.. _using:

=================================
 Using :mod:`!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
:mod:`importlib.resources` in Python 3.7
and newer (backported as :doc:`importlib_resources <importlib_resources:index>` 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 zip archives on
:data:`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 importlib_metadata
    (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 :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 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 sequence 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
<https://setuptools.readthedocs.io/en/latest/setuptools.html#dynamic-discovery-of-services-and-plugins>`_
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.


.. _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, 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.*'


.. _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 :class:`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()  # doctest: +SKIP
    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
<https://more-itertools.readthedocs.io/en/stable/api.html#more_itertools.always_iterable>`_
or otherwise guard against this condition if the target
distribution is not known to have the metadata present.

.. _requirements:

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'"]


Package distributions
---------------------

A convience method to resolve the distribution or
distributions (in the case of a namespace package) for top-level
Python packages or modules::

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


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 :data:`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 :term:`meta path finders <meta path finder>` on
:data:`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
:data:`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.


.. _`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
.. _`finders`: https://docs.python.org/3/reference/import.html#finders-and-loaders


.. rubric:: Footnotes
