Defining extension modules

A C extension for CPython is a shared library (for example, a .so file on Linux, .pyd DLL on Windows), which is loadable into the Python process (for example, it is compiled with compatible compiler settings), and which exports an export hook function (or an old-style initialization function).

To be importable by default (that is, by importlib.machinery.ExtensionFileLoader), the shared library must be available on sys.path, and must be named after the module name plus an extension listed in importlib.machinery.EXTENSION_SUFFIXES.

Note

Building, packaging and distributing extension modules is best done with third-party tools, and is out of scope of this document. One suitable tool is Setuptools, whose documentation can be found at https://setuptools.pypa.io/en/latest/setuptools.html.

The export hook must be an exported function with the following signature:

PyModuleDef_Slot *PyModExport_modulename(void)

Its name should be PyModExport_<name>, with <name> replaced by the name of the module. See Export hook name for full details.

Added in version 3.15.0a1 (unreleased): The PyModExport_<name> export hook was added in Python 3.15. The older way of defining modules is still available: consult either the PyInit function section or earlier versions of this documentation if you plan to support earlier Python versions.

The export hook returns an array of PyModuleDef_Slot entries, terminated by an entry with a slot ID of 0. These slots describe how the module should be created and initialized.

This array must remain valid and constant until interpreter shutdown. Typically, it should use static storage; for dynamic behavior you should use the Py_mod_create and Py_mod_exec slots.

The export hook may return NULL with an exception set to signal failure.

It is recommended to define the export hook function using a helper macro:

PyMODEXPORT_FUNC
Part of the Stable ABI since version 3.15.

Declare an extension module export hook. This macro:

  • specifies the PyModuleDef_Slot* return type,

  • adds any special linkage declarations required by the platform, and

  • for C++, declares the function as extern "C".

For example, a module called spam would be defined like this:

PyABIInfo_VAR(abi_info);

static PyModuleDef_Slot spam_slots[] = {
    {Py_mod_abi, &abi_info},
    {Py_mod_name, "spam"},
    {Py_mod_init, spam_init_function},
    ...
    {0, NULL},
};

PyMODEXPORT_FUNC
PyModExport_spam(void)
{
    return spam_slots;
}

The export hook is typically the only non-static item defined in the module’s C source.

The hook should be kept short – ideally, one line as above. If you do need to use Python C API in this function, it is recommended to call PyABIInfo_Check(&abi_info, "modulename") first to raise an exception, rather than crash, in common cases of ABI mismatch.

Note

It is possible to export multiple modules from a single shared library by defining multiple export hooks. However, importing them requires a custom importer or suitably named copies/links of the extension file, because Python’s import machinery only finds the function corresponding to the filename. See the Multiple modules in one library section in PEP 489 for details.

Export hook name

For modules with ASCII-only names, the export hook must be named PyModExport_<name>, with <name> replaced by the module’s name.

For non-ASCII module names, the export hook must be named PyModExportU_<name> (note the U), with <name> encoded using Python’s punycode encoding with hyphens replaced by underscores. In Python:

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

Multi-phase initialization

The process of creating an extension module follows several phases:

  • Python finds and calls the export hook to get information on how to create the module.

  • Before any substantial code is executed, Python can determine which capabilities the module supports, and it can adjust the environment or refuse loading an incompatible extension. Slots like Py_mod_abi, Py_mod_gil and Py_mod_multiple_interpreters influence this step.

  • By default, Python itself then creates the module object – that is, it does the equivalent of calling __new__() when creating an object. This step can be overridden using the Py_mod_create slot.

  • Python sets initial module attributes like __package__ and __loader__, and inserts the module object into sys.modules.

  • Afterwards, the module object is initialized in an extension-specific way – the equivalent of __init__() when creating an object, or of executing top-level code in a Python-language module. The behavior is specified using the Py_mod_exec slot.

This is called multi-phase initialization to distinguish it from the legacy (but still supported) single-phase initialization, where an initialization function returns a fully constructed module.

Changed in version 3.5: Added support for multi-phase initialization (PEP 489).

Multiple module instances

By default, extension modules are not singletons. For example, if the sys.modules entry is removed and the module is re-imported, a new module object is created and, typically, populated with fresh method and type objects. The old module is subject to normal garbage collection. This mirrors the behavior of pure-Python modules.

Additional module instances may be created in sub-interpreters or after Python runtime reinitialization (Py_Finalize() and Py_Initialize()). In these cases, sharing Python objects between module instances would likely cause crashes or undefined behavior.

