- Functional API
- Entry points
- Distribution metadata
- Distribution versions
- Distribution files
- Distribution requirements
- Extending the search algorithm
This functionality is provisional and may deviate from the usual version semantics of the standard library.
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
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
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
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) $ 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
distutils.commands and others. Each group contains a sequence 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']
This package provides the following functionality via its public API.
entry_points() function returns a dictionary of all entry points, keyed by group. Entry points are represented by
EntryPoint instances; each
EntryPoint has a
.value attributes and a
.load() method to resolve the value.
>>> eps = entry_points() # doctest: +SKIP >>> list(eps) # doctest: +SKIP ['console_scripts', 'distutils.commands', 'distutils.setup_keywords', 'egg_info.writers', 'setuptools.installation'] >>> scripts = eps['console_scripts'] # doctest: +SKIP >>> wheel = [ep for ep in scripts if ep.name == 'wheel'] # doctest: +SKIP >>> wheel # doctest: +SKIP EntryPoint(name='wheel', value='wheel.cli:main', group='console_scripts') >>> main = wheel.load() # doctest: +SKIP >>> main # doctest: +SKIP <function main at 0x103528488>
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 entrypoints, their definition, and usage.
Every distribution includes some metadata, which you can extract using the
>>> wheel_metadata = metadata('wheel')
The keys of the returned data structure 1 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() function is the quickest way to get a distribution’s version number, as a string:>>>
>>> version('wheel') '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 pathlib.Path derived object with additional
hash properties as indicated by the metadata. For example:>>>
>>> util = [p for p in files('wheel') if 'util.py' in str(p)] >>> 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
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('wheel') ["pytest (>=3.0.0) ; extra == 'test'", "pytest-cov ; extra == 'test'"]
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
>>> from importlib.metadata import distribution >>> dist = distribution('wheel')
Thus, an alternative way to get the version number is through the
>>> dist.version '0.32.3'
There are all kinds of additional metadata available on the
>>> 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 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.
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
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()): """Return an iterable of all Distribution instances capable of loading the metadata for packages for the indicated ``context``. """
DistributionFinder.Context object provides
.name properties indicating the path to search and names 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, you should derive from
Distribution and implement the
load_metadata() method. Then from your finder, return instances of this derived
Distribution in the
Technically, the returned distribution metadata object is an email.message.Message 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.