拡張モジュールの定義
********************

CPython の C 拡張は、共有ライブラリ (例えば、 Linux では ".so" ファイ
ル、 Windows では ".pyd" DLL) であり、 Python プロセスで読み込み可能 (
例えば、互換性のあるコンパイラ設定でコンパイルされている) で、 初期化
関数 をエクスポートするものです。

デフォルトで (つまり、 "importlib.machinery.ExtensionFileLoader" によ
って) インポート可能であるためには、共有ライブラリは "sys.path" 上で利
用可能である必要があり、モジュール名と
"importlib.machinery.EXTENSION_SUFFIXES". に含まれる拡張子で命名する必
要があります。

注釈:

  拡張モジュールのビルド、パッケージ化および配布には、サードパーティの
  ツールが最適であり、このドキュメントの範囲外です。適切なツールの一つ
  は Setuptools であり、ドキュメントは
  https://setuptools.pypa.io/en/latest/setuptools.html にあります。

通常、初期化関数は "PyModuleDef_Init()" を用いて初期化されたモジュール
定義を返します。これにより、作成プロセスをいくつかの段階に分割すること
ができます:

* 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.

バージョン 3.5 で変更: 多段階初期化 (**PEP 489**) のサポートを追加しま
した。


複数のモジュールインスタンス
============================

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 拡張モジュールを分離する 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.


初期化関数
==========

拡張モジュールによって定義された初期化関数は、次のシグネチャを持ちます
:

PyObject *PyInit_modulename(void)

名前が "PyInit_*<name>*" であり、 "<name>" はモジュール名です。

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 多段階初期化, 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

初期化関数は、ヘルパーマクロを使用して定義することが推奨されます:

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.


多段階初期化
============

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)
    * 次に属します: Stable ABI (バージョン 3.5 より).*

   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 多段階初期化. 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.


従来の一段階初期化
==================

注意:

  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.
