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3. Building C and C++ Extensions with distutils

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, setup.py. This is a plain Python file, which, in the most simple case, could look like this:

    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])

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

    python setup.py build

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

In the setup.py, all execution is performed by calling the "`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 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 `setup`, to better structure the driver script. In the example above, the"`ext_modules`" argument to `setup` is a list of extension modules, each of which is an instance of the `Extension`. In the example, the instance defines an extension named "`demo`" which is build by compiling a single source 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.

    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 = 'martin@v.loewis.de',
           url = 'http://www.python.org/doc/current/ext/building.html',
           long_description = '''
    This is really just a demo package.
    ''',
           ext_modules = [module1])

In this example, `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

    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

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