"importlib.metadata" を使う
***************************

**Source code:** Lib/importlib/metadata.py

バージョン 3.8 で追加.

注釈:

  This functionality is provisional and may deviate from the usual
  version semantics of the standard library.

"importlib.metadata" is a library that provides for access to
installed package metadata.  Built in part on Python's import system,
this library intends to replace similar functionality in the entry
point API and metadata API of "pkg_resources".  Along with
"importlib.resources" in Python 3.7 and newer (backported as
importlib_resources for older versions of Python), this can eliminate
the need to use the older and less efficient "pkg_resources" package.

By "installed package" we generally mean a third-party package
installed into Python's "site-packages" directory via tools such as
pip.  Specifically, it means a package with either a discoverable
"dist-info" or "egg-info" directory, and metadata defined by **PEP
566** or its older specifications. By default, package metadata can
live on the file system or in zip archives on "sys.path".  Through an
extension mechanism, the metadata can live almost anywhere.


概要
====

Let's say you wanted to get the version string for a package you've
installed using "pip".  We start by creating a virtual environment and
installing something into it:

   $ python3 -m venv example
   $ source example/bin/activate
   (example) $ pip install wheel

以下のように実行することで、"wheel" のバージョン文字列を取得することが
できます：

   (example) $ python
   >>> from importlib.metadata import version  
   >>> version('wheel')  
   '0.32.3'

You can also get the set of entry points keyed by group, such as
"console_scripts", "distutils.commands" and others.  Each group
contains a sequence of EntryPoint objects.

ディストリビューションのメタデータ:: を取得することができます。

   >>> list(metadata('wheel'))  
   ['Metadata-Version', 'Name', 'Version', 'Summary', 'Home-page', 'Author', 'Author-email', 'Maintainer', 'Maintainer-email', 'License', 'Project-URL', 'Project-URL', 'Project-URL', 'Keywords', 'Platform', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Classifier', 'Requires-Python', 'Provides-Extra', 'Requires-Dist', 'Requires-Dist']

また、 配布物のバージョン番号 を取得し、 構成ファイル をリストアップし
、配布物の 配布物の要件 のリストを取得することができます。


機能 API
========

本パッケージは、公開APIを通じて以下の機能を提供します。


エントリポイント
----------------

The "entry_points()" function returns a dictionary of all entry
points, keyed by group.  Entry points are represented by "EntryPoint"
instances; each "EntryPoint" has a ".name", ".group", and ".value"
attributes and a ".load()" method to resolve the value.  There are
also ".module", ".attr", and ".extras" attributes for getting the
components of the ".value" attribute:

   >>> eps = entry_points()  
   >>> list(eps)  
   ['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation']
   >>> scripts = eps['console_scripts']  
   >>> wheel = [ep for ep in scripts if ep.name == 'wheel'][0]  
   >>> wheel  
   EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts')
   >>> wheel.module  
   'wheel.cli'
   >>> wheel.attr  
   'main'
   >>> wheel.extras  
   []
   >>> main = wheel.load()  
   >>> main  
   <function main at 0x103528488>

The "group" and "name" are arbitrary values defined by the package
author and usually a client will wish to resolve all entry points for
a particular group.  Read the setuptools docs for more information on
entry points, their definition, and usage.


配布物メタデータ
----------------

Every distribution includes some metadata, which you can extract using
the "metadata()" function:

   >>> wheel_metadata = metadata('wheel')  

The keys of the returned data structure [1] name the metadata
keywords, and their values are returned unparsed from the distribution
metadata:

   >>> wheel_metadata['Requires-Python']  
   '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'


配布物バージョン
----------------

The "version()" function is the quickest way to get a distribution's
version number, as a string:

   >>> version('wheel')  
   '0.32.3'


配布物ファイル
--------------

You can also get the full set of files contained within a
distribution.  The "files()" function takes a distribution package
name and returns all of the files installed by this distribution.
Each file object returned is a "PackagePath", a "pathlib.PurePath"
derived object with additional "dist", "size", and "hash" properties
as indicated by the metadata.  For example:

