"importlib.metadata" -- Accessing package metadata
**************************************************

在 3.8 版新加入.

在 3.10 版的變更: "importlib.metadata" is no longer provisional.

**原始碼：**Lib/importlib/metadata/__init__.py

"importlib.metadata" is a library that provides access to the metadata
of an installed Distribution Package, such as its entry points or its
top-level names (Import Packages, modules, if any). 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", this package can
eliminate the need to use the older and less efficient "pkg_resources"
package.

"importlib.metadata" operates on third-party *distribution packages*
installed into Python's "site-packages" directory via tools such as
pip. Specifically, it works with distributions with discoverable
"dist-info" or "egg-info" directories, and metadata defined by the
Core metadata specifications.

重要:

  These are *not* necessarily equivalent to or correspond 1:1 with the
  top-level *import package* names that can be imported inside Python
  code. One *distribution package* can contain multiple *import
  packages* (and single modules), and one top-level *import package*
  may map to multiple *distribution packages* if it is a namespace
  package. You can use package_distributions() to get a mapping
  between them.

By default, distribution metadata can live on the file system or in
zip archives on "sys.path".  Through an extension mechanism, the
metadata can live almost anywhere.

也參考:

  https://importlib-metadata.readthedocs.io/
     The documentation for "importlib_metadata", which supplies a
     backport of "importlib.metadata". This includes an API reference
     for this module's classes and functions, as well as a migration
     guide for existing users of "pkg_resources".


Overview
========

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

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

You can get the version string for "wheel" by running the following:

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

You can also get a collection of entry points selectable by properties
of the EntryPoint (typically 'group' or 'name'), such as
"console_scripts", "distutils.commands" and others.  Each group
contains a collection of EntryPoint objects.

You can get the metadata for a distribution:

   >>> 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']

You can also get a distribution's version number, list its constituent
files, and get a list of the distribution's Distribution requirements.


Functional API
==============

This package provides the following functionality via its public API.


Entry points
------------

The "entry_points()" function returns a collection of entry points.
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.

Query all entry points:

   >>> eps = entry_points()  

The "entry_points()" function returns an "EntryPoints" object, a
collection of all "EntryPoint" objects with "names" and "groups"
attributes for convenience:

   >>> sorted(eps.groups)  
   ['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation']

"EntryPoints" has a "select" method to select entry points matching
specific properties. Select entry points in the "console_scripts"
group:

   >>> scripts = eps.select(group='console_scripts')  

Equivalently, since "entry_points" passes keyword arguments through to
select:

   >>> scripts = entry_points(group='console_scripts')  

Pick out a specific script named "wheel" (found in the wheel project):

   >>> 'wheel' in scripts.names  
   True
   >>> wheel = scripts['wheel']  

Equivalently, query for that entry point during selection:

   >>> (wheel,) = entry_points(group='console_scripts', name='wheel')  
   >>> (wheel,) = entry_points().select(group='console_scripts', name='wheel')  

Inspect the resolved entry point:

   >>> 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.

*Compatibility Note*

The "selectable" entry points were introduced in "importlib_metadata"
3.6 and Python 3.10. Prior to those changes, "entry_points" accepted
no parameters and always returned a dictionary of entry points, keyed
by group. For compatibility, if no parameters are passed to
entry_points, a "SelectableGroups" object is returned, implementing
that dict interface. In the future, calling "entry_points" with no
parameters will return an "EntryPoints" object. Users should rely on
the selection interface to retrieve entry points by group.


Distribution metadata
---------------------

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

   >>> wheel_metadata = metadata('wheel')  

The keys of the returned data structure, a "PackageMetadata", name the
metadata keywords, and the values are returned unparsed from the
distribution metadata:

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

"PackageMetadata" also presents a "json" attribute that returns all
the metadata in a JSON-compatible form per **PEP 566**:

   >>> wheel_metadata.json['requires_python']
   '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'

備註:

  The actual type of the object returned by "metadata()" is an
  implementation detail and should be accessed only through the
  interface described by the PackageMetadata protocol.

在 3.10 版的變更: The "Description" is now included in the metadata
when presented through the payload. Line continuation characters have
been removed.The "json" attribute was added.


Distribution versions
---------------------

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

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


Distribution files
------------------

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>

Once you have the file, you can also read its contents:

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

You can also use the "locate" method to get a the absolute path to the
file:

   >>> 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.


Distribution requirements
-------------------------

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

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


Mapping import to distribution packages
---------------------------------------

A convenience method to resolve the Distribution Package name (or
names, in the case of a namespace package) that provide each
importable top-level Python module or Import Package:

   >>> packages_distributions()
   {'importlib_metadata': ['importlib-metadata'], 'yaml': ['PyYAML'], 'jaraco': ['jaraco.classes', 'jaraco.functools'], ...}

在 3.10 版新加入.


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 Distribution Package.  You can get the "Distribution"
instance:

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

Thus, an alternative way to get the version number is through the
"Distribution" instance:

   >>> dist.version  
   '0.32.3'

There are all kinds of additional metadata available on the
"Distribution" instance:

   >>> 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 the Core
metadata specifications for additional details.


Distribution Discovery
======================

By default, this package provides built-in support for discovery of
metadata for file system and zip file Distribution Packages. This
metadata finder search defaults to "sys.path", but varies slightly in
how it interprets those values from how other import machinery does.
In particular:

* "importlib.metadata" does not honor "bytes" objects on "sys.path".

* "importlib.metadata" will incidentally honor "pathlib.Path" objects
  on "sys.path" even though such values will be ignored for imports.


Extending the search algorithm
==============================

Because Distribution Package metadata is not available through
"sys.path" searches, or package loaders directly, the metadata for a
distribution 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".

By default "importlib.metadata" installs a finder for distribution
packages found on the file system. This finder doesn't actually find
any *distributions*, but it can find their metadata.

The abstract class "importlib.abc.MetaPathFinder" defines the
interface expected of finders by Python's import system.
"importlib.metadata" extends this protocol by looking for an optional
"find_distributions" callable on the finders from "sys.meta_path" and
presents this extended interface as the "DistributionFinder" abstract
base class, which defines this abstract method:

   @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``.
       """

The "DistributionFinder.Context" object provides ".path" and ".name"
properties indicating the path to search and name to match and may
supply other relevant context.

What this means in practice is that to support finding distribution
package metadata in locations other than the file system, subclass
"Distribution" and implement the abstract methods. Then from a custom
finder, return instances of this derived "Distribution" in the
"find_distributions()" method.
