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Code Editor : cython-ep2008.txt

==============================================
The Cython Compiler for C-Extensions in Python
==============================================

Dr. Stefan Behnel
-----------------

.. class:: center

http://www.cython.org/

cython-dev@codespeak.net

.. footer:: Dr. Stefan Behnel, EuroPython 2008, Vilnius/Lietuva

.. include:: <s5defs.txt>

About myself
============

* Passionate Python developer since 2002

* after Basic, Logo, Pascal, Prolog, Scheme, Java, ...

* CS studies in Germany, Ireland, France

* PhD in distributed systems in 2007

* Language design for self-organising systems

* Darmstadt University of Technologies, Germany

* Current occupations:

* Employed by Senacor Technologies AG, Germany

* IT transformations, SOA design, Java-Development, ...

* »lxml« OpenSource XML toolkit for Python

* http://codespeak.net/lxml/

* Cython

What is Cython?
===============

Cython is

* an Open-Source project

* http://cython.org

* a Python compiler (almost)

* an enhanced, optimising fork of Pyrex

* an extended Python language for

* writing fast Python extension modules

* interfacing Python with C libraries

A little bit of history
=======================

* April 2002: release of Pyrex 0.1 by Greg Ewing

* Greg considers Pyrex a language in design phase

* Pyrex became a key language for many projects

* over the years, many people patched their Pyrex

* not all patches were answered by Greg (not in time)

* minor forks and enhanced branches followed

* March 2006: my fork of Pyrex for »lxml« XML toolkit

* November 2006: »SageX« fork of Pyrex

* by Robert Bradshaw, William Stein (Univ. Seattle, USA)

* context: »Sage«, a free mathematics software package

* 28th July 2007: official Cython launch

* integration of lxml's Pyrex fork into SageX

* the rest is in http://hg.cython.org/cython-devel/

Major Cython Developers
=======================

* Robert Bradshaw and Stefan Behnel

* lead developers

* Greg Ewing

* main developer and maintainer of Pyrex

* we happily share code and discuss ideas

* Dag Sverre Seljebotn

* Google Summer-of-Code developer

* NumPy integration, many ideas and enhancements

* many, *many* others - see

* http://cython.org/

* the mailing list archives of Cython and Pyrex

How to use Cython
=================

* you write Python code

* Cython translates it into C code

* your C compiler builds a shared library for CPython

* you import your module

* Cython has support for

* compile-time includes/imports

* with dependency tracking

* distutils

* *optionally* compile Python code from setup.py!

Example: compiling Python code
==============================

.. sourcecode:: bash

$ cat worker.py

.. sourcecode:: python

class HardWorker(object):
        u"Almost Sisyphus"
        def __init__(self, task):
            self.task = task

def work_hard(self):
            for i in range(100):
	        self.task()

.. sourcecode:: bash

$ cython worker.py

* translates to 842 line `.c file <ep2008/worker.c>`_ (Cython 0.9.8)

* lots of portability #define's

* tons of helpful C comments with Python code snippets

* a lot of code that you don't want to write yourself

Portable Code
=============

* Cython compiler generates C code that compiles

* with all major compilers (C and C++)

* on all major platforms

* in Python 2.3 to 3.0 beta1

* Cython language syntax follows Python 2.6

* optional Python 3 syntax support is on TODO list

* get involved to get it quicker!

\... the fastest way to port Python 2 code to Py3 ;-)

Python 2 feature support
========================

* most of Python 2 syntax is supported

* top-level classes and functions

* exceptions

* object operations, arithmetic, ...

* plus some Py3/2.6 features:

* keyword-only arguments

* unicode literals via ``__future__`` import

* in recent developer branch:

* ``with`` statement

* closures (i.e. local classes and functions) are close!

Speed
=====

Cython generates very efficient C code

* according to PyBench:

* conditions and loops run 2-8x faster than in Py2.5

* most benchmarks run 30%-80% faster

* overall more than 30% faster for plain Python code

* optional type declarations

* let Cython generate plain C instead of C-API calls

* make code several times faster than the above

Type declarations
=================

* »cdef« keyword declares

* local variables with C types

* functions with C signatures

* classes as builtin extension types

* Example::

def stupid_lower_case(char* s):
        cdef Py_ssize_t size, i

size = len(s)
        for i in range(size):
            if s[i] >= 'A' and s[i] <= 'Z':
                s[i] += 'a' - 'A'
        return s

Why is this stupid?
===================

Ask Cython!

.. sourcecode:: bash

$ cat stupidlowercase.py

def stupid_lower_case(char* s):
        cdef Py_ssize_t size, i

size = len(s)
        for i in range(size):
            if s[i] >= 'A' and s[i] <= 'Z':
                s[i] += 'a' - 'A'
        return s

.. sourcecode:: bash

$ cython --annotate stupidlowercase.py

=> `stupidlowercase.html <ep2008/stupidlowercase.html>`_

Calling C functions
===================

* Example::

cdef extern from "Python.h":
        # copied from the Python C-API docs:
        object PyUnicode_DecodeASCII(
            char* s, Py_ssize_t size, char* errors)

cdef extern from "string.h":
        int strlen(char *s)

cdef slightly_better_lower_case(char* s):
        cdef Py_ssize_t i, size = strlen(s)
        for i in range(size):
            if s[i] >= 'A' and s[i] <= 'Z':
                s[i] += 'a' - 'A'
	return PyUnicode_DecodeASCII(s, size, NULL)

Features in work
================

* Dynamic classes and functions with closures

.. sourcecode:: python

def factory(a,b):
        def closure_function(c):
            return a+b+c
        return closure_function

* Native support for new ``buffer`` protocol

* part of NumPy integration by Dag Seljebotn

.. sourcecode:: python

def inplace_negative_grayscale_image(
                      ndarray[unsigned char, 2] image):
        cdef int i, j
        for i in range(image.shape[0]):
            for j in range(image.shape[1]):
                arr[i, j] = 255 - arr[i, j]

Huge pile of great ideas
========================

* Cython Enhancement Proposals (CEPs)

* http://wiki.cython.org/enhancements

* native pickle support for extension classes

* meta-programming facilities

* type inference strategies

* compile-time assertions for optimisations

* object-like C-array handling

* improved C++ integration

* ...

Conclusion
==========

* Cython is a tool for

* efficiently translating Python code to C

* easily interfacing to external C libraries

* Use it to

* speed up existing Python modules

* concentrate on optimisations, not rewrites!

* write C extensions for CPython

* don't change the language just to get fast code!

* wrap C libraries *in Python*

* concentrate on the mapping, not the glue!

... but Cython is also
======================

* a great project

* a very open playground for great ideas!

Cython
======

**Cython**

**C-Extensions in Python**

\... use it, and join the project!

http://cython.org/

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Code Editor : cython-ep2008.txt