A minimal C extension

Goal:

Call the system function from the C standard library

import module
module.system(cmd)

Building

Note: Do not use tutorials that are old or for Python 2

Requirements

1. Python.h and other header files
2. libpython library
3. python3-config

In Debian:

apt install python3-dev build-essential

The above installs python3.x-dev, plus libpython3-dev and libpython3.x-dev

module.c template

#define PY_SSIZE_T_CLEAN
#include <Python.h>

static char module_doc[] = "This is a C extension\n\n"
  "A very simple extension.";

# Wrapper functions go here!

static PyMethodDef methods[] = {
    {NULL, NULL, 0, NULL}
};

static struct PyModuleDef moduledef = {
    PyModuleDef_HEAD_INIT,
    "module",
    module_doc,
    -1,
    methods
};

PyMODINIT_FUNC
PyInit_module(void)
{
    return PyModule_Create(&moduledef);
}

Based on: 1.1. A simple example, under Creating extensions without third party tools

Adding system_wrap

static PyObject *
system_wrap(PyObject *self, PyObject *args)
{
    const char *command;
    int status;

    if (!PyArg_ParseTuple(args, "s", &command))
        return NULL;
    status = system(command);
    if (status < 0) {
        return NULL;
    }
    return PyLong_FromLong(status);
}

static PyMethodDef methods[] = {
    {"system",  system_wrap, METH_VARARGS,
     "Execute a shell command.\n\nReturns the exit code."},
    {NULL, NULL, 0, NULL}
};

compiling and linking

! ls -lt module.*

! gcc -c $(python3-config --cflags) module.c -o module.o

! gcc -shared $(python3.9-config --ldflags) module.o -o module.so

! ls -lt module.*

Use the extension

import module

help(module)

Call the function

module.system('ls')

module.system('date')

module.system('foo')

Packaging

Reference: Python Packaging User Guide

pyproject.toml

[build-system]
requires = ["setuptools", "wheel"]
build-backend = "setuptools.build_meta"

setup.cfg

[metadata]
name = module
version = 0.1
author = John Smith
author_email = john@example.com
description = A minimal C extension
long_description = file: README.md
long_description_content_type = text/markdown
classifiers =
    Programming Language :: Python :: 3
    License :: OSI Approved :: LicenseName
    Operating System :: OS Independent

setup.py

from setuptools import setup, Extension

module = Extension('module',
                    sources = ['module.c'])

setup(ext_modules = [module])

README.md

# module A minimal C extension

Build source distribution and wheel

! ~/envs/jupyter/bin/python -m build --no-isolation

! ls -lt dist

Third-party tools

Name	Description
Pyrex	Pyhon-like language
SWIG	Simplified Wrapper and Interface Generator
Cython	Compile Python to C language extension modules
Numba	Use LLVM for speed
Boost.Python
PyBind11
ctypes	In the Python standard library, does not require compilation
cffi	C Foreign Function Interface, no C++