Using importlib.metadata
************************

備註: 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.


Overview
========

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

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 the set of entry points keyed by group, such as
"console_scripts", "distutils.commands" and others.  Each group
contains a sequence 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 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.

>>> eps = entry_points()  # doctest: +SKIP
>>> list(eps)  # doctest: +SKIP
['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation']
>>> scripts = eps['console_scripts']  # doctest: +SKIP
>>> wheel = [ep for ep in scripts if ep.name == 'wheel'][0]  # doctest: +SKIP
>>> wheel  # doctest: +SKIP
EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts')
>>> main = wheel.load()  # doctest: +SKIP
>>> main  # doctest: +SKIP
<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
entrypoints, their definition, and usage.


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

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.*'


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

The "version()" function is the quickest way to get a distribution'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.Path 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


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

To get the full set of requirements for a distribution, use the
"requires()" function.  Note that this returns an iterator:

   >>> list(requires('wheel'))  
   ["pytest (>=3.0.0) ; 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')  

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:

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

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


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

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.

By default "importlib.metadata" installs a finder for distribution
packages found on the file system.  This finder doesn't actually find
any *packages*, but it can find the packages' 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".  If
the finder has this method, it must return an iterator over instances
of the "Distribution" abstract class. This method must have the
signature:

   def find_distributions(name=None, path=None):
       """Return an iterable of all Distribution instances capable of
       loading the metadata for packages matching the name
       (or all names if not supplied) along the paths in the list
       of directories ``path`` (defaults to sys.path).
       """

What this means in practice is that to support finding distribution
package metadata in locations other than the file system, you should
derive from "Distribution" and implement the "load_metadata()" method.
This takes a single argument which is the name of the package whose
metadata is being found.  This instance of the "Distribution" base
abstract class is what your finder's "find_distributions()" method
should return.

-[ Footnotes ]-

[1] Technically, the returned distribution metadata object is an
    email.message.Message 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.
