"importlib.metadata" -- 存取套件的元資料
****************************************

在 3.8 版被加入.

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 provides the entry point and
metadata APIs that were previously exposed by the now-removed
"pkg_resources" package. Along with "importlib.resources", it
supersedes "pkg_resources".

"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 packages_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.

exception importlib.metadata.PackageNotFoundError

   Subclass of "ModuleNotFoundError" raised by several functions in
   this module when queried for a distribution package which is not
   installed in the current Python environment.


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

This package provides the following functionality via its public API.


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

importlib.metadata.entry_points(**select_params)

   Returns a "EntryPoints" instance describing entry points for the
   current environment. Any given keyword parameters are passed to the
   "select()" method for comparison to the attributes of the
   individual entry point definitions.

   Note: to query for entry points based on "EntryPoint.dist"
   attribute, use "Distribution.entry_points()" instead (as different
   "Distribution" instances do not currently compare equal, even if
   they have the same attributes)

class importlib.metadata.EntryPoints

   Details of a collection of installed entry points.

   Also provides a ".groups" attribute that reports all identified
   entry point groups, and a ".names" attribute that reports all
   identified entry point names.

class importlib.metadata.EntryPoint

   Details of an installed entry point.

   Each "EntryPoint" instance has ".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, and ".dist" for obtaining
   information regarding the distribution package that provides the
   entry point.

Query all entry points:

   >>> eps = entry_points()

The "entry_points()" function returns a "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.

在 3.12 版的變更: 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. With "importlib_metadata" 5.0 and
Python 3.12, "entry_points" always returns an "EntryPoints" object.
See backports.entry_points_selectable for compatibility options.

在 3.13 版的變更: "EntryPoint" objects no longer present a tuple-like
interface ("__getitem__()").


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

importlib.metadata.metadata(distribution_name)

   Return the distribution metadata corresponding to the named
   distribution package as a "PackageMetadata" instance.

   Raises "PackageNotFoundError" if the named distribution package is
   not installed in the current Python environment.

class importlib.metadata.PackageMetadata

   A concrete implementation of the PackageMetadata protocol.

   In addition to providing the defined protocol methods and
   attributes, subscripting the instance is equivalent to calling the
   "get()" method.

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 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.*'

The full set of available metadata is not described here. See the PyPA
Core metadata specification for additional details.

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.新增 "json" 屬性。


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

importlib.metadata.version(distribution_name)

   Return the installed distribution package version for the named
   distribution package.

   Raises "PackageNotFoundError" if the named distribution package is
   not installed in the current Python environment.

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
------------------

importlib.metadata.files(distribution_name)

   Return the full set of files contained within the named
   distribution package as "PackagePath" instances.

   Raises "PackageNotFoundError" if the named distribution package is
   not installed in the current Python environment.

   Returns "None" if the distribution is found but the installation
   database records reporting the files associated with the
   distribution package are missing.

class importlib.metadata.PackagePath

   A "pathlib.PurePath" derived object with additional "dist", "size",
   and "hash" properties corresponding to the distribution package's
   installation metadata for that file, also:

   locate()

      If possible, return the concrete "SimplePath" allowing to access
      data, or raise a "NotImplementedError" otherwise.

class importlib.metadata.SimplePath

   A protocol representing a minimal subset of "pathlib.Path" that
   allows to check if it "exists()", to traverse using "joinpath()"
   and "parent", and to retrieve data using "read_text()" and
   "read_bytes()".

The "files()" function takes a Distribution Package name and returns
all of the files installed by this distribution. 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 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
-------------------------

importlib.metadata.requires(distribution_name)

   Return the declared dependency specifiers for the named
   distribution package.

   Raises "PackageNotFoundError" if the named distribution package is
   not installed in the current Python environment.

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
---------------------------------------

importlib.metadata.packages_distributions()

   Return a mapping from the top level module and import package names
   found via "sys.meta_path" to the names of the distribution packages
   (if any) that provide the corresponding files.

   To allow for namespace packages (which may have members provided by
   multiple distribution packages), each top level import name maps to
   a list of distribution names rather than mapping directly to a
   single name.

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'], ...}

Some editable installs, do not supply top-level names, and thus this
function is not reliable with such installs.

在 3.10 版被加入.


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

While the module level API described above is the most common and
convenient usage, all that information is accessible from the
"Distribution" class. "Distribution" is an abstract object that
represents the metadata for a Python Distribution Package. Get the
concrete "Distribution" subclass instance for an installed
distribution package by calling the "distribution()" function:

   >>> from importlib.metadata import distribution
   >>> dist = distribution('wheel')
   >>> type(dist)
   <class 'importlib.metadata.PathDistribution'>

importlib.metadata.distribution(distribution_name)

   Return a "Distribution" instance describing the named distribution
   package.

   Raises "PackageNotFoundError" if the named distribution package is
   not installed in the current Python environment.

Thus, an alternative way to get e.g. the version number is through the
"Distribution.version" attribute:

   >>> dist.version
   '0.32.3'

The same applies for "entry_points()" and "files()".

class importlib.metadata.Distribution

   Details of an installed distribution package.

   Note: different "Distribution" instances do not currently compare
   equal, even if they relate to the same installed distribution and
   accordingly have the same attributes.

   static at(path)

   classmethod from_name(name)

      Return a "Distribution" instance at the given path or with the
      given name.

   classmethod discover(*, context=None, **kwargs)

      Returns an iterable of "Distribution" instances for all packages
      (see distribution-discovery).

