building.rst

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 :envvar:`PYTHONPATH`, and must be named after the module name, with an appropriate extension. When using setuptools, the correct filename is generated automatically.

The initialization function has the signature:

.. c:function:: PyObject* PyInit_modulename(void)

It returns either a fully initialized module, or a :c:type:`PyModuleDef` instance. See :ref:`initializing-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 :ref:`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.

Building C and C++ Extensions with setuptools

Python 3.12 and newer no longer come with distutils. Please refer to the setuptools documentation at https://setuptools.readthedocs.io/en/latest/setuptools.html to learn more about how build and distribute C/C++ extensions with setuptools.