To avoid such issues, each instance of an extension module should be isolated: changes to one instance should not implicitly affect the others, and all state owned by the module, including references to Python objects, should be specific to a particular module instance. See Isolating Extension Modules for more details and a practical guide.

A simpler way to avoid these issues is raising an error on repeated initialization.

All modules are expected to support sub-interpreters, or otherwise explicitly signal a lack of support. This is usually achieved by isolation or blocking repeated initialization, as above. A module may also be limited to the main interpreter using the Py_mod_multiple_interpreters slot.

PyInit function

Deprecated since version 3.15.0a1 (unreleased): This functionality is soft deprecated. It will not get new features, but there are no plans to remove it.

Instead of PyModExport_modulename(), an extension module can define an older-style initialization function with the signature:

PyObject *PyInit_modulename(void)

Its name should be PyInit_<name>, with <name> replaced by the name of the module. For non-ASCII module names, use PyInitU_<name> instead, with <name> encoded as for the export hook.

If a module exports both PyInit_<name> and PyModExport_<name>, the PyInit_<name> function is ignored.

Like with PyMODEXPORT_FUNC, it is recommended to define the initialization function using a helper macro:

PyMODINIT_FUNC

Declare an extension module initialization function. This macro:

  • specifies the PyObject* return type,

  • adds any special linkage declarations required by the platform, and

  • for C++, declares the function as extern "C".

Normally, the initialization function (PyInit_modulename) returns a PyModuleDef instance with non-NULL m_slots. This allows Python to use multi-phase initialization.

Before it is returned, the PyModuleDef instance must be initialized using the following function:

PyObject *PyModuleDef_Init(PyModuleDef *def)
Part of the Stable ABI since version 3.5.

Ensure a module definition is a properly initialized Python object that correctly reports its type and a reference count.

Return def cast to PyObject*, or NULL if an error occurred.

Calling this function is required before returning a PyModuleDef from a module initialization function. It should not be used in other contexts.

Note that Python assumes that PyModuleDef structures are statically allocated. This function may return either a new reference or a borrowed one; this reference must not be released.

Added in version 3.5.

For example, a module called spam would be defined like this:

static struct PyModuleDef spam_module = {
    .m_base = PyModuleDef_HEAD_INIT,
    .m_name = "spam",
    ...
};

PyMODINIT_FUNC
PyInit_spam(void)
{
    return PyModuleDef_Init(&spam_module);
}

Legacy single-phase initialization

Deprecated since version 3.15.0a1 (unreleased): Single-phase initialization is soft deprecated. It is a legacy mechanism to initialize extension modules, with known drawbacks and design flaws. Extension module authors are encouraged to use multi-phase initialization instead.

However, there are no plans to remove support for it.

In single-phase initialization, the old-style initializaton function (PyInit_modulename) should create, populate and return a module object. This is typically done using PyModule_Create() and functions like PyModule_AddObjectRef().

Single-phase initialization differs from the default in the following ways:

  • Single-phase modules are, or rather contain, “singletons”.

    When the module is first initialized, Python saves the contents of the module’s __dict__ (that is, typically, the module’s functions and types).

    For subsequent imports, Python does not call the initialization function again. Instead, it creates a new module object with a new __dict__, and copies the saved contents to it. For example, given a single-phase module _testsinglephase [1] that defines a function sum and an exception class error:

    >>> import sys
>>> import _testsinglephase as one
>>> del sys.modules['_testsinglephase']
>>> import _testsinglephase as two
>>> one is two
False
>>> one.__dict__ is two.__dict__
False
>>> one.sum is two.sum
True
>>> one.error is two.error
True

    The exact behavior should be considered a CPython implementation detail.

  • To work around the fact that PyInit_modulename does not take a spec argument, some state of the import machinery is saved and applied to the first suitable module created during the PyInit_modulename call. Specifically, when a sub-module is imported, this mechanism prepends the parent package name to the name of the module.

    A single-phase PyInit_modulename function should create “its” module object as soon as possible, before any other module objects can be created.

  • Attempts to import a single-phase initialization module reentrantly from its own initialization function are likely to cause infinite recursion. (The extension author may prevent this by manually inserting a partially initialized module object in sys.modules.)

  • Non-ASCII module names (PyInitU_modulename) are not supported.

  • Single-phase modules support module lookup functions like PyState_FindModule().

  • The module’s PyModuleDef.m_slots must be NULL.