Python 3 – Extension Programming with C

Python is one of the most commonly used programming languages in the world. It’s well-known for its simplicity and versatility, which makes it an excellent choice for both beginners and seasoned professionals. One of the most powerful features of Python is its ability to extend itself through the use of C extensions. This allows you to take advantage of C’s speed and efficiency while still enjoying the benefits of Python’s high-level abstractions. In this article, we’ll explore the basics of Python 3 extension programming with C.

What is a C extension?

A C extension is a module that is written in C and can be loaded into Python through the use of the ctypes, cffi, or Cython libraries. To create a C extension, you should have a good understanding of both C and Python, as well as the ability to build C code using a compiler.

The primary advantage of writing a C extension is that it can significantly increase the speed and efficiency of your Python code. Since C is a compiled language, it can execute much faster than Python’s interpreted bytecode. This is particularly important when working with computationally intensive tasks or large data sets.

Building a Simple C Extension

Let’s start off by writing a simple C extension. We’re going to create a module that implements the Pythagorean theorem for calculating the length of the hypotenuse of a right-angled triangle.

#include <Python.h>
#include <math.h>

static PyObject *pythagoras(PyObject *self, PyObject *args) {
    double leg1, leg2;

    if (!PyArg_ParseTuple(args, "dd", &leg1, &leg2)) {
        return NULL;
    }

    double hypotenuse = sqrt(pow(leg1, 2) + pow(leg2, 2));

    return Py_BuildValue("d", hypotenuse);
}

static PyMethodDef PythagorasMethods[] = {
    {"calculate", pythagoras, METH_VARARGS, "Calculate the length of the hypotenuse of a right-angle triangle using the Pythagorean theorem."},
    {NULL, NULL, 0, NULL}
};

static struct PyModuleDef pythagoras_module = {
    PyModuleDef_HEAD_INIT,
    "pythagoras",
    "A simple Python module that implements the Pythagorean theorem.",
    -1,
    PythagorasMethods
};

PyMODINIT_FUNC PyInit_pythagoras(void) {
    return PyModule_Create(&pythagoras_module);
}

Let’s break down what’s happening in this code. First, we include the necessary headers for working with Python and math functions. The pythagoras function is our main function that will perform the calculation. It takes two arguments, the lengths of the two legs of the right-angle triangle, and returns the length of the hypotenuse using Py_BuildValue.

The PyMethodDef struct defines the method that will be available to Python. We call this method calculate and give it a description that will be visible in the Python documentation. The PyModuleDef struct defines our module and lists the PyMethodDef struct. Finally, the PyInit_pythagoras function initializes the module and returns a reference to it.

Compiling the C Extension

Now that we have our C code, we need to compile it into a shared library that Python can load. We can do this using the gcc compiler.

$ gcc -fpic -O -c pythagoras.c -o pythagoras.o
$ gcc -shared pythagoras.o -o pythagoras.so

This will create a shared library called pythagoras.so that Python can load. We’re now ready to test out our extension in Python.

import pythagoras

print(pythagoras.calculate(3, 4))

This should output 5.0, which is the length of the hypotenuse of a right-angled triangle with legs of length 3 and 4.

Conclusion

Writing C extensions for Python can significantly improve the performance of your code, especially when working with computationally intensive tasks. While it can be challenging to write C code, Python’s simplicity and its various libraries can make the process much more manageable. With the knowledge and know-how of Python 3 extension programming with C described above, we hope you can confidently create C extensions in your next programming project.