Building Cython code

Cython code must, unlike Python, be compiled. This happens in two stages:

  • A .pyx file is compiled by Cython to a .c file, containing the code of a Python extension module
  • The .c file is compiled by a C compiler to a .so file (or .pyd on Windows) which can be import-ed directly into a Python session.

There are several ways to build Cython code:

  • Write a distutils setup.py.
  • Use pyximport, importing Cython .pyx files as if they were .py files (using distutils to compile and build the background).
  • Run the cython command-line utility manually to produce the .c file from the .pyx file, then manually compiling the .c file into a shared object library or .dll suitable for import from Python. (This is mostly for debugging and experimentation.)
  • Use the [Sage] notebook which allows Cython code inline.

Currently, distutils is the most common way Cython files are built and distributed. The other methods are described in more detail in the Source Files and Compilation section of the reference manual.

Building a Cython module using distutils

Imagine a simple “hello world” script in a file hello.pyx:

def say_hello_to(name):
    print("Hello %s!" % name)

The following could be a corresponding setup.py script:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext_modules = [Extension("hello", ["hello.pyx"])]

setup(
  name = 'Hello world app',
  cmdclass = {'build_ext': build_ext},
  ext_modules = ext_modules
)

To build, run python setup.py build_ext --inplace. Then simply start a Python session and do from hello import say_hello_to and use the imported function as you see fit.

../../_images/sage.png

The Sage notebook allows transparently editing and compiling Cython code simply by typing %cython at the top of a cell and evaluate it. Variables and functions defined in a Cython cell imported into the running session.

[Sage]
  1. Stein et al., Sage Mathematics Software, http://sagemath.org