mirror of https://github.com/python/cpython
399 lines
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ReStructuredText
399 lines
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ReStructuredText
.. _using:
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=================================
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Using :mod:`!importlib.metadata`
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=================================
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.. module:: importlib.metadata
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:synopsis: The implementation of the importlib metadata.
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.. versionadded:: 3.8
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.. versionchanged:: 3.10
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``importlib.metadata`` is no longer provisional.
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**Source code:** :source:`Lib/importlib/metadata/__init__.py`
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``importlib_metadata`` is a library that provides access to
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the metadata of an installed `Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_,
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such as its entry points
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or its top-level names (`Import Package <https://packaging.python.org/en/latest/glossary/#term-Import-Package>`_\s, modules, if any).
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Built in part on Python's import system, this library
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intends to replace similar functionality in the `entry point
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API`_ and `metadata API`_ of ``pkg_resources``. Along with
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:mod:`importlib.resources`,
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this package can eliminate the need to use the older and less efficient
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``pkg_resources`` package.
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``importlib_metadata`` operates on third-party *distribution packages*
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installed into Python's ``site-packages`` directory via tools such as
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`pip <https://pypi.org/project/pip/>`_.
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Specifically, it works with distributions with discoverable
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``dist-info`` or ``egg-info`` directories,
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and metadata defined by the `Core metadata specifications <https://packaging.python.org/en/latest/specifications/core-metadata/#core-metadata>`_.
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.. important::
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These are *not* necessarily equivalent to or correspond 1:1 with
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the top-level *import package* names
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that can be imported inside Python code.
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One *distribution package* can contain multiple *import packages*
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(and single modules),
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and one top-level *import package*
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may map to multiple *distribution packages*
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if it is a namespace package.
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You can use :ref:`package_distributions() <package-distributions>`
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to get a mapping between them.
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By default, distribution metadata can live on the file system
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or in zip archives on
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:data:`sys.path`. Through an extension mechanism, the metadata can live almost
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anywhere.
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.. seealso::
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https://importlib-metadata.readthedocs.io/
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The documentation for ``importlib_metadata``, which supplies a
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backport of ``importlib.metadata``.
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This includes an `API reference
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<https://importlib-metadata.readthedocs.io/en/latest/api.html>`__
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for this module's classes and functions,
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as well as a `migration guide
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<https://importlib-metadata.readthedocs.io/en/latest/migration.html>`__
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for existing users of ``pkg_resources``.
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Overview
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========
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Let's say you wanted to get the version string for a
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`Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_ you've installed
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using ``pip``. We start by creating a virtual environment and installing
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something into it:
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.. code-block:: shell-session
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$ python3 -m venv example
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$ source example/bin/activate
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(example) $ python -m pip install wheel
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You can get the version string for ``wheel`` by running the following:
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.. code-block:: pycon
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(example) $ python
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>>> from importlib.metadata import version # doctest: +SKIP
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>>> version('wheel') # doctest: +SKIP
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'0.32.3'
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You can also get a collection of entry points selectable by properties of the EntryPoint (typically 'group' or 'name'), such as
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``console_scripts``, ``distutils.commands`` and others. Each group contains a
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collection of :ref:`EntryPoint <entry-points>` objects.
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You can get the :ref:`metadata for a distribution <metadata>`::
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>>> list(metadata('wheel')) # doctest: +SKIP
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['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']
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You can also get a :ref:`distribution's version number <version>`, list its
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:ref:`constituent files <files>`, and get a list of the distribution's
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:ref:`requirements`.
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Functional API
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==============
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This package provides the following functionality via its public API.
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.. _entry-points:
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Entry points
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------------
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The ``entry_points()`` function returns a collection of entry points.
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Entry points are represented by ``EntryPoint`` instances;
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each ``EntryPoint`` has a ``.name``, ``.group``, and ``.value`` attributes and
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a ``.load()`` method to resolve the value. There are also ``.module``,
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``.attr``, and ``.extras`` attributes for getting the components of the
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``.value`` attribute.
