\chapter{Building C and \Cpp{} Extensions with distutils
     \label{building}}

\sectionauthor{Martin v. L\"owis}{[email protected]}

Starting in Python 1.4, Python provides, on \UNIX{}, a special make
file for building make files for building dynamically-linked
extensions and custom interpreters.  Starting with Python 2.0, this
mechanism (known as related to Makefile.pre.in, and Setup files) is no
longer supported. Building custom interpreters was rarely used, and
extension modules can be built using distutils.

Building an extension module using distutils requires that distutils
is installed on the build machine, which is included in Python 2.x and
available separately for Python 1.5. Since distutils also supports
creation of binary packages, users don't necessarily need a compiler
and distutils to install the extension.

A distutils package contains a driver script, \file{setup.py}. This is
a plain Python file, which, in the most simple case, could look like
this:

\begin{verbatim}
from distutils.core import setup, Extension

module1 = Extension('demo',
                    sources = ['demo.c'])

setup (name = 'PackageName',
       version = '1.0',
       description = 'This is a demo package',
       ext_modules = [module1])

\end{verbatim}

With this \file{setup.py}, and a file \file{demo.c}, running

\begin{verbatim}
python setup.py build 
\end{verbatim}

will compile \file{demo.c}, and produce an extension module named
\samp{demo} in the \file{build} directory. Depending on the system,
the module file will end up in a subdirectory \file{build/lib.system},
and may have a name like \file{demo.so} or \file{demo.pyd}.

In the \file{setup.py}, all execution is performed by calling the
\samp{setup} function. This takes a variable number of keyword 
arguments, of which the example above uses only a
subset. Specifically, the example specifies meta-information to build
packages, and it specifies the contents of the package.  Normally, a
package will contain of addition modules, like Python source modules,
documentation, subpackages, etc. Please refer to the distutils
documentation in \citetitle[../dist/dist.html]{Distributing Python
Modules} to learn more about the features of distutils; this section
explains building extension modules only.

It is common to pre-compute arguments to \function{setup}, to better
structure the driver script. In the example above,
the\samp{ext_modules} argument to \function{setup} is a list of
extension modules, each of which is an instance of the
\class{Extension}. In the example, the instance defines an extension
named \samp{demo} which is build by compiling a single source file,
\file{demo.c}.

In many cases, building an extension is more complex, since additional
preprocessor defines and libraries may be needed. This is demonstrated
in the example below.

\begin{verbatim}
from distutils.core import setup, Extension

module1 = Extension('demo',
                    define_macros = [('MAJOR_VERSION', '1'),
                                     ('MINOR_VERSION', '0')],
                    include_dirs = ['/usr/local/include'],
                    libraries = ['tcl83'],
                    library_dirs = ['/usr/local/lib'],
                    sources = ['demo.c'])

setup (name = 'PackageName',
       version = '1.0',
       description = 'This is a demo package',
       author = 'Martin v. Loewis',
       author_email = '[email protected]',
       url = 'http://www.python.org/doc/current/ext/building.html',
       long_description = '''
This is really just a demo package.
''',
       ext_modules = [module1])

\end{verbatim}

In this example, \function{setup} is called with additional
meta-information, which is recommended when distribution packages have
to be built. For the extension itself, it specifies preprocessor
defines, include directories, library directories, and libraries.
Depending on the compiler, distutils passes this information in
different ways to the compiler. For example, on \UNIX{}, this may
result in the compilation commands

\begin{verbatim}
gcc -DNDEBUG -g -O3 -Wall -Wstrict-prototypes -fPIC -DMAJOR_VERSION=1 -DMINOR_VERSION=0 -I/usr/local/include -I/usr/local/include/python2.2 -c demo.c -o build/temp.linux-i686-2.2/demo.o

gcc -shared build/temp.linux-i686-2.2/demo.o -L/usr/local/lib -ltcl83 -o build/lib.linux-i686-2.2/demo.so
\end{verbatim}

These lines are for demonstration purposes only; distutils users
should trust that distutils gets the invocations right.

\section{Distributing your extension modules
     \label{distributing}}

When an extension has been successfully build, there are three ways to
use it.

End-users will typically want to install the module, they do so by
running

\begin{verbatim}
python setup.py install
\end{verbatim}

Module maintainers should produce source packages; to do so, they run

\begin{verbatim}
python setup.py sdist
\end{verbatim}

In some cases, additional files need to be included in a source
distribution; this is done through a \file{MANIFEST.in} file; see the
distutils documentation for details.

If the source distribution has been build successfully, maintainers
can also create binary distributions. Depending on the platform, one
of the following commands can be used to do so.

\begin{verbatim}
python setup.py bdist_wininst
python setup.py bdist_rpm
python setup.py bdist_dumb
\end{verbatim}