In the previous three recipes, we have discussed Cython, Boost.Python, and pybind11 as tools to interface Python and C++ providing a modern and clean approach. The main interface in the previous recipes was a C++ interface. However, we may be in a situation where we do not have a C++ interface to hook on to and where we would like to interface Python with Fortran or other languages.

In this recipe, we will demonstrate an alternative approach for interfacing Python using the Python C Foreign Function Interface (CFFI; see also https://cffi.readthedocs.io). Since C is the lingua franca of programming languages and most programming languages (including Fortran) are able to talk to a C interface, Python CFFI is a tool to couple Python with a large number of languages. A very nice feature of Python CFFI is that the resulting interface is thin and non-intrusive, meaning that it neither restricts the Python layer in language features, nor does it impose any restrictions on the code below the C layer, apart from requiring a C interface.

In this recipe, we will apply Python CFFI to couple Python and C++ via a C interface using the bank account example introduced in preceding recipe. Our goal is to arrive at a context-aware interface where we can instantiate several bank accounts, each carrying its internal state. We will conclude this recipe by commenting on how to couple Python and Fortran using Python CFFI. In Chapter 11, Packaging Projects, Recipe 3, Distributing a C/Fortran/Python project built with CMake/CFFI via PyPI, we will revisit this example and show how to package it and make it installable with pip.