Call a Python function from within a C program

Question

I have an application in C and at some point I need to solve a non-linear optimization problem. Unfortunately AFAIK there are very limited resources to do that in C (please let me know otherwise). However it is quite simple to do it in Python, eg scipy.optimize.minimize .

While I was trying to do that I encountered some of what it seems to be very frequent pitfalls, eg Python.h not found, module not loading, segmentation fault on function call, etc.

What is a quick and easy first-timer's way to link the two programs?

Answer 1

There are some things that you have to make sure are in place in order to make this work:

1. Make sure you have Python installed (you may need the python-dev package).
2. Locate your Python.h file, eg by locate Python.h . One of the occurrences should be in a sub(sub)folder in the include folder, eg the path should be something like ../include/python2.7/Python.h .
3. Insert #include “<path_to_Python.h>" in your C code in order to be able to use the Python API.

4. Use any tutorial to call your Python function. I used this one and it did the trick. However there were a couple of small points missing:

   • Whenever you use any Py<Name> function, eg PyImport_Import() , always check the result to make sure there was no error, eg

      // Load the module object pModule = PyImport_Import(pName); if (!pModule) { PyErr_Print(); printf("ERROR in pModule\\n"); exit(1); } 

   • Immediately after initializing the Python interpreter, ie after Py_Initialize(); , you have to append the current path to sys.path in order to be able to load your module (assuming it is located in your current directory):

      PyObject *sys = PyImport_ImportModule("sys"); PyObject *path = PyObject_GetAttrString(sys, "path"); PyList_Append(path, PyString_FromString(".")); 

5. Keep in mind that when you give the name of your Python file, it has to be without the extension .py .
6. Lastly, you have to do the following during compiling/linking:
   • Remember the ../include/python2.7/Python.h file you used before? Include the include folder in the list of the header files directories with the -I option in the gcc options during compilation, eg -I /System/Library/Frameworks/Python.framework/Versions/2.7/include .
   • Also pass to the linker the folder with the required libraries. It should be inside the same folder where the include folder is located, eg -L /System/Library/Frameworks/Python.framework/Versions/2.7/lib , along with the -lpython2.7 option (of course adjusting it accordingly to your Python version).

Now you must be able to successfully compile and execute your C program that calls in it your Python program.

I hope this was helpful and good luck!

Sources:

• How do you call Python code from C code?
• http://www.linuxjournal.com/article/8497?page=0,1
• http://www.codeproject.com/Articles/11805/Embedding-Python-in-CC-Part-I
• http://www.codeproject.com/Articles/11843/Embedding-Python-in-CC-Part-II
• Python C API doesn't load module
• What sets up sys.path with Python, and when?
• http://linux.die.net/man/1/gcc
• PyObject segfault on function call
• I have Python on my Ubuntu system, but gcc can't find Python.h
• How do you call Python code from C code?