      The optional argument *context* is a
      "DistributionFinder.Context" instance, used to modify the search
      for distributions. Alternatively, *kwargs* may contain keyword
      arguments for constructing a new "DistributionFinder.Context".

   metadata: PackageMetadata

      There are all kinds of additional metadata available on
      "Distribution" instances as a "PackageMetadata" 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 PyPA Core metadata specification for additional details.

   name: str

   requires: list[str]

   version: str

      A few metadata fields are also available as shortcut properties.

      在 3.10 版被加入: The "name" shortcut was added.

   origin

      For editable packages, an "origin" property may present **PEP
      610** metadata (for non-editable packages, "origin" is "None"):

         >>> dist.origin.url
         'file:///path/to/wheel-0.32.3.editable-py3-none-any.whl'

      The "origin" object follows the Direct URL Data Structure.

      在 3.13 版被加入.

   entry_points: EntryPoints

      The entry points provided by this distribution package.

   files: list[PackagePath] | None

      All files contained in this distribution package. Like
      "files()", this returns "None" if there are no records.

   The following two abstract methods need to be implemented when
   implementing-custom-providers:

   locate_file(path)

      Like "PackagePath.locate()", return a "SimplePath" for the given
      path. Takes a "os.PathLike" or a "str".

   read_text(filename)

      A shortcut for "distribution.locate_file(filename).read_text()".


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.

class importlib.metadata.DistributionFinder

   A "MetaPathFinder" subclass capable of discovering installed
   distributions.

   Custom providers should implement this interface in order to supply
   metadata.

      class Context(**kwargs)

         A "Context" gives a custom provider a means to solicit
         additional details from the callers of distribution discovery
         functions like "distributions()" or "Distribution.discover()"
         beyond ".name" and ".path" when searching for distributions.

         For example, a provider could expose suites of packages in
         either a "public" or "private" "realm". A caller of
         distribution discovery functions may wish to query only for
         distributions in a particular realm and could call
         "distributions(realm="private")" to signal to the custom
         provider to only include distributions from that realm.

         Each "DistributionFinder" must expect any parameters and
         should attempt to honor the canonical parameters defined
         below when appropriate.

         See the section on Implementing Custom Providers for more
         details.

         name

            Specific name for which a distribution finder should
            match.

            A ".name" of "None" matches all distributions.

         path

            A property providing the sequence of directory paths that
            a distribution finder should search.

            Typically refers to Python installed package paths such as
            "site-packages" directories and defaults to "sys.path".

importlib.metadata.distributions(**kwargs)

   Returns an iterable of "Distribution" instances for all packages.

   The *kwargs* argument may contain either a keyword argument
   "context", a "DistributionFinder.Context" instance, or pass keyword
   arguments for constructing a new "DistributionFinder.Context". The
   "DistributionFinder.Context" is used to modify the search for
   distributions.


Implementing Custom Providers
=============================

"importlib.metadata" address two API surfaces, one for *consumers* and
another for *providers*. Most users are consumers, consuming metadata
provided by the packages. There are other use-cases, however, where
users wish to expose metadata through some other mechanism, such as
alongside a custom importer. Such a use case calls for a *custom
provider*.

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".

The implementation has hooks integrated into the "PathFinder", serving
metadata for distribution packages found on the file system.

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()) -> Iterable[Distribution]:
       """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 sought by the consumer.

In practice, 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.


範例
----

Imagine a custom finder that loads Python modules from a database:

   class DatabaseImporter(importlib.abc.MetaPathFinder):
       def __init__(self, db):
           self.db = db

       def find_spec(self, fullname, target=None) -> ModuleSpec:
           return self.db.spec_from_name(fullname)

   sys.meta_path.append(DatabaseImporter(connect_db(...)))

That importer now presumably provides importable modules from a
database, but it provides no metadata or entry points. For this custom
importer to provide metadata, it would also need to implement
"DistributionFinder":

   from importlib.metadata import DistributionFinder

   class DatabaseImporter(DistributionFinder):
       ...

       def find_distributions(self, context=DistributionFinder.Context()):
           query = dict(name=context.name) if context.name else {}
           for dist_record in self.db.query_distributions(query):
               yield DatabaseDistribution(dist_record)

In this way, "query_distributions" would return records for each
distribution served by the database matching the query. For example,
if "requests-1.0" is in the database, "find_distributions" would yield
a "DatabaseDistribution" for "Context(name='requests')" or
"Context(name=None)".

For the sake of simplicity, this example ignores "context.path". The
"path" attribute defaults to "sys.path" and is the set of import paths
to be considered in the search. A "DatabaseImporter" could potentially
function without any concern for a search path. Assuming the importer
does no partitioning, the "path" would be irrelevant. In order to
illustrate the purpose of "path", the example would need to illustrate
a more complex "DatabaseImporter" whose behavior varied depending on
"sys.path"/"PYTHONPATH". In that case, the "find_distributions" should
honor the "context.path" and only yield "Distribution"s pertinent to
that path.

"DatabaseDistribution", then, would look something like:

   class DatabaseDistribution(importlib.metadata.Distribution):
       def __init__(self, record):
           self.record = record

       def read_text(self, filename):
           """
           Read a file like "METADATA" for the current distribution.
           """
           if filename == "METADATA":
               return f"""Name: {self.record.name}
   Version: {self.record.version}
   """
           if filename == "entry_points.txt":
               return "\n".join(
                 f"""[{ep.group}]\n{ep.name}={ep.value}"""
                 for ep in self.record.entry_points)

       def locate_file(self, path):
           raise RuntimeError("This distribution has no file system")

This basic implementation should provide metadata and entry points for
packages served by the "DatabaseImporter", assuming that the "record"
supplies suitable ".name", ".version", and ".entry_points" attributes.

The "DatabaseDistribution" may also provide other metadata files, like
"RECORD" (required for "Distribution.files") or override the
implementation of "Distribution.files". See the source for more
inspiration.
