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
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:
For modules with ASCII-only names, the function must be named
<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
<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])
setup.py, and a file
python setup.py build
demo.c, and produce an extension module named
build directory. Depending on the system, the module file will end
up in a subdirectory
build/lib.system, and may have a name like
setup.py, all execution is performed by calling the
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
Extension. In the example, the instance
defines an extension named
demo which is build by compiling a single source
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 = 'email@example.com', 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
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