Python Cookbook 2Nd Edition Jun 1002005 [Electronic resources] نسخه متنی

Recipe 17.2. Implementing a Simple Extension Type with Pyrex

Credit: Alex Martelli

Problem

You
want to code and build an extension type for Python with a minimal
amount of hard work.

Solution

The Pyrex language is the simplest and fastest way to build Python
extensions. Once we have Pyrex installed, the next step is to create
a setup.py file to use the
distutils package to build and install our module:

from distutils.core import setup, Extension
from Pyrex.Distutils import build_ext
setup(name = "elemlist",
version = "1.0",
maintainer = "Alex Martelli",
maintainer_email = "amcx@aleax.it",
description = "Simple Python extension module in Pyrex",
ext_modules = [Extension('elemlist',sources=['elemlist.pyx'])],
cmdclass = {'build_ext': build_ext},
)

Then, we need a file elemlist.pyx with our
module's source code:

cdef class cons:
cdef public object car, cdr
def _ _init_ _(self, car, cdr):
self.car = car
self.cdr = cdr
def _ _repr_ _(self):
return 'cons(%r, %r)' % (self.car, self.cdr)

Discussion

Pyrex is a language for writing Python extension modules. It was
developed by Greg Ewing and is freely downloadable and installable.
The Pyrex language is a large subset of Python, with the addition of
a few language constructs to allow easy generation of fast C code. In
this recipe, the only special Pyrex construct we use is the
cdef statement, which lets us express C-level
declarations.

This module is roughly equivalent to the Python-coded module:

class cons(object):
_ _slots_ _ = ('car', 'cdr')
def _ _init_ _(self, car, cdr):
self.car = car
self.cdr = cdr
def _ _repr_ _(self):
return 'cons(%r, %r)' % (self.car, self.cdr)

As you see, Pyrex code is very similar to Python code. Pyrex code
gets compiled into C, which in turn gets compiled to machine code,
while Python code is compiled into bytecode instead. For such a
simple type as cons, the Pyrex version is not much
faster than the Python version, but a pure C version, such as the one
that I demonstrated previously in Recipe 17.1, despite having 25 times as
much code, wouldn't be any faster either.

Building a compiled extension module is just as simple when
it's coded in Pyrex as when it's
coded directly in C, thanks to the distutils
package, which does all of the hard work. (You need to have Pyrex
installed.) When you build a Pyrex-coded module, you may get warnings
from your C compiler about symbols with names starting with
_ _pyx or _ _pyx that are
defined but not used, or declared but not defined. Do not let these
warning messages worry you: your C compiler is running with the
highest possible level of warnings, and the little anomalies
it's warning about are just perfectly normal and
innocuous artifacts in the C sources that Pyrex automatically
generates. Pyrex is not quite finished yet (the Pyrex version at the
time of writing is 0.9.3), so no attention has yet been spent on
purely cosmetic warts. (By the time you read this, a mature version
of Pyrex may have been released, with all i's dotted
and all t's crossed. Nevertheless, I would recommend
Pyrex even if the latest version still causes numerous warnings.)

See Also

Abundant documentation on Pyrex, as well as examples, can be found in
the directory (and particularly in subdirectories
Doc and Demos) where you
unpacked Pyrex's .tar.gz file;
essentially the same documentation can also be read online, starting
from the Pyrex web site at http://nz.cosc.canterbury.ac.nz/~greg/python/Pyrex/.