# 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 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.

Normally, the initialization function returns a module definition initialized using `PyModuleDef_Init()`. This allows splitting the creation process into several phases:

- 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.

- By default, Python itself creates the module object – that is, it does the equivalent of `object.__new__()` for classes. It also sets initial attributes like `__package__` and `__loader__`.

- Afterwards, the module object is initialized using extension-specific code – the equivalent of `__init__()` on classes.

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

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.

## Initialization function

The initialization function defined by an extension module has the following signature:

PyObject *PyInit_modulename(void)

Its name should be `PyInit_<name>`, with `<name>` replaced by the name of the module.

For modules with ASCII-only names, the function must instead be named `PyInit_<name>`, with `<name>` 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_<name>`, with `<name>` 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 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"`.

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);
    }

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.

The initialization function is typically the only non-`static` item defined in the module’s C source.

## Multi-phase initialization

Normally, the initialization function (`PyInit_modulename`) returns a `PyModuleDef` instance with non-`NULL` `m_slots`. 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 for Multi-phase initialization. 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.

## Legacy single-phase initialization

Attention

Single-phase initialization 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.

In single-phase initialization, the initialization 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.

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

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

[1]

`_testsinglephase` is an internal module used in CPython’s self-test suite; your installation may or may not include it.