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Query all entry points::
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>>> eps = entry_points() # doctest: +SKIP
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The ``entry_points()`` function returns an ``EntryPoints`` object,
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a collection of all ``EntryPoint`` objects with ``names`` and ``groups``
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attributes for convenience::
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>>> sorted(eps.groups) # doctest: +SKIP
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['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation']
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``EntryPoints`` has a ``select`` method to select entry points
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matching specific properties. Select entry points in the
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``console_scripts`` group::
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>>> scripts = eps.select(group='console_scripts') # doctest: +SKIP
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Equivalently, since ``entry_points`` passes keyword arguments
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through to select::
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>>> scripts = entry_points(group='console_scripts') # doctest: +SKIP
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Pick out a specific script named "wheel" (found in the wheel project)::
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>>> 'wheel' in scripts.names # doctest: +SKIP
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True
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>>> wheel = scripts['wheel'] # doctest: +SKIP
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Equivalently, query for that entry point during selection::
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>>> (wheel,) = entry_points(group='console_scripts', name='wheel') # doctest: +SKIP
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>>> (wheel,) = entry_points().select(group='console_scripts', name='wheel') # doctest: +SKIP
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Inspect the resolved entry point::
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>>> wheel # doctest: +SKIP
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EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts')
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>>> wheel.module # doctest: +SKIP
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'wheel.cli'
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>>> wheel.attr # doctest: +SKIP
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'main'
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>>> wheel.extras # doctest: +SKIP
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[]
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>>> main = wheel.load() # doctest: +SKIP
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>>> main # doctest: +SKIP
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<function main at 0x103528488>
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The ``group`` and ``name`` are arbitrary values defined by the package author
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and usually a client will wish to resolve all entry points for a particular
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group. Read `the setuptools docs
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<https://setuptools.pypa.io/en/latest/userguide/entry_point.html>`_
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for more information on entry points, their definition, and usage.
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*Compatibility Note*
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The "selectable" entry points were introduced in ``importlib_metadata``
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3.6 and Python 3.10. Prior to those changes, ``entry_points`` accepted
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no parameters and always returned a dictionary of entry points, keyed
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by group. With ``importlib_metadata`` 5.0 and Python 3.12,
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``entry_points`` always returns an ``EntryPoints`` object. See
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`backports.entry_points_selectable <https://pypi.org/project/backports.entry_points_selectable>`_
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for compatibility options.
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.. _metadata:
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Distribution metadata
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---------------------
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Every `Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_ includes some metadata,
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which you can extract using the
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``metadata()`` function::
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>>> wheel_metadata = metadata('wheel') # doctest: +SKIP
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The keys of the returned data structure, a ``PackageMetadata``,
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name the metadata keywords, and
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the values are returned unparsed from the distribution metadata::
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>>> wheel_metadata['Requires-Python'] # doctest: +SKIP
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'>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
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``PackageMetadata`` also presents a ``json`` attribute that returns
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all the metadata in a JSON-compatible form per :PEP:`566`::
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>>> wheel_metadata.json['requires_python']
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'>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
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.. note::
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The actual type of the object returned by ``metadata()`` is an
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implementation detail and should be accessed only through the interface
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described by the
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`PackageMetadata protocol <https://importlib-metadata.readthedocs.io/en/latest/api.html#importlib_metadata.PackageMetadata>`_.
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.. versionchanged:: 3.10
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The ``Description`` is now included in the metadata when presented
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through the payload. Line continuation characters have been removed.
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.. versionadded:: 3.10
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The ``json`` attribute was added.
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.. _version:
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Distribution versions
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---------------------
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The ``version()`` function is the quickest way to get a
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`Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_'s version
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number, as a string::
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>>> version('wheel') # doctest: +SKIP
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'0.32.3'
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.. _files:
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Distribution files
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------------------
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You can also get the full set of files contained within a distribution. The
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``files()`` function takes a `Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_ name
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and returns all of the
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files installed by this distribution. Each file object returned is a
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``PackagePath``, a :class:`pathlib.PurePath` derived object with additional ``dist``,
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``size``, and ``hash`` properties as indicated by the metadata. For example::
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>>> util = [p for p in files('wheel') if 'util.py' in str(p)][0] # doctest: +SKIP
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>>> util # doctest: +SKIP
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PackagePath('wheel/util.py')
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>>> util.size # doctest: +SKIP
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859
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>>> util.dist # doctest: +SKIP
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<importlib.metadata._hooks.PathDistribution object at 0x101e0cef0>
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>>> util.hash # doctest: +SKIP
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<FileHash mode: sha256 value: bYkw5oMccfazVCoYQwKkkemoVyMAFoR34mmKBx8R1NI>
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Once you have the file, you can also read its contents::
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>>> print(util.read_text()) # doctest: +SKIP
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import base64
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import sys
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...
