Call python code from c via cython

I needed to do this using CMake and ended up recreating this sample. You can find the repository with complete working example here.

You can build and run the example using either Docker on the CLI or a Visual Studio devcontainer.

Maybe this is not what you want but I got it working by the following changes:

in quacker.pyx I added

cdef public int i

To force Cython to generate the .h file.

An then in the main:

#include <Python.h>
#include "caller.h"
#include "quacker.h"

int main() {
  Py_Initialize();
  initquacker();
  initcaller();
  call_quack();
  Py_Finalize();
  return 0;
}

In case there's anyone wondering how would it work in Python 3, here's my solution after struggling a bit as a Cython newbie.

main.c

#include <Python.h>
#include "caller.h"

int
main() 
{
    PyImport_AppendInittab("caller", PyInit_caller);
    Py_Initialize();
    PyImport_ImportModule("caller");
    call_quack();
    Py_Finalize();
    return 0;
}

caller.pyx

# cython: language_level=3
import sys
sys.path.insert(0, '')

from quacker import quack

cdef public void call_quack():
    quack()

quacker.py

def quack():
    print("Quack!")

Finally, here's the Makefile that compiles everything:

target=main
cybridge=caller

CC=gcc
CFLAGS= `python3-config --cflags`
LDFLAGS=`python3-config --ldflags`

all:
        cython $(cybridge).pyx
        $(CC) $(CFLAGS) -c *.c
        $(CC) $(LDFLAGS) *.o -o $(target)

clean:
        rm -f $(cybridge).{c,h,o} $(target).o $(target)
        rm -rf __pycache__

If you rename the quacker.pyx to quacker.py, everything is actually correct. The only problem is that your program won't search for python modules in the current directory, resulting in the output:

Exception NameError: "name 'quack' is not defined" in 'caller.call_quack' ignored

If you add the current directory to the PYTHONPATH environment variable however, the output becomes the one you'd expect:

$ PYTHONPATH=".:$PYTHONPATH" ./main 
Quack!

When running the python shell, according to the documentation the current directory (or the directory containing the script) is added to the sys.path variable automatically, but when creating a simple program using Py_Initialize and Py_Finalize this does not seem to happen. Since the PYTHONPATH variable is also used to populate the sys.path python variable, the workaround above produces the correct result.

Alternatively, below the Py_Intialize line, you could add an empty string to sys.path as follows by just executing some python code, specified as a string:

PyRun_SimpleString("import sys\nsys.path.insert(0,'')");

After recompiling, just running ./main should then work.

Edit

It's actually interesting to see what's going on if you run the code as specified in the question, so without renaming the quacker.pyx file. In that case, the initcaller() function tries to import the quacker module, but since no quacker.py or quacker.pyc exists, the module cannot be found, and the initcaller() function produces an error.

Now, this error is reported the python way, by raising an exception. But the code in the main.c file doesn't check for this. I'm no expert in this, but in my tests adding the following code below initcaller() seemed to work:

if (PyErr_Occurred())
{
    PyErr_Print();
    return -1;
}

The output of the program then becomes the following:

Traceback (most recent call last):
  File "caller.pyx", line 1, in init caller (caller.c:836)
    from quacker import quack
ImportError: No module named quacker

By calling the initquacker() function before initcaller(), the module name quacker already gets registered so the import call that's done inside initcaller() will detect that it's already loaded and the call will succeed.