.. highlight:: c

.. _extension-modules:

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 :dfn:`export hook` function (or an
old-style :ref:`initialization function <extension-pyinit>`).

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

.. _extension-export-hook:

Extension export hook
.....................

.. versionadded:: 3.15

   Support for the :samp:`PyModExport_{<name>}` export hook was added in Python
   3.15. The older way of defining modules is still available: consult either
   the :ref:`extension-pyinit` section or earlier versions of this
   documentation if you plan to support earlier Python versions.

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

.. c:function:: PySlot *PyModExport_modulename(void)

For modules with ASCII-only names, the :ref:`export hook <extension-export-hook>`
must be named :samp:`PyModExport_{<name>}`,
with ``<name>`` replaced by the module's name.

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

.. code-block:: 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

The export hook returns an array of :c:type:`PySlot` 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.
Prefer using the :c:macro:`Py_mod_create` and :c:macro:`Py_mod_exec` slots
for any dynamic behavior.

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:

.. c:macro:: PyMODEXPORT_FUNC

   Declare an extension module export hook.
   This macro:

   * specifies the :c:expr:`PySlot*` 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 PySlot spam_slots[] = {
       PySlot_STATIC_DATA(Py_mod_abi, &abi_info),
       PySlot_STATIC_DATA(Py_mod_name, "spam"),
       PySlot_FUNC(Py_mod_init, spam_init_function),
       ...
       PySlot_END
   };

   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.

.. _pymodexport-api-caveats:

The hook should be kept short.
If it does more than ``return`` a static array, several caveats apply:

- If you need to use any Python C API, it is recommended to call
  :c:func:`PyABIInfo_Check` first to raise an exception,
  rather than crash, in common cases of ABI mismatch.
- Code in the export hook must never rely on the :term:`GIL`:
  :term:`free-threaded builds <free-threaded build>` of Python can only check
  the :c:macro:`Py_mod_gil` slot (or the lack of it) after the hook returns,
- Similarly, the hook may be called in any subinterpreter, since the
  :c:macro:`Py_mod_multiple_interpreters` slot (or lack of it)
  is only checked after the hook returns.

For example::

   PyMODEXPORT_FUNC
   PyModExport_modulename(void)
   {
      if (PyABIInfo_Check(&abi_info, "modulename") < 0) {
         /* ABI mismatch. It's not safe to examine the raised exception. */
         return NULL;
      }

      /* use Python API (as little as possible); don't rely on GIL */

      return modulename_slots;
   }

.. 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 <https://peps.python.org/pep-0489/#multiple-modules-in-one-library>`__
   section in :pep:`489` for details.


.. _multi-phase-initialization:

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 :c:data:`Py_mod_abi`, :c:data:`Py_mod_gil` and
  :c:data:`Py_mod_multiple_interpreters` influence this step.
- By default, Python itself then creates the module object -- that is, it does
  the equivalent of calling :py:meth:`~object.__new__` when creating an object.
  This step can be overridden using the :c:data:`Py_mod_create` slot.
- Python sets initial module attributes like :attr:`~module.__package__` and
  :attr:`~module.__loader__`, and inserts the module object into
  :py:attr:`sys.modules`.
- Afterwards, the module object is initialized in an extension-specific way
  -- the equivalent of :py:meth:`~object.__init__` when creating an object,
  or of executing top-level code in a Python-language module.
  The behavior is specified using the :c:data:`Py_mod_exec` slot.

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

.. versionchanged:: 3.5

   Added support for multi-phase initialization (:pep:`489`).


Multiple module instances
.........................

By default, extension modules are not singletons.
For example, if the :py:attr:`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
:ref:`sub-interpreters <sub-interpreter-support>`
or after Python runtime reinitialization
(:c:func:`Py_Finalize` and :c:func:`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 :ref:`isolating-extensions-howto` for more details and a practical guide.

A simpler way to avoid these issues is
:ref:`raising an error on repeated initialization <isolating-extensions-optout>`.

All modules are expected to support
:ref:`sub-interpreters <sub-interpreter-support>`, 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 :c:data:`Py_mod_multiple_interpreters` slot.


.. _extension-pyinit:

``PyInit`` function
...................

.. soft-deprecated:: 3.15

   This functionality will not get new features,
   but there are no plans to remove it.

Instead of :c:func:`PyModExport_modulename`, an extension module can define
an older-style :dfn:`initialization function` with the signature:

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

Its name should be :samp:`PyInit_{<name>}`, with ``<name>`` replaced by the
name of the module.
For non-ASCII module names, use :samp:`PyInitU_{<name>}` instead, with
``<name>`` encoded in the same way as for the
:ref:`export hook <extension-export-hook>` (that is, using Punycode
with underscores).

If a module exports both :samp:`PyInit_{<name>}` and
:samp:`PyModExport_{<name>}`, the :samp:`PyInit_{<name>}` function
is ignored.

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

.. c:macro:: PyMODINIT_FUNC

   Declare an extension module initialization function.
   This macro:

   * specifies the :c:expr:`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 :c:type:`PyModuleDef` instance with non-``NULL``
:c:member:`~PyModuleDef.m_slots`. This allows Python to use
:ref:`multi-phase initialization <multi-phase-initialization>`.

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

.. c:function:: PyObject* PyModuleDef_Init(PyModuleDef *def)

   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 :c:type:`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.

   .. versionadded:: 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);
   }


.. _single-phase-initialization:

Legacy single-phase initialization
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. soft-deprecated:: 3.15

   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.

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

In single-phase initialization, the old-style
:ref:`initialization function <extension-pyinit>` (``PyInit_modulename``)
should create, populate and return a module object.
This is typically done using :c:func:`PyModule_Create` and functions like
:c:func:`PyModule_AddObjectRef`.

Single-phase initialization differs from the :ref:`default <multi-phase-initialization>`
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``
  [#testsinglephase]_ that defines a function ``sum`` and an exception class
  ``error``:

  .. code-block:: python

     >>> 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
  :c:func:`PyState_FindModule`.

* The module's :c:member:`PyModuleDef.m_slots` must be NULL.

.. [#testsinglephase] ``_testsinglephase`` is an internal module used
   in CPython's self-test suite; your installation may or may not
   include it.
