bpo-34632: Add importlib.metadata

Doc/library/importlib.metadata.rst

@@ -0,0 +1,260 @@
+.. _using:
+
+==========================
+ Using importlib.metadata
+==========================
+
+``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
+<https://pypi.org/project/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 wheels on
+``sys.path``.  Through an extension mechanism, the metadata can live almost
+anywhere.
+
+.. note::  Although this package supports loading metadata from wheels
+   on ``sys.path``, that support is provisional and does not serve to
+   contravene the `PEP 427 directive
+   <https://www.python.org/dev/peps/pep-0427/#is-it-possible-to-import-python-code-directly-from-a-wheel-file>`_,
+   which states that relying on this format is discouraged, and use is
+   at your own risk.
+
+
+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::
+
+.. highlight:: none
+
+    $ python3 -m venv example
+    $ source example/bin/activate
+    (example) $ pip install wheel
+
+You can get the version string for ``wheel`` by running the following::
+
+.. highlight:: none
+
+    (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 :ref:`EntryPoint <entry-points>` objects.
+
+You can get the :ref:`metadata for a distribution <metadata>`::
+
+    >>> 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 :ref:`distribution's version number <version>`, list its
+:ref:`constituent files <files>`, and get a list of the distribution's
+:ref:`requirements`.
+
+
+Functional API
+==============
+
+This package provides the following functionality via its public API.
+
+
+.. _entry-points:
+
+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()
+    >>> 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')
+    >>> 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
+<https://setuptools.readthedocs.io/en/latest/setuptools.html#dynamic-discovery-of-services-and-plugins>`_
+for more information on entrypoints, their definition, and usage.
+
+
+.. _metadata:
+
+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 [#f1]_ 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.*'
+
+
+.. _version:
+
+Distribution versions
+---------------------
+
+The ``version()`` function is the quickest way to get a distribution's version
+number, as a string::
+
+    >>> version('wheel')
+    '0.32.3'
+
+
+.. _files:
+
+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
+
+
+.. _requirements:
+
+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
+<https://www.python.org/dev/peps/pep-0566/>`_ 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 :py: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=sys.path):
+        """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.
+
+
+.. _`entry point API`: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points
+.. _`metadata API`: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#metadata-api
+.. _`Python 3.7 and newer`: https://docs.python.org/3/library/importlib.html#module-importlib.resources
+.. _`importlib_resources`: https://importlib-resources.readthedocs.io/en/latest/index.html
+.. _`PEP 566`: https://www.python.org/dev/peps/pep-0566/
+.. _`finders`: https://docs.python.org/3/reference/import.html#finders-and-loaders
+.. _`meta path finders`: https://docs.python.org/3/glossary.html#term-meta-path-finder
+.. _`sys.meta_path`: https://docs.python.org/3/library/sys.html#sys.meta_path
+.. _`pathlib.Path`: https://docs.python.org/3/library/pathlib.html#pathlib.Path
+
+
+.. rubric:: Footnotes
+
+.. [#f1] Technically, the returned distribution metadata object is an
+         `email.message.Message
+         <https://docs.python.org/3/library/email.message.html#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.

Doc/library/modules.rst

@@ -17,3 +17,4 @@ The full list of modules described in this chapter is:
    modulefinder.rst
    runpy.rst
    importlib.rst
+   importlib.metadata.rst

Lib/importlib/metadata/__init__.py

@@ -0,0 +1,17 @@
+from .api import distribution, Distribution, PackageNotFoundError  # noqa: F401
+from .api import (
+    metadata, entry_points, version, files, requires, distributions,
+    )
+
+# Import for installation side-effects.
+from . import _hooks  # noqa: F401
+
+
+__all__ = [
+    'entry_points',
+    'files',
+    'metadata',
+    'requires',
+    'version',
+    'distributions',
+    ]