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def as_bytes(s):
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if isinstance(s, text_type):
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return s.encode('utf-8')
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return s
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You can also use the ``locate`` method to get a the absolute path to the
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file::
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>>> util.locate() # doctest: +SKIP
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PosixPath('/home/gustav/example/lib/site-packages/wheel/util.py')
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In the case where the metadata file listing files
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(RECORD or SOURCES.txt) is missing, ``files()`` will
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return ``None``. The caller may wish to wrap calls to
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``files()`` in `always_iterable
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<https://more-itertools.readthedocs.io/en/stable/api.html#more_itertools.always_iterable>`_
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or otherwise guard against this condition if the target
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distribution is not known to have the metadata present.
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.. _requirements:
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Distribution requirements
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-------------------------
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To get the full set of requirements for a `Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_,
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use the ``requires()``
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function::
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>>> requires('wheel') # doctest: +SKIP
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["pytest (>=3.0.0) ; extra == 'test'", "pytest-cov ; extra == 'test'"]
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.. _package-distributions:
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.. _import-distribution-package-mapping:
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Mapping import to distribution packages
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---------------------------------------
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A convenience method to resolve the `Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_
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name (or names, in the case of a namespace package)
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that provide each importable top-level
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Python module or `Import Package <https://packaging.python.org/en/latest/glossary/#term-Import-Package>`_::
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>>> packages_distributions()
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{'importlib_metadata': ['importlib-metadata'], 'yaml': ['PyYAML'], 'jaraco': ['jaraco.classes', 'jaraco.functools'], ...}
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.. versionadded:: 3.10
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.. _distributions:
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Distributions
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=============
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While the above API is the most common and convenient usage, you can get all
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of that information from the ``Distribution`` class. A ``Distribution`` is an
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abstract object that represents the metadata for
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a Python `Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_. You can
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get the ``Distribution`` instance::
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>>> from importlib.metadata import distribution # doctest: +SKIP
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>>> dist = distribution('wheel') # doctest: +SKIP
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Thus, an alternative way to get the version number is through the
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``Distribution`` instance::
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>>> dist.version # doctest: +SKIP
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'0.32.3'
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There are all kinds of additional metadata available on the ``Distribution``
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instance::
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>>> dist.metadata['Requires-Python'] # doctest: +SKIP
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'>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
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>>> dist.metadata['License'] # doctest: +SKIP
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'MIT'
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The full set of available metadata is not described here.
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See the `Core metadata specifications <https://packaging.python.org/en/latest/specifications/core-metadata/#core-metadata>`_ for additional details.
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Distribution Discovery
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======================
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By default, this package provides built-in support for discovery of metadata
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for file system and zip file `Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_\s.
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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:
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- ``importlib.metadata`` does not honor :class:`bytes` objects on ``sys.path``.
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- ``importlib.metadata`` will incidentally honor :py:class:`pathlib.Path` objects on ``sys.path`` even though such values will be ignored for imports.
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Extending the search algorithm
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==============================
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Because `Distribution Package <https://packaging.python.org/en/latest/glossary/#term-Distribution-Package>`_ metadata
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is not available through :data:`sys.path` searches, or
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package loaders directly,
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the metadata for a distribution is found through import
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system :ref:`finders <finders-and-loaders>`. To find a distribution package's metadata,
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``importlib.metadata`` queries the list of :term:`meta path finders <meta path finder>` on
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:data:`sys.meta_path`.
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By default ``importlib_metadata`` installs a finder for distribution packages
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found on the file system.
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This finder doesn't actually find any *distributions*,
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but it can find their metadata.
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The abstract class :py:class:`importlib.abc.MetaPathFinder` defines the
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interface expected of finders by Python's import system.
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``importlib.metadata`` extends this protocol by looking for an optional
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``find_distributions`` callable on the finders from
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:data:`sys.meta_path` and presents this extended interface as the
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``DistributionFinder`` abstract base class, which defines this abstract
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method::
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@abc.abstractmethod
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def find_distributions(context=DistributionFinder.Context()):
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"""Return an iterable of all Distribution instances capable of
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loading the metadata for packages for the indicated ``context``.
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"""
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The ``DistributionFinder.Context`` object provides ``.path`` and ``.name``
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properties indicating the path to search and name to match and may
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supply other relevant context.
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What this means in practice is that to support finding distribution package
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metadata in locations other than the file system, subclass
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``Distribution`` and implement the abstract methods. Then from
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a custom finder, return instances of this derived ``Distribution`` in the
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``find_distributions()`` method.
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.. _`entry point API`: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points
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.. _`metadata API`: https://setuptools.readthedocs.io/en/latest/pkg_resources.html#metadata-api
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