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 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.
   (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 :ref:`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 IPython notebook
--------------------------

Cython can be used conveniently and interactively from a web browser
through the IPython notebook.  To install IPython, e.g. into a virtualenv,
use pip:

.. sourcecode:: bash

    (venv)$ pip install "ipython[notebook]"
    (venv)$ ipython notebook

To enable support for Cython compilation, install Cython and load the
``cythonmagic`` extension from within IPython::

    %load_ext cythonmagic

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
-----------------------

.. figure:: sage.png

  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.

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