Core Python Programming (2nd Edition)

22.2. Extending Python by Writing Extensions
Creating extensions for Python involves three main steps:
1.
2.
3.
Creating application code
Wrapping code with boilerplates
Compilation and testing
In this section, we will break out all three pieces and expose them all to you.
22.2.1. Create Your Application Code
First, before any code becomes an extension, create a standalone "library." In other words, create your
code keeping in mind that it is going to turn into a Python module. Design your functions and objects
with the vision that Python code will be communicating and sharing data with your C code and vice
versa.
Next, create test code to bulletproof your software. You may even use the "Pythonic" development
method of designating your main() function in C as the testing application so that if your code is
compiled, linked, and loaded into an executable (as opposed to just a shared object), invocation of such
an executable will result in a regression test of your software library. For our extension example below,
this is exactly what we do.
The test case involves two C functions that we want to bring to the world of Python programming. The
first is the recursive factorial function, fac(). The second, reverse(), is a simple string reverse
algorithm, whose main purpose is to reverse a string "in place," that is, to return a string whose
characters are all reversed from their original positions, all without allocating a separate string to copy in
reverse order. Because this involves the use of pointers, we need to carefully design and debug our code
before bringing Python into the picture.
Our first version, Extest1.c, is presented in Example 22.1.
This code consists of a pair of functions, fac() and reverse(), which are implementations of the
functionality we described above. fac() takes a single integer argument and recursively calculates the
result, which is eventually returned to the caller once it exits the outermost call.
Example 22.1. Pure C Version of Library (Extest1.c)