.. module:: importlib.metadata :synopsis: The implementation of the importlib metadata.
.. versionadded:: 3.8
.. versionchanged:: 3.10 ``importlib.metadata`` is no longer provisional.
Source code: :source:`Lib/importlib/metadata/__init__.py`
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` (with new features backported to the
importlib_resources package), 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
:data:`sys.path`. Through an extension mechanism, the metadata can live almost
anywhere.
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) $ python -m pip install wheel
You can get the version string for wheel by running the following:
(example) $ python
>>> from importlib.metadata import version # doctest: +SKIP
>>> version('wheel') # doctest: +SKIP
'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')) # doctest: +SKIP
['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`.
This package provides the following functionality via its public API.
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() # doctest: +SKIP
The entry_points() function returns an EntryPoints object,
a sequence of all EntryPoint objects with names and groups
attributes for convenience:
>>> sorted(eps.groups) # doctest: +SKIP ['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') # doctest: +SKIP
Equivalently, since entry_points passes keyword arguments
through to select:
>>> scripts = entry_points(group='console_scripts') # doctest: +SKIP
Pick out a specific script named "wheel" (found in the wheel project):
>>> 'wheel' in scripts.names # doctest: +SKIP True >>> wheel = scripts['wheel'] # doctest: +SKIP
Equivalently, query for that entry point during selection:
>>> (wheel,) = entry_points(group='console_scripts', name='wheel') # doctest: +SKIP >>> (wheel,) = entry_points().select(group='console_scripts', name='wheel') # doctest: +SKIP
Inspect the resolved entry point:
>>> wheel # doctest: +SKIP EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts') >>> wheel.module # doctest: +SKIP 'wheel.cli' >>> wheel.attr # doctest: +SKIP 'main' >>> wheel.extras # doctest: +SKIP [] >>> main = wheel.load() # doctest: +SKIP >>> main # doctest: +SKIP <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.
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.
Every distribution includes some metadata, which you can extract using the
metadata() function:
>>> wheel_metadata = metadata('wheel') # doctest: +SKIP
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'] # doctest: +SKIP '>=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.*'
.. versionchanged:: 3.10 The ``Description`` is now included in the metadata when presented through the payload. Line continuation characters have been removed.
.. versionadded:: 3.10 The ``json`` attribute was added.
The version() function is the quickest way to get a distribution's version
number, as a string:
>>> version('wheel') # doctest: +SKIP
'0.32.3'
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] # doctest: +SKIP
>>> util # doctest: +SKIP
PackagePath('wheel/util.py')
>>> util.size # doctest: +SKIP
859
>>> util.dist # doctest: +SKIP
<importlib.metadata._hooks.PathDistribution object at 0x101e0cef0>
>>> util.hash # doctest: +SKIP
<FileHash mode: sha256 value: bYkw5oMccfazVCoYQwKkkemoVyMAFoR34mmKBx8R1NI>
Once you have the file, you can also read its contents:
>>> print(util.read_text()) # doctest: +SKIP
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
or otherwise guard against this condition if the target
distribution is not known to have the metadata present.
To get the full set of requirements for a distribution, use the requires()
function:
>>> requires('wheel') # doctest: +SKIP
["pytest (>=3.0.0) ; extra == 'test'", "pytest-cov ; extra == 'test'"]
A convenience 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'], ...}
.. versionadded:: 3.10
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 # doctest: +SKIP
>>> dist = distribution('wheel') # doctest: +SKIP
Thus, an alternative way to get the version number is through the
Distribution instance:
>>> dist.version # doctest: +SKIP '0.32.3'
There are all kinds of additional metadata available on the Distribution
instance:
>>> dist.metadata['Requires-Python'] # doctest: +SKIP '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*' >>> dist.metadata['License'] # doctest: +SKIP 'MIT'
The full set of available metadata is not described here. See PEP 566 for additional details.
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 :ref:`finders <finders-and-loaders>`. 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`.
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 :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.