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 beimport
-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 in 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. (These manual steps are mostly for debugging and experimentation.)- Use the [Jupyter] notebook or the [Sage] notebook, both of which allow 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 Cython.Build import cythonize
setup(
name = 'Hello world app',
ext_modules = cythonize("hello.pyx"),
)
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.
Using the Jupyter notebook¶
Cython can be used conveniently and interactively from a web browser through the Jupyter notebook. To install Jupyter notebook, e.g. into a virtualenv, use pip:
(venv)$ pip install jupyter
(venv)$ jupyter notebook
To enable support for Cython compilation, install Cython and load the
Cython
extension from within the Jupyter notebook:
%load_ext Cython
Then, prefix a cell with the %%cython
marker to compile it:
%%cython
cdef int a = 0
for i in range(10):
a += i
print(a)
You can show Cython’s code analysis by passing the --annotate
option:
%%cython --annotate
...

Using the Sage notebook¶

For users of the Sage math distribution, 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.
[Jupyter] | http://jupyter.org/ |
[Sage] |
|