4. Building C and C++ Extensions
********************************

A C extension for CPython is a shared library (e.g. a ".so" file on
Linux, ".pyd" on Windows), which exports an *initialization function*.

To be importable, the shared library must be available on
"PYTHONPATH", and must be named after the module name, with an
appropriate extension. When using distutils, the correct filename is
generated automatically.

The initialization function has the signature:

PyObject *PyInit_modulename(void)

It returns either a fully initialized module, or a "PyModuleDef"
instance. See Initializing C modules for details.

For modules with ASCII-only names, the function must be named
"PyInit_<modulename>", with "<modulename>" replaced by the name of the
module. When using Multi-phase initialization, non-ASCII module names
are allowed. In this case, the initialization function name is
"PyInitU_<modulename>", with "<modulename>" encoded using Python's
*punycode* encoding with hyphens replaced by underscores. In Python:

   def initfunc_name(name):
       try:
           suffix = b'_' + name.encode('ascii')
       except UnicodeEncodeError:
           suffix = b'U_' + name.encode('punycode').replace(b'-', b'_')
       return b'PyInit' + suffix

It is possible to export multiple modules from a single shared library
by defining multiple initialization functions. However, importing them
requires using symbolic links or a custom importer, because by default
only the function corresponding to the filename is found. See the
*"Multiple modules in one library"* section in **PEP 489** for
details.


4.1. Building C and C++ Extensions with distutils
=================================================

Extension modules can be built using distutils,  which is included in
Python. 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 additional
modules, like Python source modules, documentation, subpackages, etc.
Please refer to the distutils documentation in Distributing Python
Modules (Legacy version) 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 = 'https://docs.python.org/extending/building',
          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.


4.2. Distributing your extension modules
========================================

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

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

   python setup.py install

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

   python setup.py sdist

In some cases, additional files need to be included in a source
distribution; this is done through a "MANIFEST.in" file; see
Specifying the files to distribute for details.

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

   python setup.py bdist_rpm
   python setup.py bdist_dumb
