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

3.8 新版功能.

注解:

  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.


概述
====

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.


可用 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.  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.


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

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, 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')  

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 **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".

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

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.

-[ 备注 ]-

[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.