   >>> util = [p for p in files('wheel') if 'util.py' in str(p)][0]  
   >>> util  
   PackagePath('wheel/util.py')
   >>> util.size  
   859
   >>> util.dist  
   <importlib.metadata._hooks.PathDistribution object at 0x101e0cef0>
   >>> util.hash  
   <FileHash mode: sha256 value: bYkw5oMccfazVCoYQwKkkemoVyMAFoR34mmKBx8R1NI>

ファイルを取得したら、その内容を読むこともできます:

   >>> print(util.read_text())  
   import base64
   import sys
   ...
   def as_bytes(s):
       if isinstance(s, text_type):
           return s.encode('utf-8')
       return s

また、 "locate" メソッドを使用すると、ファイルへの絶対パスを取得するこ
とができます:

   >>> util.locate()  
   PosixPath('/home/gustav/example/lib/site-packages/wheel/util.py')

In the case where the metadata file listing files (RECORD or
SOURCES.txt) is missing, "files()" will return "None". The caller may
wish to wrap calls to "files()" in always_iterable or otherwise guard
against this condition if the target distribution is not known to have
the metadata present.


配布物の要件
------------

To get the full set of requirements for a distribution, use the
"requires()" function:

   >>> requires('wheel')  
   ["pytest (>=3.0.0) ; extra == 'test'", "pytest-cov ; extra == 'test'"]


Distributions
=============

While the above API is the most common and convenient usage, you can
get all of that information from the "Distribution" class.  A
"Distribution" is an abstract object that represents the metadata for
a Python package.  You can get the "Distribution" instance:

   >>> from importlib.metadata import distribution  
   >>> dist = distribution('wheel')  

したがって、バージョン情報を取得する別の方法として、 "Distribution" イ
ンスタンスを使用します:

   >>> dist.version  
   '0.32.3'

"Distribution" インスタンスには、あらゆる種類の追加メタデータが用意さ
れています:

   >>> dist.metadata['Requires-Python']  
   '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
   >>> dist.metadata['License']  
   'MIT'

The full set of available metadata is not described here.  See **PEP
566** for additional details.


検索アルゴリズムの拡張
======================

Because package metadata is not available through "sys.path" searches,
or package loaders directly, the metadata for a package is found
through import system finders.  To find a distribution package's
metadata, "importlib.metadata" queries the list of *meta path finders*
on "sys.meta_path".

The default "PathFinder" for Python includes a hook that calls into
"importlib.metadata.MetadataPathFinder" for finding distributions
loaded from typical file-system-based paths.

抽象クラス "importlib.abc.MetaPathFinder" はPythonの importシステムに
よってファインダーに期待されるインターフェイスを定義しています。
"importlib.metadata" はこのプロトコルを拡張し、 "sys.meta_path" からフ
ァインダーにオプションの "find_distributions" を呼び出すことができるよ
うにし、この拡張インターフェースを "DistributionFinder" 抽象基底クラス
として提示し、この抽象メソッドを定義しています:

   @abc.abstractmethod
   def find_distributions(context=DistributionFinder.Context()):
       """Return an iterable of all Distribution instances capable of
       loading the metadata for packages for the indicated ``context``.
       """

"DistributionFinder.Context" オブジェクトは、検索するパスと一致する名
前を示す ".path" と ".name" のプロパティを提供し、その他の関連するコン
テキストを提供することもできます。

つまり、ファイルシステム以外の場所にある配布パッケージのメタデータを見
つけるには、 "Distribution" をサブクラス化して抽象メソッドを実装します
。そして、カスタムファインダーから "find_distributions()" メソッドで、
派生した "Distribution" のインスタンスを返します。

-[ 脚注 ]-

[1] Technically, the returned distribution metadata object is an
    "email.message.EmailMessage" instance, but this is an
    implementation detail, and not part of the stable API.  You should
    only use dictionary-like methods and syntax to access the metadata
    contents.
