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 setuptools, 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):
        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 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.