The code should be compiled with

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g++ pythonRun.cpp -lpython2.7 -lpthread -ldl -lutil -O2 -L/home/me/pythonPKG/lib

The Program will print “Hello World.” on the screen.

Run Python File

Run python file is almost the same with run some code since we just read the code file and use exec to run it. Here we go:

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#include <python2.7/Python.h>
int main(){
    Py_SetProgramName("myPythonProgram");
    Py_Initialize();
    PyRun_SimpleString("exec(open('cpptest.py').read())");
    Py_Finalize();
}

code in cpptest.py:

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print "hello world."

Pass Parameters to Python Function

Passing parameters to python is a little bit difficult since we should make the parameters in C++ environment. Fortunately, Python’s C-API provide several useful apis to construct nearly all types of python data with C++.

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#include <python2.7/Python.h>
int main(){
    Py_SetProgramName("myPythonProgram");
    Py_Initialize();
    // parDict is a parameter to send to python function
    PyObject * parDict;
    parDict = PyDict_New();
    PyDict_SetItemString(parDict, "x0", PyFloat_FromDouble(1.0));
    // run python code to load functions
    PyRun_SimpleString("exec(open('cpptest.py').read())");
    // get function showval from __main__
    PyObject* main_module = PyImport_AddModule("__main__");
    PyObject* global_dict = PyModule_GetDict(main_module);
    PyObject* func = PyDict_GetItemString(global_dict, "showval");
    // parameter should be a tuple
    PyObject* par = PyTuple_Pack(1, parDict);
    // call the function
    PyObject_CallObject(func, par);
    Py_Finalize();
}

Function defined in cpptest.py is:

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import numpy as np
def showval(par):
    print "in function showval"
    print par
    print np.array([1, 2, 3])

we get the result:

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in function showval
{'x0': 1.0}
[1 2 3]

Conclusion

In this post we illustrate variety of ways to invoke python code in C++ environment. Besides, the method to compile a useable python library is also presented.

Reference

[1] https://docs.python.org/2/c-api/index.html